writeback: fix range_cyclic writeback vs writepages deadlock

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

range_cyclic
writeback Process 1 Process 2

xfs_vm_writepages
write_cache_pages
writeback_index = 2
cycled = 0
....
find page 2 dirty
lock Page 2
->writepage
page 2 writeback
page 2 clean
page 2 added to bio
no more pages
write()
locks page 1
dirties page 1
locks page 2
dirties page 1
fsync()
....
xfs_vm_writepages write_cache_pages
start index 0
find page 1 towrite
lock Page 1
->writepage
page 1 writeback
page 1 clean
page 1 added to bio
find page 2 towrite
lock Page 2
page 2 is writeback
<blocks>
write() locks page 1 dirties page 1 fsync()
....
xfs_vm_writepages write_cache_pages
start index 0

!done && !cycled
sets index to 0, restarts lookup
find page 1 dirty
find page 1 towrite lock Page 1 page 1 is writeback <blocks>

lock Page 1
<blocks>

DEADLOCK because:

- process 1 needs page 2 writeback to complete to make enough progress to issue IO pending for page 1
- writeback needs page 1 writeback to complete so process 2 can progress and unlock the page it is blocked on, then it can issue the IO pending for page 2
- process 2 can't make progress until process 1 issues IO for page 1

The underlying cause of the problem here is that range_cyclic writeback is processing pages in descending index order as we hold higher index pages in a structure controlled from above write_cache_pages(). The write_cache_pages() caller needs to be able to submit these pages for IO before write_cache_pages restarts writeback at mapping index 0 to avoid wcp inverting the page lock/writeback wait order.

generic_writepages() is not susceptible to this bug as it has no private context held across write_cache_pages() - filesystems using this infrastructure always submit pages in ->writepage immediately and so there is no problem with range_cyclic going back to mapping index 0.

However:
mpage_writepages() has a private bio context, exofs_writepages() has page_collect fuse_writepages() has fuse_fill_wb_data nfs_writepages() has nfs_pageio_descriptor xfs_vm_writepages() has xfs_writepage_ctx

All of these ->writepages implementations can hold pages under writeback in their private structures until write_cache_pages() returns, and hence they are all susceptible to this deadlock.

Also worth noting is that ext4 has it's own bastardised version of write_cache_pages() and so it /may/ have an equivalent deadlock. I looked at the code long enough to understand that it has a similar retry loop for range_cyclic writeback reaching the end of the file and then promptly ran away before my eyes bled too much. I'll leave it for the ext4 developers to determine if their code is actually has this deadlock and how to fix it if it has.

There's a few ways I can see avoid this deadlock. There's probably more, but these are the first I've though of:

1. get rid of range_cyclic altogether

2. range_cyclic always stops at EOF, and we start again from writeback index 0 on the next call into write_cache_pages()

2a. wcp also returns EAGAIN to ->writepages implementations to indicate range cyclic has hit EOF. writepages implementations can then flush the current context and call wpc again to continue. i.e. lift the retry into the ->writepages implementation

3. range_cyclic uses trylock_page() rather than lock_page(), and it skips pages it can't lock without blocking. It will already do this for pages under writeback, so this seems like a no-brainer

3a. all non-WB_SYNC_ALL writeback uses trylock_page() to avoid blocking as per pages under writeback.

I don't think #1 is an option - range_cyclic prevents frequently dirtied lower file offset from starving background writeback of rarely touched higher file offsets.

#2 is simple, and I don't think it will have any impact on performance as going back to the start of the file implies an immediate seek. We'll have exactly the same number of seeks if we switch writeback to another inode, and then come back to this one later and restart from index 0.

#2a is pretty much "status quo without the deadlock". Moving the retry loop up into the wcp caller means we can issue IO on the pending pages before calling wcp again, and so avoid locking or waiting on pages in the wrong order. I'm not convinced we need to do this given that we get the same thing from #2 on the next writeback call from the writeback infrastructure.

#3 is really just a band-aid - it doesn't fix the access/wait inversion problem, just prevents it from becoming a deadlock situation. I'd prefer we fix the inversion, not sweep it under the carpet like this.

#3a is really an optimisation that just so happens to include the band-aid fix of #3.

So I'm really tending towards #2 as the simplest way to fix this, and that's what's the patch below implements.

Signed-off-by Dave Chinner <[email protected]>

---

 mm/page-writeback.c | 33 +++++++++++++++------------------
 1 file changed, 15 insertions(+), 18 deletions(-)

mm/page-writeback.c +15 -18

index 337c6afb3345..c10e70ed3515 100644
@@ -2150,6 +2150,13 @@ EXPORT_SYMBOL(tag_pages_for_writeback);
  * not miss some pages (e.g., because some other process has cleared TOWRITE
  * tag we set). The rule we follow is that TOWRITE tag can be cleared only
  * by the process clearing the DIRTY tag (and submitting the page for IO).
+ *
+ * To avoid deadlocks between range_cyclic writeback and callers that hold
+ * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
+ * we do not loop back to the start of the file. Doing so causes a page
+ * lock/page writeback access order inversion - we should only ever lock
+ * multiple pages in ascending page->index order, and looping back to the start
+ * of the file violates that rule and causes deadlocks.
  */
 int write_cache_pages(struct address_space *mapping,
 		      struct writeback_control *wbc, writepage_t writepage,

