btrfs: try to allocate larger folios for metadata
Subsystems (4)
v7 cover btrfs: try to allocate larger folios for metadata
[CHANGELOG]
v7:
- Fix an accidentially removed line caused by previous modification attempt
Previously I was moving that line to the common branch to unconditionally define root_mem_cgroup pointer.
But that's later discarded and changed to use macro definition, but forgot to add back the original line.
v6:
- Add a new root_mem_cgroup definition for CONFIG_MEMCG=n cases So that users of root_mem_cgroup no longer needs to check CONFIG_MEMCG.
This is to fix the compile error for CONFIG_MEMCG=n cases.
- Slight rewording of the 2nd patch
v5:
- Use root memcgroup to attach folios to btree inode filemap
- Only try higher order folio once without NOFAIL nor extra retry
v4:
- Hide the feature behind CONFIG_BTRFS_DEBUG
So that end users won't be affected (aka, still per-page based allocation) meanwhile we can do more testing on this new behavior.
v3:
- Rebased to the latest for-next branch
- Use PAGE_ALLOC_COSTLY_ORDER to determine whether to use __GFP_NOFAIL
- Add a dependency MM patch "mm/page_alloc: unify the warning on NOFAIL and high order allocation"
This allows us to use NOFAIL up to 32K nodesize, and makes sure for default 16K nodesize, all metadata would go 16K folios
v2:
- Rebased to handle the change in "btrfs: cache folio size and shift in extent_buffer"
This is the latest update on the attempt to utilize larger folios for btrfs metadata.
The previous version exposed a reproducibe hang at btrfs/187, where we hang at filemap_add_folio() around its memcgroup charge code.
Even without the problem, I still believe for btree inode we do not really need all the memcgroup charge, nor using __GFP_NOFAIL to work around the possible memcgroup limits.
So in this update, suggested by the memcgroup people from SUSE, there is a new patch to make btree inode filemap folio attaching to use the root memcgroup, so that we won't be limited by the memcgroup.
Then for the patch enabling the larger folio, I reverted back to the old behavior that we only try larger folio once without extra retry, just to be extra safe.
Qu Wenruo (3):
memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases
btrfs: always uses root memcgroup for filemap_add_folio()
btrfs: prefer to allocate larger folio for metadata
fs/btrfs/extent_io.c | 112 ++++++++++++++++++++++++++----------- include/linux/memcontrol.h | 6 ++ 2 files changed, 84 insertions(+), 34 deletions(-)
Signature · 1 line
2.45.2
PATCH v7 1/3 memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases
There is an incoming btrfs patchset, which will use @root_mem_cgroup as the active cgroup to attach metadata folios to its internal btree inode, so that btrfs can skip the possibly costly charge for the internal inode which is only accessible by btrfs itself.
However @root_mem_cgroup is not always defined (not defined for CONFIG_MEMCG=n case), thus all such callers need to do the extra handling for different CONFIG_MEMCG settings.
So here we add a special macro definition of root_mem_cgroup, making it to always be NULL.
The advantage of this, other than pulling the pointer definition out, is that we will avoid wasting global data section space for such pointer.
--- include/linux/memcontrol.h | 6 ++++++ 1 file changed, 6 insertions(+)
include/linux/memcontrol.h +6 -0
@@ -346,6 +346,12 @@ enum page_memcg_data_flags {#define __FIRST_OBJEXT_FLAG (1UL << 0) +/* +* For CONFIG_MEMCG=n case, still define a root_mem_cgroup, but that will +* always be NULL and not taking any global data section space. +*/ +#define root_mem_cgroup (NULL) +#endif /* CONFIG_MEMCG */ enum objext_flags {
Signature · 1 line
2.45.2
Re: [PATCH v7 1/3] memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases
Quoted 11 lines · Qu Wenruo
There is an incoming btrfs patchset, which will use @root_mem_cgroup as the active cgroup to attach metadata folios to its internal btree inode, so that btrfs can skip the possibly costly charge for the internal inode which is only accessible by btrfs itself.
However @root_mem_cgroup is not always defined (not defined for
CONFIG_MEMCG=ncase), thus all such callers need to do the extra handling for differentCONFIG_MEMCGsettings.So here we add a special macro definition of
root_mem_cgroup, making it to always be NULL.
Isn't just a declaration sufficient? Nothing should really dereference the pointer anyway.
