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PagePrivate handling

DISCUSSION Matthew Wilcox 5 participants 7 messages Oct 14, 2020

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Matthew Wilcox

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#0 lore raw

Our handling of PagePrivate, page->private and PagePrivate2 is a giant mess. Let's recap.

Filesystems which use bufferheads (ie most of them) set page->private to point to a buffer_head, set the PagePrivate bit and increment the refcount on the page.

The vmscan pageout code (*) needs to know whether a page is freeable:

        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
... where is_page_cache_freeable() contains ...
        return page_count(page) - page_has_private(page) == 1 + page_cache_pins;

That's a little inscrutable, but the important thing is that if page_has_private() is true, then the page's reference count is supposed to be one higher than it would be otherwise. And that makes sense given how "having bufferheads" means "page refcount ges incremented".

But page_has_private() doesn't actually mean "PagePrivate is set". It means "PagePrivate or PagePrivate2 is set". And I don't understand how filesystems are supposed to keep that straight -- if we're setting PagePrivate2, and PagePrivate is clear, increment the refcount? If we're clearing PagePrivate, decrement the refcount if PagePrivate2 is also clear?

We introduced attach_page_private() and detach_page_private() earlier this year to help filesystems get the refcount right. But we still have a few filesystems using PagePrivate themselves (afs, btrfs, ceph, crypto, erofs, f2fs, jfs, nfs, orangefs & ubifs) and I'm not convinced they're all getting it right.

Here's a bug I happened on while looking into this:

        if (page_has_private(page))
                attach_page_private(newpage, detach_page_private(page));

        if (PagePrivate2(page)) {
                ClearPagePrivate2(page);
                SetPagePrivate2(newpage);
        }

The aggravating thing is that this doesn't even look like a bug. You have to be in the kind of mood where you're thinking "What if page has Private2 set and Private clear?" and the answer is that newpage ends up with PagePrivate set, but page->private set to NULL. And I don't know whether this is a situation that can ever happen with btrfs, but we shouldn't have code like this lying around in the tree because it _looks_ right and somebody else might copy it.

So what shold we do about all this? First, I want to make the code snippet above correct, because it looks right. So page_has_private() needs to test just PagePrivate and not PagePrivate2. Now we need a new function to call to determine whether the filesystem needs its invalidatepage callback invoked. Not sure what that should be named.

I think I also want to rename PG_private_2 to PG_owner_priv_2. There's a clear relationship between PG_private and page->private. There is no relationship between PG_private_2 and page->private, so it's a misleading name. Or maybe it should just be PG_fscache and btrfs can find some other way to mark the pages?

Also ... do we really need to increment the page refcount if we have PagePrivate set? I'm not awfully familiar with the buffercache -- is it possible we end up in a situation where a buffer, perhaps under I/O, has the last reference to a struct page? It seems like that reference is always put from drop_buffers() which is called from try_to_free_buffers() which is always called by someone who has a reference to a struct page that they got from the pagecache. So what is this reference count for?

(*) Also THP split and page migration. But maybe you don't care about those things ... you definitely care about pageout!

#1 lore raw
Quoted 19 lines · Matthew Wilcox

Our handling of PagePrivate, page->private and PagePrivate2 is a giant mess. Let's recap.

Filesystems which use bufferheads (ie most of them) set page->private to point to a buffer_head, set the PagePrivate bit and increment the refcount on the page.

The vmscan pageout code (*) needs to know whether a page is freeable:

        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
... where is_page_cache_freeable() contains ...
        return page_count(page) - page_has_private(page) == 1 +

page_cache_pins;

That's a little inscrutable, but the important thing is that if page_has_private() is true, then the page's reference count is supposed to be one higher than it would be otherwise. And that makes sense given how "having bufferheads" means "page refcount ges incremented".

Quoted 6 lines

But page_has_private() doesn't actually mean "PagePrivate is set". It means "PagePrivate or PagePrivate2 is set". And I don't understand how filesystems are supposed to keep that straight -- if we're setting PagePrivate2, and PagePrivate is clear, increment the refcount? If we're clearing PagePrivate, decrement the refcount if PagePrivate2 is also clear?