@@ -2163,7 +2170,6 @@ int write_cache_pages(struct address_space *mapping,
 	pgoff_t index;
 	pgoff_t end;		/* Inclusive */
 	pgoff_t done_index;
-	int cycled;
 	int range_whole = 0;
 	int tag;
 

@@ -2171,23 +2177,17 @@ int write_cache_pages(struct address_space *mapping,
 	if (wbc->range_cyclic) {
 		writeback_index = mapping->writeback_index; /* prev offset */
 		index = writeback_index;
-		if (index == 0)
-			cycled = 1;
-		else
-			cycled = 0;
 		end = -1;
 	} else {
 		index = wbc->range_start >> PAGE_SHIFT;
 		end = wbc->range_end >> PAGE_SHIFT;
 		if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
 			range_whole = 1;
-		cycled = 1; /* ignore range_cyclic tests */
 	}
 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
 		tag = PAGECACHE_TAG_TOWRITE;
 	else
 		tag = PAGECACHE_TAG_DIRTY;
-retry:
 	if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
 		tag_pages_for_writeback(mapping, index, end);
 	done_index = index;

@@ -2273,17 +2273,14 @@ int write_cache_pages(struct address_space *mapping,
 		pagevec_release(&pvec);
 		cond_resched();
 	}
-	if (!cycled && !done) {
-		/*
-		 * range_cyclic:
-		 * We hit the last page and there is more work to be done: wrap
-		 * back to the start of the file
-		 */
-		cycled = 1;
-		index = 0;
-		end = writeback_index - 1;
-		goto retry;
-	}
+
+	/*
+	 * If we hit the last page and there is more work to be done: wrap
+	 * back the index back to the start of the file for the next
+	 * time we are called.
+	 */
+	if (wbc->range_cyclic && !done)
+		done_index = 0;
 	if (wbc->range_cyclic || (range_whole && wbc->nr_to_write > 0))
 		mapping->writeback_index = done_index;
 

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Quoted 6 lines · Dave Chinner

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

Quoted 24 lines

#2 is simple, and I don't think it will have any impact on performance as going back to the start of the file implies an immediate seek. We'll have exactly the same number of seeks if we switch writeback to another inode, and then come back to this one later and restart from index 0.

#2a is pretty much "status quo without the deadlock". Moving the retry loop up into the wcp caller means we can issue IO on the pending pages before calling wcp again, and so avoid locking or waiting on pages in the wrong order. I'm not convinced we need to do this given that we get the same thing from #2 on the next writeback call from the writeback infrastructure.

#3 is really just a band-aid - it doesn't fix the access/wait inversion problem, just prevents it from becoming a deadlock situation. I'd prefer we fix the inversion, not sweep it under the carpet like this.

#3a is really an optimisation that just so happens to include the band-aid fix of #3.

So I'm really tending towards #2 as the simplest way to fix this, and that's what's the patch below implements.

Quoted 2 lines

Signed-off-by Dave Chinner <[email protected]>

---

FWIW, this seems like a reasonable approach to me. One thing I'm not sure about is whether the higher level plug in wb_writeback() could cause the same problem even with the lower level cycle restart out of the picture.

It looks to me that if the inode still has dirty pages after the write_cache_pages(), it ends up on wb->b_more_io via writeback_sb_inodes() -> requeue_inode(). The caller (wb_writeback()) repopulates ->b_io from ->b_more_io (via queue_io()) if the former is empty (and we haven't otherwise stopped) before finishing the plug. There is a blk_flush_plug() in writeback_sb_inodes(), but that appears to be non-deterministic. That call aside, could that plug effectively hold the page in writeback long enough for background writeback to spin around and sit on the page 1 lock?

Quoted 7 lines · Brian Foster

Quoted 6 lines · Dave Chinner

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

[...]

Quoted 24 lines

Quoted 24 lines

#2 is simple, and I don't think it will have any impact on performance as going back to the start of the file implies an immediate seek. We'll have exactly the same number of seeks if we switch writeback to another inode, and then come back to this one later and restart from index 0.

#2a is pretty much "status quo without the deadlock". Moving the retry loop up into the wcp caller means we can issue IO on the pending pages before calling wcp again, and so avoid locking or waiting on pages in the wrong order. I'm not convinced we need to do this given that we get the same thing from #2 on the next writeback call from the writeback infrastructure.

#3 is really just a band-aid - it doesn't fix the access/wait inversion problem, just prevents it from becoming a deadlock situation. I'd prefer we fix the inversion, not sweep it under the carpet like this.

#3a is really an optimisation that just so happens to include the band-aid fix of #3.

So I'm really tending towards #2 as the simplest way to fix this, and that's what's the patch below implements.