Signature · 2 lines
Michal Hocko SUSE Labs
Re: [PATCH v7 1/3] memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases
Quoted 10 lines · Michal Hocko
Quoted 9 lines · Qu Wenruo
There is an incoming btrfs patchset, which will use @root_mem_cgroup as the active cgroup to attach metadata folios to its internal btree inode, so that btrfs can skip the possibly costly charge for the internal inode which is only accessible by btrfs itself.
However @root_mem_cgroup is not always defined (not defined for
CONFIG_MEMCG=ncase), thus all such callers need to do the extra handling for differentCONFIG_MEMCGsettings.
Quoted 6 lines
Quoted 2 lines
So here we add a special macro definition of
root_mem_cgroup, making it to always be NULL.Isn't just a declaration sufficient? Nothing should really dereference the pointer anyway.
That can pass the compile, but waste the extra bytes for the pointer in the data section, even if no one is utilizing that pointer.
Thanks,
Qu
Re: [PATCH v7 1/3] memcontrol: define root_mem_cgroup for CONFIG_MEMCG=n cases
Quoted 13 lines · Qu Wenruo
Quoted 10 lines · Michal Hocko
Quoted 9 lines · Qu Wenruo
There is an incoming btrfs patchset, which will use @root_mem_cgroup as the active cgroup to attach metadata folios to its internal btree inode, so that btrfs can skip the possibly costly charge for the internal inode which is only accessible by btrfs itself.
However @root_mem_cgroup is not always defined (not defined for
CONFIG_MEMCG=ncase), thus all such callers need to do the extra handling for differentCONFIG_MEMCGsettings.
Quoted 8 lines
Quoted 6 lines
Quoted 2 lines
So here we add a special macro definition of
root_mem_cgroup, making it to always be NULL.Isn't just a declaration sufficient? Nothing should really dereference the pointer anyway.
That can pass the compile, but waste the extra bytes for the pointer in the data section, even if no one is utilizing that pointer.
Are you sure that the mere declaration will be defined in the data section?
Signature · 2 lines
Michal Hocko SUSE Labs
PATCH v7 2/3 btrfs: always uses root memcgroup for filemap_add_folio()
In reply to Qu Wenruo[BACKGROUND]
The function filemap_add_folio() charges the memory cgroup, as we assume all page caches are accessible by user space progresses thus needs the cgroup accounting.
However btrfs is a special case, it has a very large metadata thanks to its support of data csum (by default it's 4 bytes per 4K data, and can be as large as 32 bytes per 4K data).
This means btrfs has to go page cache for its metadata pages, to take advantage of both cache and reclaim ability of filemap.
This has a tiny problem, that all btrfs metadata pages have to go through the memcgroup charge, even all those metadata pages are not accessible by the user space, and doing the charging can introduce some latency if there is a memory limits set.
Btrfs currently uses __GFP_NOFAIL flag as a workaround for this cgroup charge situation so that metadata pages won't really be limited by memcgroup.
[ENHANCEMENT]
Instead of relying on __GFP_NOFAIL to avoid charge failure, use root memory cgroup to attach metadata pages.
With root memory cgroup, we directly skip the charging part, and only rely on __GFP_NOFAIL for the real memory allocation part.
--- fs/btrfs/extent_io.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
fs/btrfs/extent_io.c +10 -0
@@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct btrfs_fs_info *fs_info = eb->fs_info;struct address_space *mapping = fs_info->btree_inode->i_mapping; +struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ;int ret;
@@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,ASSERT (eb->folios[i]); retry: +/* +* Btree inode is a btrfs internal inode, and not exposed to any +* user. +* Furthermore we do not want any cgroup limits on this inode. +* So we always use root_mem_cgroup as our active memcg when attaching +* the folios. +*/ + old_memcg =set_active_memcg (root_mem_cgroup); ret =filemap_add_folio (mapping, eb->folios[i], index + i,GFP_NOFS | __GFP_NOFAIL); +set_active_memcg (old_memcg);if (!ret) goto finish;
Signature · 1 line
2.45.2
Johannes Weiner
maintainerRe: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
Quoted 51 lines · Qu Wenruo
[BACKGROUND]
The functionfilemap_add_folio()charges the memory cgroup, as we assume all page caches are accessible by user space progresses thus needs the cgroup accounting.However btrfs is a special case, it has a very large metadata thanks to its support of data csum (by default it's 4 bytes per 4K data, and can be as large as 32 bytes per 4K data).