At least for btrfs, only PagePrivate elevates the refcount on the page. PagePrivate2 means:

This page has been properly setup for COW’d IO, and it went through the normal path of page_mkwrite() or file_write() instead of being silently dirtied by a deep corner of the MM.

Quoted 18 lines

We introduced attach_page_private() and detach_page_private() earlier this year to help filesystems get the refcount right. But we still have a few filesystems using PagePrivate themselves (afs, btrfs, ceph, crypto, erofs, f2fs, jfs, nfs, orangefs & ubifs) and I'm not convinced they're all getting it right.

Here's a bug I happened on while looking into this:

if (page_has_private(page))
attach_page_private(newpage,
detach_page_private(page));

        if (PagePrivate2(page)) {
                ClearPagePrivate2(page);
                SetPagePrivate2(newpage);
        }
Quoted 4 lines

The aggravating thing is that this doesn't even look like a bug. You have to be in the kind of mood where you're thinking "What if page has Private2 set and Private clear?" and the answer is that newpage ends up with PagePrivate set, but page->private set to NULL.

Do you mean PagePrivate2 set but page->private NULL? Btrfs should only hage PagePrivate2 set on pages that are formally in our writeback state machine, so it’ll get cleared as we unwind through normal IO or truncate etc. For data pages, btrfs page->private is simply set to 1 so the MM will kindly call releasepage for us.

Quoted 11 lines

And I
don't know whether this is a situation that can ever happen with btrfs, but we shouldn't have code like this lying around in the tree because it _looks_ right and somebody else might copy it.

So what shold we do about all this? First, I want to make the code snippet above correct, because it looks right. So page_has_private() needs to test just PagePrivate and not PagePrivate2. Now we need a new function to call to determine whether the filesystem needs its invalidatepage callback invoked. Not sure what that should be named.

I haven’t checked all the page_has_private() callers, but maybe page_has_private() should stay the same and add page_private_count() for times where we need to get out our fingers and toes for the refcount math.

Quoted 5 lines

I think I also want to rename PG_private_2 to PG_owner_priv_2. There's a clear relationship between PG_private and page->private. There is no relationship between PG_private_2 and page->private, so it's a misleading name. Or maybe it should just be PG_fscache and btrfs can find some other way to mark the pages?

Btrfs should be able to flip bits in page->private to cover our current usage of PG_private_2. If we ever attach something real to page->private, we can flip bits in that instead. It’s kinda messy though and we’d have to change attach_page_private a little to reflect its new life as a bit setting machine.

Quoted 8 lines

Also ... do we really need to increment the page refcount if we have PagePrivate set? I'm not awfully familiar with the buffercache -- is it possible we end up in a situation where a buffer, perhaps under I/O, has the last reference to a struct page? It seems like that reference is always put from drop_buffers() which is called from try_to_free_buffers() which is always called by someone who has a reference to a struct page that they got from the pagecache. So what is this reference count for?

I’m not sure what we gain by avoiding the refcount bump? Many filesystems use the pattern of: “put something in page->private, free that thing in releasepage.” Without the refcount bump it feels like we’d have more magic to avoid freeing the page without leaking things in page->private. I think the extra ref lets the FS crowd keep our noses out of the MM more often, so it seems like a net positive to me.

Quoted 3 lines

(*) Also THP split and page migration. But maybe you don't care about those things ... you definitely care about pageout!

-chris

author
#2 lore raw
Quoted 27 lines · Chris Mason
Quoted 19 lines · Matthew Wilcox

Our handling of PagePrivate, page->private and PagePrivate2 is a giant mess. Let's recap.

Filesystems which use bufferheads (ie most of them) set page->private to point to a buffer_head, set the PagePrivate bit and increment the refcount on the page.

The vmscan pageout code (*) needs to know whether a page is freeable:

        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
... where is_page_cache_freeable() contains ...
        return page_count(page) - page_has_private(page) == 1 +

page_cache_pins;

That's a little inscrutable, but the important thing is that if page_has_private() is true, then the page's reference count is supposed to be one higher than it would be otherwise. And that makes sense given how "having bufferheads" means "page refcount ges incremented".