This means btrfs has to go page cache for its metadata pages, to take advantage of both cache and reclaim ability of filemap.This has a tiny problem, that all btrfs metadata pages have to go through the memcgroup charge, even all those metadata pages are not accessible by the user space, and doing the charging can introduce some latency if there is a memory limits set.
Btrfs currently uses
__GFP_NOFAILflag as a workaround for this cgroup charge situation so that metadata pages won't really be limited by memcgroup.[ENHANCEMENT]
Instead of relying on__GFP_NOFAILto avoid charge failure, use root memory cgroup to attach metadata pages.With root memory cgroup, we directly skip the charging part, and only rely on
__GFP_NOFAILfor the real memory allocation part.Suggested-by Michal Hocko <[email protected]>Suggested-by Vlastimil Babka (SUSE) <[email protected]>Signed-off-by Qu Wenruo <[email protected]>--- fs/btrfs/extent_io.c | 10 ++++++++++ 1 file changed, 10 insertions(+)fs/btrfs/extent_io.c +2
-0@@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct btrfs_fs_info *fs_info = eb->fs_info;struct address_space *mapping = fs_info->btree_inode->i_mapping; +struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ;int ret;@@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,ASSERT (eb->folios[i]); retry: +/*
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2995
Quoted 10 lines
fs/btrfs/extent_io.c +8
-0+* Btree inode is a btrfs internal inode, and not exposed to any +* user. +* Furthermore we do not want any cgroup limits on this inode. +* So we always use root_mem_cgroup as our active memcg when attaching +* the folios. +*/ + old_memcg =set_active_memcg (root_mem_cgroup); ret =filemap_add_folio (mapping, eb->folios[i], index + i,GFP_NOFS | __GFP_NOFAIL); +set_active_memcg (old_memcg);
It looks correct. But it's going through all dance to set up current->active_memcg, then have the charge path look that up, css_get(), call try_charge() only to bail immediately, css_put(), then update current->active_memcg again. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for.
Wouldn't it be a lot simpler (and cheaper) to have a filemap_add_folio_nocharge()?
Roman Gushchin
maintainerRe: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
Quoted 70 lines · Johannes Weiner
Quoted 51 lines · Qu Wenruo
[BACKGROUND]
The functionfilemap_add_folio()charges the memory cgroup, as we assume all page caches are accessible by user space progresses thus needs the cgroup accounting.However btrfs is a special case, it has a very large metadata thanks to its support of data csum (by default it's 4 bytes per 4K data, and can be as large as 32 bytes per 4K data).
This means btrfs has to go page cache for its metadata pages, to take advantage of both cache and reclaim ability of filemap.This has a tiny problem, that all btrfs metadata pages have to go through the memcgroup charge, even all those metadata pages are not accessible by the user space, and doing the charging can introduce some latency if there is a memory limits set.
Btrfs currently uses
__GFP_NOFAILflag as a workaround for this cgroup charge situation so that metadata pages won't really be limited by memcgroup.[ENHANCEMENT]
Instead of relying on__GFP_NOFAILto avoid charge failure, use root memory cgroup to attach metadata pages.With root memory cgroup, we directly skip the charging part, and only rely on
__GFP_NOFAILfor the real memory allocation part.Suggested-by Michal Hocko <[email protected]>Suggested-by Vlastimil Babka (SUSE) <[email protected]>Signed-off-by Qu Wenruo <[email protected]>--- fs/btrfs/extent_io.c | 10 ++++++++++ 1 file changed, 10 insertions(+)fs/btrfs/extent_io.c +2
-0@@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct btrfs_fs_info *fs_info = eb->fs_info;struct address_space *mapping = fs_info->btree_inode->i_mapping; +struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ;int ret;@@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,ASSERT (eb->folios[i]); retry: +/* fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2995
Quoted 10 lines
fs/btrfs/extent_io.c +8
-0+* Btree inode is a btrfs internal inode, and not exposed to any +* user. +* Furthermore we do not want any cgroup limits on this inode. +* So we always use root_mem_cgroup as our active memcg when attaching +* the folios. +*/ + old_memcg =set_active_memcg (root_mem_cgroup); ret =filemap_add_folio (mapping, eb->folios[i], index + i,GFP_NOFS | __GFP_NOFAIL); +set_active_memcg (old_memcg);It looks correct. But it's going through all dance to set up
current->active_memcg, then have the charge path look that up,css_get(), calltry_charge()only to bail immediately,css_put(), then updatecurrent->active_memcgagain. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for.
Quoted 2 lines
Wouldn't it be a lot simpler (and cheaper) to have a
filemap_add_folio_nocharge()?
Time to restore GFP_NOACCOUNT? I think it might be useful for allocating objects which are shared across the entire system and/or unlikely will go away under the memory pressure.
Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
In reply to Johannes WeinerQuoted 50 lines · Johannes Weiner
Quoted 49 lines · Qu Wenruo
[BACKGROUND]
The functionfilemap_add_folio()charges the memory cgroup, as we assume all page caches are accessible by user space progresses thus needs the cgroup accounting.However btrfs is a special case, it has a very large metadata thanks to its support of data csum (by default it's 4 bytes per 4K data, and can be as large as 32 bytes per 4K data).
This means btrfs has to go page cache for its metadata pages, to take advantage of both cache and reclaim ability of filemap.This has a tiny problem, that all btrfs metadata pages have to go through the memcgroup charge, even all those metadata pages are not accessible by the user space, and doing the charging can introduce some latency if there is a memory limits set.
Btrfs currently uses
__GFP_NOFAILflag as a workaround for this cgroup charge situation so that metadata pages won't really be limited by memcgroup.[ENHANCEMENT]
Instead of relying on__GFP_NOFAILto avoid charge failure, use root memory cgroup to attach metadata pages.With root memory cgroup, we directly skip the charging part, and only rely on
__GFP_NOFAILfor the real memory allocation part.Suggested-by Michal Hocko <[email protected]>Suggested-by Vlastimil Babka (SUSE) <[email protected]>Signed-off-by Qu Wenruo <[email protected]>--- fs/btrfs/extent_io.c | 10 ++++++++++ 1 file changed, 10 insertions(+)fs/btrfs/extent_io.c +1
-0@@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct btrfs_fs_info *fs_info = eb->fs_info;struct address_space *mapping = fs_info->btree_inode->i_mapping; +struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ;int ret;@@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,ASSERT (eb->folios[i]);
Quoted 11 lines
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2994
Quoted 11 lines
fs/btrfs/extent_io.c +8
-0retry: +/* +* Btree inode is a btrfs internal inode, and not exposed to any +* user. +* Furthermore we do not want any cgroup limits on this inode. +* So we always use root_mem_cgroup as our active memcg when attaching +* the folios. +*/ + old_memcg =set_active_memcg (root_mem_cgroup); ret =filemap_add_folio (mapping, eb->folios[i], index + i,GFP_NOFS | __GFP_NOFAIL);
I thoutght you've said that NOFAIL was added to workaround memcg charges. Can you remove it when memcg is out of the picture?
It would be great to add some background about how much memory are we talking about. Because this might require memcg configuration in some setups.
Quoted 11 lines
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2995
Quoted 1 line
fs/btrfs/extent_io.c +1
-0+set_active_memcg (old_memcg);It looks correct. But it's going through all dance to set up
current->active_memcg, then have the charge path look that up,css_get(), calltry_charge()only to bail immediately,css_put(), then updatecurrent->active_memcgagain. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for.Wouldn't it be a lot simpler (and cheaper) to have a
filemap_add_folio_nocharge()?
Yes, that would certainly simplify things. From the previous discussion I understood that there would be broader scopes which would opt-out from charging. If this is really about a single filemap_add_folio call then having a variant without doesn't call mem_cgroup_charge sounds like a much more viable option and also it doesn't require to make any memcg specific changes.
Signature · 2 lines
Michal Hocko SUSE Labs
Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
Quoted 51 lines · Michal Hocko
Quoted 50 lines · Johannes Weiner
Quoted 49 lines · Qu Wenruo
[BACKGROUND]
The functionfilemap_add_folio()charges the memory cgroup, as we assume all page caches are accessible by user space progresses thus needs the cgroup accounting.However btrfs is a special case, it has a very large metadata thanks to its support of data csum (by default it's 4 bytes per 4K data, and can be as large as 32 bytes per 4K data).