Quoted 6 lines

But page_has_private() doesn't actually mean "PagePrivate is set". It means "PagePrivate or PagePrivate2 is set". And I don't understand how filesystems are supposed to keep that straight -- if we're setting PagePrivate2, and PagePrivate is clear, increment the refcount? If we're clearing PagePrivate, decrement the refcount if PagePrivate2 is also clear?

Quoted 6 lines

At least for btrfs, only PagePrivate elevates the refcount on the page. PagePrivate2 means:

This page has been properly setup for COW’d IO, and it went through the normal path of page_mkwrite() or file_write() instead of being silently dirtied by a deep corner of the MM.

What's not clear to me is whether btrfs can be in the situation where PagePrivate2 is set and PagePrivate is clear. If so, then we have a bug to fix.

Quoted 16 lines
Quoted 16 lines

We introduced attach_page_private() and detach_page_private() earlier this year to help filesystems get the refcount right. But we still have a few filesystems using PagePrivate themselves (afs, btrfs, ceph, crypto, erofs, f2fs, jfs, nfs, orangefs & ubifs) and I'm not convinced they're all getting it right.

Here's a bug I happened on while looking into this:

        if (page_has_private(page))
                attach_page_private(newpage, detach_page_private(page));

        if (PagePrivate2(page)) {
                ClearPagePrivate2(page);
                SetPagePrivate2(newpage);
        }
Quoted 6 lines
Quoted 4 lines

The aggravating thing is that this doesn't even look like a bug. You have to be in the kind of mood where you're thinking "What if page has Private2 set and Private clear?" and the answer is that newpage ends up with PagePrivate set, but page->private set to NULL.

Do you mean PagePrivate2 set but page->private NULL?

Sorry, I skipped a step of the explanation.

page_has_private returns true if Private or Private2 is set. So if page has PagePrivate clear and PagePrivate2 set, newpage will end up with both PagePrivate and PagePrivate2 set -- attach_page_private() doesn't check whether the pointer is NULL (and IMO, it shouldn't).

Given our current macros, what was _meant_ here was:

         if (PagePrivate(page))
                 attach_page_private(newpage, detach_page_private(page));

but that's not obviously right.

Quoted 5 lines

Btrfs should only hage
PagePrivate2 set on pages that are formally in our writeback state machine, so it’ll get cleared as we unwind through normal IO or truncate etc. For data pages, btrfs page->private is simply set to 1 so the MM will kindly call releasepage for us.

That's not what I'm seeing here:

static void attach_extent_buffer_page(struct extent_buffer *eb,
struct page *page)

{
        if (!PagePrivate(page))
                attach_page_private(page, eb);
        else
                WARN_ON(page->private != (unsigned long)eb);
}

Or is that not a data page?

Quoted 9 lines
Quoted 5 lines

So what shold we do about all this? First, I want to make the code snippet above correct, because it looks right. So page_has_private() needs to test just PagePrivate and not PagePrivate2. Now we need a new function to call to determine whether the filesystem needs its invalidatepage callback invoked. Not sure what that should be named.

I haven’t checked all the page_has_private() callers, but maybe page_has_private() should stay the same and add page_private_count() for times where we need to get out our fingers and toes for the refcount math.

I was thinking about page_expected_count() which returns the number of references from the page cache plus the number of references from the various page privates. So is_page_cache_freeable() becomes:

	return page_count(page) == page_expected_count(page) + 1;

can_split_huge_page() becomes:

	if (page_has_private(page))
		return false;
	return page_count(page) == page_expected_count(page) +
			total_mapcount(page) + 1;
Quoted 11 lines
Quoted 5 lines

I think I also want to rename PG_private_2 to PG_owner_priv_2. There's a clear relationship between PG_private and page->private. There is no relationship between PG_private_2 and page->private, so it's a misleading name. Or maybe it should just be PG_fscache and btrfs can find some other way to mark the pages?

Btrfs should be able to flip bits in page->private to cover our current usage of PG_private_2. If we ever attach something real to page->private, we can flip bits in that instead. It’s kinda messy though and we’d have to change attach_page_private a little to reflect its new life as a bit setting machine.

It's not great, but with David wanting to change how PageFsCache is used,
it may be unavoidable (I'm not sure if he's discussed that with you yet)
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=fscache-iter&id=6f10fd7766ed6d87c3f696bb7931281557b389f5 shows part of it -- essentially he wants to make PagePrivate2 mean that I/O is currently
ongoing to an fscache, and so truncate needs to wait on it being finished.

Quoted 11 lines
Quoted 10 lines

Also ... do we really need to increment the page refcount if we have PagePrivate set? I'm not awfully familiar with the buffercache -- is it possible we end up in a situation where a buffer, perhaps under I/O, has the last reference to a struct page? It seems like that reference is
always put from drop_buffers() which is called from try_to_free_buffers()
which is always called by someone who has a reference to a struct page that they got from the pagecache. So what is this reference count for?

Quoted 6 lines

I’m not sure what we gain by avoiding the refcount bump? Many filesystems use the pattern of: “put something in page->private, free that thing in releasepage.” Without the refcount bump it feels like we’d have more magic to avoid freeing the page without leaking things in page->private. I think the extra ref lets the FS crowd keep our noses out of the MM more often, so it seems like a net positive to me.

The question is whether the "thing" in page->private can ever have the last reference on a struct page. Gao says erofs can be in that situation, so never mind this change.

#3 lore raw
Quoted 2 lines · Matthew Wilcox
Quoted 1 line · Chris Mason

On 14 Oct 2020, at 9:49, Matthew Wilcox wrote:

...

Quoted 18 lines
Quoted 11 lines
Quoted 10 lines

Also ... do we really need to increment the page refcount if we have PagePrivate set? I'm not awfully familiar with the buffercache -- is it possible we end up in a situation where a buffer, perhaps under I/O, has the last reference to a struct page? It seems like that reference is
always put from drop_buffers() which is called from try_to_free_buffers()
which is always called by someone who has a reference to a struct page that they got from the pagecache. So what is this reference count for?

Quoted 6 lines

I’m not sure what we gain by avoiding the refcount bump? Many filesystems use the pattern of: “put something in page->private, free that thing in releasepage.†Without the refcount bump it feels like we’d have more magic to avoid freeing the page without leaking things in page->private. I think the extra ref lets the FS crowd keep our noses out of the MM more often, so it seems like a net positive to me.

Quoted 3 lines

The question is whether the "thing" in page->private can ever have the last reference on a struct page. Gao says erofs can be in that situation, so never mind this change.

Add some words, just my thought... we have a management structure which could store PagePrivate page cache pages, !PagePrivate page cache pages, and non-page cache pages which are directly from buddy system.

and I knew the extra refcount rule for PagePrivate from the beginning (since the rule is quite stable for many many years (I remembered from 200x introduced by akpm?) so I designed the whole workflow to handle these different types of pages in the management structure based on this rule and to make reclaim & migrate work for all page cache pages properly.). I think many modules think the rule is stable as well ... anyway, I think there's always be another way to handle the same thing if the refcount rule is changed, yet I need to revisit all current logic and do proper changes. And I think many modules (including out-of-tree modules) could be impacted as well... anyway...

Thanks,
Gao Xiang

Quoted 1 line
#4 lore raw
In reply to Matthew Wilcox
Quoted 5 lines · Matthew Wilcox

It's not great, but with David wanting to change how PageFsCache is used,
it may be unavoidable (I'm not sure if he's discussed that with you yet)
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=fscache-iter&id=6f10fd7766ed6d87c3f696bb7931281557b389f5 shows part of it -- essentially he wants to make PagePrivate2 mean that I/O is currently
ongoing to an fscache, and so truncate needs to wait on it being finished.

->invalidatepage() and ->releasepage() had to wait anyway.

PG_fscache used to mean that the cache might have some knowledge of the page and it might have I/O in progress on it - entirely as and when the cache felt like doing it.

Now it just means that there's write I/O in progress on it at the netfs's behest (though it might be issued by a cache helper). The main part of the cache doesn't know about the page and doesn't care about the page flag, it just sees an iov_iter.

In a sense, it's now a second PG_writeback flag. A page can be being written to two locations at the same time (the server and the cache). Each write is independent, though they may be started from the same place, and one of the writes may be being handled by another filesystem entirely (the cache may have started a DIO write to ext4, for example).

It's not needed for reading, since the PG_locked flag entirely suffices for that. The read helper doesn't do parallel reads from the server and the cache to the same chunk of pagecache.