This means btrfs has to go page cache for its metadata pages, to take advantage of both cache and reclaim ability of filemap.This has a tiny problem, that all btrfs metadata pages have to go through the memcgroup charge, even all those metadata pages are not accessible by the user space, and doing the charging can introduce some latency if there is a memory limits set.
Btrfs currently uses
__GFP_NOFAILflag as a workaround for this cgroup charge situation so that metadata pages won't really be limited by memcgroup.[ENHANCEMENT]
Instead of relying on__GFP_NOFAILto avoid charge failure, use root memory cgroup to attach metadata pages.With root memory cgroup, we directly skip the charging part, and only rely on
__GFP_NOFAILfor the real memory allocation part.Suggested-by Michal Hocko <[email protected]>Suggested-by Vlastimil Babka (SUSE) <[email protected]>Signed-off-by Qu Wenruo <[email protected]>--- fs/btrfs/extent_io.c | 10 ++++++++++ 1 file changed, 10 insertions(+)fs/btrfs/extent_io.c +1
-0@@ -2971,6 +2971,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct btrfs_fs_info *fs_info = eb->fs_info;struct address_space *mapping = fs_info->btree_inode->i_mapping; +struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ;int ret;@@ -2981,8 +2982,17 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,ASSERT (eb->folios[i]);
Quoted 14 lines
Quoted 11 lines
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2994
Quoted 11 lines
fs/btrfs/extent_io.c +8
-0retry: +/* +* Btree inode is a btrfs internal inode, and not exposed to any +* user. +* Furthermore we do not want any cgroup limits on this inode. +* So we always use root_mem_cgroup as our active memcg when attaching +* the folios. +*/ + old_memcg =set_active_memcg (root_mem_cgroup); ret =filemap_add_folio (mapping, eb->folios[i], index + i,GFP_NOFS | __GFP_NOFAIL);I thoutght you've said that NOFAIL was added to workaround memcg charges. Can you remove it when memcg is out of the picture?
Sure, but that would be a dedicated patch, as we need to add the -ENOMEM handling.
I already have such a patch before:
https://lore.kernel.org/linux-btrfs/d6a9c038e12f1f2dae353f1ba657ba0666f0aaaa.1720159494.git.wqu@suse.com/
But that's before the memcgroup change.
I'd prefer to have all the larger folio fully tested and merged, then cleanup the NOFAIL flags.
Quoted 17 lines
It would be great to add some background about how much memory are we talking about. Because this might require memcg configuration in some setups.
Quoted 11 lines
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2995
Quoted 1 line
fs/btrfs/extent_io.c +1
-0+set_active_memcg (old_memcg);It looks correct. But it's going through all dance to set up
current->active_memcg, then have the charge path look that up,css_get(), calltry_charge()only to bail immediately,css_put(), then updatecurrent->active_memcgagain. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for.Wouldn't it be a lot simpler (and cheaper) to have a
filemap_add_folio_nocharge()?
Quoted 7 lines
Yes, that would certainly simplify things. From the previous discussion I understood that there would be broader scopes which would opt-out from charging. If this is really about a single
filemap_add_foliocall then having a variant without doesn't callmem_cgroup_chargesounds like a much more viable option and also it doesn't require to make any memcg specific changes.
I'm not 100% sure if the VFS guys are happy with that.
The current filemap folio interfaces are already much concentraced, other than all the various page based interfaces for different situations.
E.g. we have the following wrappers related to filemap page cache search/creation:
- find_get_page() and find_get_page_flags()
- find_lock_page()
- find_or_create_page()
- grab_cache_page_nowait()
- grab_cache_page()
Meanwhile just two folio interfaces:
- filemap_get_folio()
- __fielmap_get_folio()
So according to the trend, I'm pretty sure VFS people will reject such new interface just to skip accounting.
Thus the GFP_NO_ACCOUNT solution looks more feasible.
Thanks,
Qu
Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
On Sat 20-07-24 07:41:19, Qu Wenruo wrote:
[...]
Quoted 2 lines
So according to the trend, I'm pretty sure VFS people will reject such new interface just to skip accounting.
I would just give it a try with your usecase described. If this is a nogo then the root cgroup workaround is still available.
Quoted 1 line
Thus the
GFP_NO_ACCOUNTsolution looks more feasible.
So we have GFP_ACCOUNT to opt in for accounting and now we should be adding GFP_NO_ACCOUNT to override it? This doesn't sound like a good use of gfp flags (which we do not have infinitely) and it is also quite confusing TBH.