Willy has asked that I either make PG_writeback cover both cases or that PG_fscache can only be set if PG_writeback is also set. However, both of these require extra state to be attached to page->private to be able to coordinate this as there may be (as mentioned above) two parallel writes from the same data that may finish at different times.

David

#5 lore raw
In reply to Matthew Wilcox
Quoted 31 lines · Matthew Wilcox
Quoted 30 lines · Chris Mason
Quoted 19 lines · Matthew Wilcox

Our handling of PagePrivate, page->private and PagePrivate2 is a giant mess. Let's recap.

Filesystems which use bufferheads (ie most of them) set page->private to point to a buffer_head, set the PagePrivate bit and increment the refcount on the page.

The vmscan pageout code (*) needs to know whether a page is freeable:

        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
... where is_page_cache_freeable() contains ...
        return page_count(page) - page_has_private(page) == 1 +

page_cache_pins;

That's a little inscrutable, but the important thing is that if page_has_private() is true, then the page's reference count is supposed to be one higher than it would be otherwise. And that makes sense given how "having bufferheads" means "page refcount ges incremented".

Quoted 6 lines

But page_has_private() doesn't actually mean "PagePrivate is set". It means "PagePrivate or PagePrivate2 is set". And I don't understand how filesystems are supposed to keep that straight -- if we're setting PagePrivate2, and PagePrivate is clear, increment the refcount? If we're clearing PagePrivate, decrement the refcount if PagePrivate2 is also clear?

At least for btrfs, only PagePrivate elevates the refcount on the page. PagePrivate2 means:

Quoted 7 lines
Quoted 3 lines

This page has been properly setup for COW’d IO, and it went through the normal path of page_mkwrite() or file_write() instead of being silently dirtied by a deep corner of the MM.

What's not clear to me is whether btrfs can be in the situation where PagePrivate2 is set and PagePrivate is clear. If so, then we have a bug to fix.

I don’t think it’ll happen. Everyone in btrfs setting PagePrivate2 seems to have already called attach_page_private(). It’s possible I missed a corner but I don’t think so.

Quoted 23 lines
Quoted 23 lines
Quoted 22 lines

We introduced attach_page_private() and detach_page_private() earlier this year to help filesystems get the refcount right. But we still have a few filesystems using PagePrivate themselves (afs, btrfs, ceph, crypto, erofs, f2fs, jfs, nfs, orangefs & ubifs) and I'm not convinced they're all getting it right.

Here's a bug I happened on while looking into this:

if (page_has_private(page))
attach_page_private(newpage,
detach_page_private(page));

        if (PagePrivate2(page)) {
                ClearPagePrivate2(page);
                SetPagePrivate2(newpage);
        }

The aggravating thing is that this doesn't even look like a bug. You have to be in the kind of mood where you're thinking "What if page has Private2 set and Private clear?" and the answer is that newpage ends up with PagePrivate set, but page->private set to NULL.

Quoted 9 lines
Quoted 1 line

Do you mean PagePrivate2 set but page->private NULL?

Sorry, I skipped a step of the explanation.

page_has_private returns true if Private or Private2 is set. So if page has PagePrivate clear and PagePrivate2 set, newpage will end up with both PagePrivate and PagePrivate2 set -- attach_page_private() doesn't check whether the pointer is NULL (and IMO, it shouldn't).

I see what you mean, given how often we end up calling btrfs_migratepage() in the fleet, I’m willing to say this is probably fine. But it’s easy enough to toss in a warning.

Quoted 18 lines

Given our current macros, what was _meant_ here was:

if (PagePrivate(page))
attach_page_private(newpage,
detach_page_private(page));

but that's not obviously right.

Quoted 5 lines

Btrfs should only hage
PagePrivate2 set on pages that are formally in our writeback state machine, so it’ll get cleared as we unwind through normal IO or truncate etc. For data pages, btrfs page->private is simply set to 1 so the MM will kindly call releasepage for us.

That's not what I'm seeing here:

static void attach_extent_buffer_page(struct extent_buffer *eb,
struct page *page)

Quoted 8 lines
{
        if (!PagePrivate(page))
                attach_page_private(page, eb);
        else
                WARN_ON(page->private != (unsigned long)eb);
}

Or is that not a data page?