Signature · 2 lines
Michal Hocko SUSE Labs
Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
Quoted 7 lines · Michal Hocko
On Sat 20-07-24 07:41:19, Qu Wenruo wrote:
[...]Quoted 2 lines
So according to the trend, I'm pretty sure VFS people will reject such new interface just to skip accounting.
I would just give it a try with your usecase described. If this is a nogo then the root cgroup workaround is still available.
I have submitted a patchset doing exactly that, and thankfully that's the series where I got all the helpful feedbacks:
Unfortunately I haven't get any feedback from the VFS guys.
Quoted 7 lines
Quoted 2 lines
Thus the
GFP_NO_ACCOUNTsolution looks more feasible.So we have
GFP_ACCOUNTto opt in for accounting and now we should be addingGFP_NO_ACCOUNTto override it? This doesn't sound like a good use of gfp flags (which we do not have infinitely) and it is also quite confusing TBH.
The problem is, for filemap_add_folio(), we didn't specify GFP_ACCOUNT (nor any other caller) but it is still doing the charge, due to the mostly-correct assumption that all filemap page caches are accessible to user space programs.
So one can argue that, cgroup is still charged even if no GFP_ACCOUNT is specified.
But I get your point, indeed it's not that a good idea to introduce GFP_NO_ACCOUNT.
Thanks,
Qu
Re: [PATCH v7 2/3] btrfs: always uses root memcgroup for filemap_add_folio()
In reply to Michal Hocko在 2024/7/20 03:43, Michal Hocko 写道:
[...]
Quoted 13 lines
Quoted 10 lines
fs/btrfs/extent_io.c static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, · line 2995
Quoted 2 lines
fs/btrfs/extent_io.c +1
-0+set_active_memcg (old_memcg);It looks correct. But it's going through all dance to set up
current->active_memcg, then have the charge path look that up,css_get(), calltry_charge()only to bail immediately,css_put(), then updatecurrent->active_memcgagain. All those branches are necessary when we want to charge to a "real" other cgroup. But in this case, we always know we're not charging, so it seems uncalled for.Quoted 2 lines
Wouldn't it be a lot simpler (and cheaper) to have a
filemap_add_folio_nocharge()?
Quoted 7 lines
Yes, that would certainly simplify things. From the previous discussion I understood that there would be broader scopes which would opt-out from charging. If this is really about a single
filemap_add_foliocall then having a variant without doesn't callmem_cgroup_chargesounds like a much more viable option and also it doesn't require to make any memcg specific changes.
Talking about skipping mem cgroup charging, I still have a question.
[MEMCG AT FOLIO EVICTION TIME]
Even we completely skip the mem cgroup charging, we cannot really escape the eviction time handling.
In fact if we just skip the mem_cgroup_charge(), kernel would crash when evicting the folio.
As in lru_gen_eviction(), folio_memcg() would just return NULL, and mem_cgroup_id(memcg) would trigger a NULL pointer dereference.
That's why I sent out a patch fixing that first:
https://lore.kernel.org/linux-mm/e1036b9cc8928be9a7dec150ab3a0317bd7180cf.1720572937.git.wqu@suse.com/
I'm not sure if it's going to cause any extra problems even with the above fix.
And just for the sake of consistency, it looks more sane to have root_mem_cgroup for the filemap_add_folio() operation, other than leave it empty, especially since most filemaps still need proper memcg handling.
[REALLY EXPENSIVE?]
Another question is, is the set_active_memcg() and later handling really that expensive?
set_active_memcg() is small enough to be an inline function, so is the active_memcg(), css_get() and the root memcg path of try_charge().
Later commit part is not that expensive either, mostly simple member or per-cpu assignment.
According to my very little knowledge about mem cgroup, most of the heavy lifting part is in the slow path of try_charge_memcg().
Even with all the set_active_memcg(), the whole extra overhead still look very tiny.
And it should already be a big win for btrfs to opt-out the regular charging routine.
Thanks,
Qu
PATCH v7 3/3 btrfs: prefer to allocate larger folio for metadata
In reply to Qu WenruoFor btrfs metadata, the high order folios are only utilized when all the following conditions are met:
- The extent buffer start is aligned to nodesize
This should be the common case for any btrfs in the last 5 years.
- The nodesize is larger than page size
Or there is no need to use larger folios at all.
- MM layer can fulfill our folio allocation request
- The larger folio must exactly cover the extent buffer No longer no smaller, must be an exact fit.
This is to make extent buffer accessors much easier.
They only need to check the first slot in eb->folios[], to determine their access unit (need per-page handling or a large folio covering the whole eb).
There is another small blockage, filemap APIs can not guarantee the folio size.
For example, by default we go 16K nodesize on x86_64, meaning a larger folio we expect would be with order 2 (size 16K).
We don't accept 2 order 1 (size 8K) folios, or we fall back to 4 order 0 (page sized) folios.
So here we go a different workaround, allocate a order 2 folio first, then attach them to the filemap of metadata.
Thus here comes several results related to the attach attempt of eb folios:
1) We can attach the pre-allocated eb folio to filemap This is the most simple and hot path, we just continue our work setting up the extent buffer.
2) There is an existing folio in the filemap
2.0) Subpage case
We would reuse the folio no matter what, subpage is doing a different way handling folio->private (a bitmap other than a pointer to an existing eb).
2.1) There is already a live extent buffer attached to the filemap
folio
This should be more or less hot path, we grab the existing eb and free the current one.
2.2) No live eb.
2.2.1) The filemap folio is larger than eb folio
This is a better case, we can reuse the filemap folio, but we need to cleanup all the pre-allocated folios of the new eb before reusing.
Later code should take the folio size change into consideration.
2.2.2) The filemap folio is the same size of eb folio
We just free the current folio, and reuse the filemap one. No other special handling needed.
2.2.3) The filemap folio is smaller than eb folio
This is the most tricky corner case, we can not easily replace the folio in filemap using our eb folio.
Thus here we return -EAGAIN, to inform our caller to re-try with order 0 (of course with our larger folio freed).
Otherwise all the needed infrastructure is already here, we only need to try allocate larger folio as our first try in alloc_eb_folio_array().
For now, the higher order allocation is only a preferable attempt for debug build, before we had enough test coverage and push it to end users.
--- fs/btrfs/extent_io.c | 102 ++++++++++++++++++++++++++++--------------- 1 file changed, 68 insertions(+), 34 deletions(-)
fs/btrfs/extent_io.c +68 -34
@@ -719,12 +719,28 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,* * For now, the folios populated are always in order 0 (aka, single page). */ -static int alloc_eb_folio_array (struct extent_buffer *eb,bool nofail) +static int alloc_eb_folio_array (struct extent_buffer *eb,int order, +bool nofail) {struct page *page_array[INLINE_EXTENT_BUFFER_PAGES ] = {0 };int num_pages =num_extent_pages (eb);int ret; +if (order) { +gfp_t gfp; + +if (order >0 ) + gfp =GFP_NOFS | __GFP_NORETRY | __GFP_NOWARN; +else + gfp = nofail ? (GFP_NOFS | __GFP_NOFAIL) :GFP_NOFS ; + eb->folios[0 ] =folio_alloc (gfp, order); +if (likely (eb->folios[0 ])) { + eb->folio_size =folio_size (eb->folios[0 ]); + eb->folio_shift =folio_shift (eb->folios[0 ]); +return 0 ; + } +/* Fallback to 0 order (single page) allocation. */ + } ret =btrfs_alloc_page_array (num_pages, page_array, nofail);if (ret <0 )return ret;
@@ -2707,7 +2723,7 @@ struct extent_buffer *btrfs_clone_extent_buffer(const struct extent_buffer *src)*/ set_bit (EXTENT_BUFFER_UNMAPPED , &new->bflags); - ret =alloc_eb_folio_array (new, false); + ret =alloc_eb_folio_array (new,0 , false);if (ret) {btrfs_release_extent_buffer (new);return NULL ;
@@ -2740,7 +2756,7 @@ struct extent_buffer *__alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info,if (!eb)return NULL ; - ret =alloc_eb_folio_array (eb, false); + ret =alloc_eb_folio_array (eb,0 , false);if (ret) goto err;
@@ -2955,6 +2971,14 @@ static int check_eb_alignment(struct btrfs_fs_info *fs_info, u64 start)return 0 ; } +static void free_all_eb_folios (struct extent_buffer *eb) +{ +for (int i =0 ; i <INLINE_EXTENT_BUFFER_PAGES ; i++) { +if (eb->folios[i]) +folio_put (eb->folios[i]); + eb->folios[i] =NULL ; + } +}/* * Return 0 if eb->folios[i] is attached to btree inode successfully.