Correct, extent_buffers are metadata only. They wouldn’t be Private2.

Quoted 36 lines
Quoted 10 lines
Quoted 6 lines

So what shold we do about all this? First, I want to make the code snippet above correct, because it looks right. So page_has_private() needs to test just PagePrivate and not PagePrivate2. Now we need a new function to call to determine whether the filesystem needs its invalidatepage callback invoked. Not sure what that should be named.

I haven’t checked all the page_has_private() callers, but maybe page_has_private() should stay the same and add page_private_count() for times where we need to get out our fingers and toes for the refcount math.

I was thinking about page_expected_count() which returns the number of references from the page cache plus the number of references from the various page privates. So is_page_cache_freeable() becomes:

	return page_count(page) == page_expected_count(page) + 1;

can_split_huge_page() becomes:

	if (page_has_private(page))
		return false;
	return page_count(page) == page_expected_count(page) +
			total_mapcount(page) + 1;
Quoted 11 lines
Quoted 5 lines

I think I also want to rename PG_private_2 to PG_owner_priv_2. There's a clear relationship between PG_private and page->private. There is no relationship between PG_private_2 and page->private, so it's a misleading name. Or maybe it should just be PG_fscache and btrfs can find some other way to mark the pages?

Btrfs should be able to flip bits in page->private to cover our current usage of PG_private_2. If we ever attach something real to page->private, we can flip bits in that instead. It’s kinda messy though and we’d have to change attach_page_private a little to reflect its new life as a bit setting machine.

Quoted 7 lines

It's not great, but with David wanting to change how PageFsCache is used,
it may be unavoidable (I'm not sure if he's discussed that with you yet)
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=fscache-iter&id=6f10fd7766ed6d87c3f696bb7931281557b389f5 shows part of it -- essentially he wants to make PagePrivate2 mean that I/O is
currently
ongoing to an fscache, and so truncate needs to wait on it being finished.

It’s fine-ish for btrfs since we’re setting Private2 only on pages that are either writeback or will be writeback really soon. But, I’d prefer this end up in a callback so that we don’t have magic meaning for magic flags in magic places.

-chris

#6 lore raw
In reply to Matthew Wilcox
Quoted 155 lines · Matthew Wilcox
Quoted 27 lines · Chris Mason
Quoted 19 lines · Matthew Wilcox

Our handling of PagePrivate, page->private and PagePrivate2 is a giant mess. Let's recap.

Filesystems which use bufferheads (ie most of them) set page->private to point to a buffer_head, set the PagePrivate bit and increment the refcount on the page.

The vmscan pageout code (*) needs to know whether a page is freeable:

        if (!is_page_cache_freeable(page))
                return PAGE_KEEP;
... where is_page_cache_freeable() contains ...
        return page_count(page) - page_has_private(page) == 1 +

page_cache_pins;

That's a little inscrutable, but the important thing is that if page_has_private() is true, then the page's reference count is supposed to be one higher than it would be otherwise. And that makes sense given how "having bufferheads" means "page refcount ges incremented".

Quoted 6 lines

But page_has_private() doesn't actually mean "PagePrivate is set". It means "PagePrivate or PagePrivate2 is set". And I don't understand how filesystems are supposed to keep that straight -- if we're setting PagePrivate2, and PagePrivate is clear, increment the refcount? If we're clearing PagePrivate, decrement the refcount if PagePrivate2 is also clear?

Quoted 6 lines

At least for btrfs, only PagePrivate elevates the refcount on the page. PagePrivate2 means:

This page has been properly setup for COW’d IO, and it went through the normal path of page_mkwrite() or file_write() instead of being silently dirtied by a deep corner of the MM.

What's not clear to me is whether btrfs can be in the situation where PagePrivate2 is set and PagePrivate is clear. If so, then we have a bug to fix.

Quoted 16 lines
Quoted 16 lines

We introduced attach_page_private() and detach_page_private() earlier this year to help filesystems get the refcount right. But we still have a few filesystems using PagePrivate themselves (afs, btrfs, ceph, crypto, erofs, f2fs, jfs, nfs, orangefs & ubifs) and I'm not convinced they're all getting it right.