@@ -2974,6 +2998,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,struct mem_cgroup *old_memcg;const unsigned long index = eb->start >>PAGE_SHIFT ;struct folio *existing_folio =NULL ; +const int eb_order =folio_order (eb->folios[0 ]);int ret;ASSERT (found_eb_ret);
@@ -3003,15 +3028,6 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, goto retry; } -/* For now, we should only have single-page folios for btree inode. */ -ASSERT (folio_nr_pages (existing_folio) ==1 ); - -if (folio_size (existing_folio) != eb->folio_size) { -folio_unlock (existing_folio); -folio_put (existing_folio); -return -EAGAIN ; - } - finish:spin_lock (&mapping->i_private_lock);if (existing_folio && fs_info->nodesize <PAGE_SIZE ) {
@@ -3020,6 +3036,7 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i, eb->folios[i] = existing_folio; }else if (existing_folio) {struct extent_buffer *existing_eb; +int existing_order =folio_order (existing_folio); existing_eb =grab_extent_buffer (fs_info,folio_page (existing_folio,0 ));
@@ -3031,9 +3048,34 @@ static int attach_eb_folio_to_filemap(struct extent_buffer *eb, int i,folio_put (existing_folio);return 1 ; } -/* The extent buffer no longer exists, we can reuse the folio. */ -__free_page (folio_page (eb->folios[i],0 )); - eb->folios[i] = existing_folio; +if (existing_order > eb_order) { +/* +* The existing one has higher order, we need to drop +* all eb folios before resuing it. +* And this should only happen for the first folio. +*/ +ASSERT (i ==0 ); +free_all_eb_folios (eb); + eb->folios[i] = existing_folio; + }else if (existing_order == eb_order) { +/* +* Can safely reuse the filemap folio, just +* release the eb one. +*/ +folio_put (eb->folios[i]); + eb->folios[i] = existing_folio; + }else { +/* +* The existing one has lower order. +* +* Just retry and fallback to order 0. +*/ +ASSERT (i ==0 ); +folio_unlock (existing_folio); +folio_put (existing_folio); +spin_unlock (&mapping->i_private_lock); +return -EAGAIN ; + } } eb->folio_size =folio_size (eb->folios[i]); eb->folio_shift =folio_shift (eb->folios[i]);
@@ -3066,6 +3108,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,u64 lockdep_owner = owner_root;bool page_contig = true;int uptodate =1 ; +int order =0 ;int ret;if (check_eb_alignment (fs_info, start))
@@ -3082,6 +3125,10 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,btrfs_warn_32bit_limit (fs_info);#endif +if (IS_ENABLED (CONFIG_BTRFS_DEBUG ) && fs_info->nodesize >PAGE_SIZE && +IS_ALIGNED (start, fs_info->nodesize)) + order =ilog2 (fs_info->nodesize >>PAGE_SHIFT ); + eb =find_extent_buffer (fs_info, start);if (eb)return eb;
@@ -3116,7 +3163,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info, reallocate:/* Allocate all pages first. */ - ret =alloc_eb_folio_array (eb, true); + ret =alloc_eb_folio_array (eb, order, true);if (ret <0 ) {btrfs_free_subpage (prealloc); goto out;
@@ -3134,26 +3181,12 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info, }/* -* TODO: Special handling for a corner case where the order of -* folios mismatch between the new eb and filemap. -* -* This happens when: -* -* - the new eb is using higher order folio -* -* - the filemap is still using 0-order folios for the range -* This can happen at the previous eb allocation, and we don't -* have higher order folio for the call. -* -* - the existing eb has already been freed -* -* In this case, we have to free the existing folios first, and -* re-allocate using the same order. -* Thankfully this is not going to happen yet, as we're still -* using 0-order folios. +* Got a corner case where the existing folio is lower order, +* fallback to 0 order and retry. */ if (unlikely (ret == -EAGAIN )) { -ASSERT (0 ); + order =0 ; +free_all_eb_folios (eb); goto reallocate; } attached++;
@@ -3164,6 +3197,7 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,* and free the allocated page. */ folio = eb->folios[i]; + num_folios =num_extent_folios (eb);WARN_ON (btrfs_folio_test_dirty (fs_info, folio, eb->start, eb->len));/*
Signature · 1 line
2.45.2