Here's a bug I happened on while looking into this:

        if (page_has_private(page))
                attach_page_private(newpage, detach_page_private(page));

        if (PagePrivate2(page)) {
                ClearPagePrivate2(page);
                SetPagePrivate2(newpage);
        }
Quoted 6 lines
Quoted 4 lines

The aggravating thing is that this doesn't even look like a bug. You have to be in the kind of mood where you're thinking "What if page has Private2 set and Private clear?" and the answer is that newpage ends up with PagePrivate set, but page->private set to NULL.

Do you mean PagePrivate2 set but page->private NULL?

Sorry, I skipped a step of the explanation.

page_has_private returns true if Private or Private2 is set. So if page has PagePrivate clear and PagePrivate2 set, newpage will end up with both PagePrivate and PagePrivate2 set -- attach_page_private() doesn't check whether the pointer is NULL (and IMO, it shouldn't).

Given our current macros, what was _meant_ here was:

         if (PagePrivate(page))
                 attach_page_private(newpage, detach_page_private(page));

but that's not obviously right.

Quoted 5 lines

Btrfs should only hage
PagePrivate2 set on pages that are formally in our writeback state machine, so it’ll get cleared as we unwind through normal IO or truncate etc. For data pages, btrfs page->private is simply set to 1 so the MM will kindly call releasepage for us.

That's not what I'm seeing here:

static void attach_extent_buffer_page(struct extent_buffer *eb,
struct page *page)

{
        if (!PagePrivate(page))
                attach_page_private(page, eb);
        else
                WARN_ON(page->private != (unsigned long)eb);
}

Or is that not a data page?

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So what shold we do about all this? First, I want to make the code snippet above correct, because it looks right. So page_has_private() needs to test just PagePrivate and not PagePrivate2. Now we need a new function to call to determine whether the filesystem needs its invalidatepage callback invoked. Not sure what that should be named.

I haven’t checked all the page_has_private() callers, but maybe page_has_private() should stay the same and add page_private_count() for times where we need to get out our fingers and toes for the refcount math.

I was thinking about page_expected_count() which returns the number of references from the page cache plus the number of references from the various page privates. So is_page_cache_freeable() becomes:

        return page_count(page) == page_expected_count(page) + 1;

can_split_huge_page() becomes:

        if (page_has_private(page))
                return false;
        return page_count(page) == page_expected_count(page) +
                        total_mapcount(page) + 1;
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I think I also want to rename PG_private_2 to PG_owner_priv_2. There's a clear relationship between PG_private and page->private. There is no relationship between PG_private_2 and page->private, so it's a misleading name. Or maybe it should just be PG_fscache and btrfs can find some other way to mark the pages?

Btrfs should be able to flip bits in page->private to cover our current usage of PG_private_2. If we ever attach something real to page->private, we can flip bits in that instead. It’s kinda messy though and we’d have to change attach_page_private a little to reflect its new life as a bit setting machine.

It's not great, but with David wanting to change how PageFsCache is used,
it may be unavoidable (I'm not sure if he's discussed that with you yet)
https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit/?h=fscache-iter&id=6f10fd7766ed6d87c3f696bb7931281557b389f5 shows part of it -- essentially he wants to make PagePrivate2 mean that I/O is currently
ongoing to an fscache, and so truncate needs to wait on it being finished.

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Also ... do we really need to increment the page refcount if we have PagePrivate set? I'm not awfully familiar with the buffercache -- is it possible we end up in a situation where a buffer, perhaps under I/O, has the last reference to a struct page? It seems like that reference is
always put from drop_buffers() which is called from try_to_free_buffers()
which is always called by someone who has a reference to a struct page that they got from the pagecache. So what is this reference count for?

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I’m not sure what we gain by avoiding the refcount bump? Many filesystems use the pattern of: “put something in page->private, free that thing in releasepage.” Without the refcount bump it feels like we’d have more magic to avoid freeing the page without leaking things in page->private. I think the extra ref lets the FS crowd keep our noses out of the MM more often, so it seems like a net positive to me.

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The question is whether the "thing" in page->private can ever have the last reference on a struct page. Gao says erofs can be in that situation, so never mind this change.

I recall when truncation is failed to remove private data, we may end up having page->private as last reference. Anyway vmscan can handle such case.

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