Quoted 7 lines

Quoted 2 lines

Signed-off-by Dave Chinner <[email protected]>

---

FWIW, this seems like a reasonable approach to me. One thing I'm not sure about is whether the higher level plug in wb_writeback() could cause the same problem even with the lower level cycle restart out of the picture.

Plugging can't cause this because it captures bios that have been released from the caller context vi submit_bio(). The plug list has hooks in the scheduler that flush it on context switch precisely so that we don't get deadlock problems with bios being stuck on the plug list when we block for some reason.

Quoted 9 lines

It looks to me that if the inode still has dirty pages after the write_cache_pages(), it ends up on wb->b_more_io via writeback_sb_inodes() -> requeue_inode(). The caller (wb_writeback()) repopulates ->b_io from ->b_more_io (via queue_io()) if the former is empty (and we haven't otherwise stopped) before finishing the plug. There is a blk_flush_plug() in writeback_sb_inodes(), but that appears to be non-deterministic. That call aside, could that plug effectively hold the page in writeback long enough for background writeback to spin around and sit on the page 1 lock?

Right, that could happen, but the plug list will be flushed before we context switch away and sleep after failng to get the page lock, so there's no problem here.

Cheers,

Dave.

Signature · 2 lines

Dave Chinner
[email protected]

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

The underlying cause of the problem here is that range_cyclic writeback is processing pages in descending index order as we hold higher index pages in a structure controlled from above write_cache_pages(). The write_cache_pages() caller needs to be able to submit these pages for IO before write_cache_pages restarts writeback at mapping index 0 to avoid wcp inverting the page lock/writeback wait order.

generic_writepages() is not susceptible to this bug as it has no private context held across write_cache_pages() - filesystems using this infrastructure always submit pages in ->writepage immediately and so there is no problem with range_cyclic going back to mapping index 0.

However:
mpage_writepages() has a private bio context, exofs_writepages() has page_collect fuse_writepages() has fuse_fill_wb_data nfs_writepages() has nfs_pageio_descriptor xfs_vm_writepages() has xfs_writepage_ctx

All of these ->writepages implementations can hold pages under writeback in their private structures until write_cache_pages() returns, and hence they are all susceptible to this deadlock.

Also worth noting is that ext4 has it's own bastardised version of write_cache_pages() and so it /may/ have an equivalent deadlock. I looked at the code long enough to understand that it has a similar retry loop for range_cyclic writeback reaching the end of the file and then promptly ran away before my eyes bled too much. I'll leave it for the ext4 developers to determine if their code is actually has this deadlock and how to fix it if it has.

There's a few ways I can see avoid this deadlock. There's probably more, but these are the first I've though of:

1. get rid of range_cyclic altogether

2. range_cyclic always stops at EOF, and we start again from writeback index 0 on the next call into write_cache_pages()

2a. wcp also returns EAGAIN to ->writepages implementations to indicate range cyclic has hit EOF. write-pages implementations can then flush the current context and call wpc again to continue. i.e. lift the retry into the ->writepages implementation

Quoted 46 lines · Dave Chinner

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

The underlying cause of the problem here is that range_cyclic writeback is processing pages in descending index order as we hold higher index pages in a structure controlled from above write_cache_pages(). The write_cache_pages() caller needs to be able to submit these pages for IO before write_cache_pages restarts writeback at mapping index 0 to avoid wcp inverting the page lock/writeback wait order.

generic_writepages() is not susceptible to this bug as it has no private context held across write_cache_pages() - filesystems using this infrastructure always submit pages in ->writepage immediately and so there is no problem with range_cyclic going back to mapping index 0.

However:
mpage_writepages() has a private bio context, exofs_writepages() has page_collect fuse_writepages() has fuse_fill_wb_data nfs_writepages() has nfs_pageio_descriptor xfs_vm_writepages() has xfs_writepage_ctx

All of these ->writepages implementations can hold pages under writeback in their private structures until write_cache_pages() returns, and hence they are all susceptible to this deadlock.

Also worth noting is that ext4 has it's own bastardised version of write_cache_pages() and so it /may/ have an equivalent deadlock. I looked at the code long enough to understand that it has a similar retry loop for range_cyclic writeback reaching the end of the file and then promptly ran away before my eyes bled too much. I'll leave it for the ext4 developers to determine if their code is actually has this deadlock and how to fix it if it has.

There's a few ways I can see avoid this deadlock. There's probably more, but these are the first I've though of:

1. get rid of range_cyclic altogether

Quoted 7 lines

2. range_cyclic always stops at EOF, and we start again from writeback index 0 on the next call into write_cache_pages()

2a. wcp also returns EAGAIN to ->writepages implementations to indicate range cyclic has hit EOF. write-pages implementations can then flush the current context and call wpc again to continue. i.e. lift the retry into the ->writepages implementation

Btrfs has a variation of 2a in our bastardized copy of write_cache_pages(), it has a call to flush the bios we've built up before doing operations that might deadlock, including waiting for writeback, locking pages etc:

                        if (wbc->sync_mode != WB_SYNC_NONE) {
                                if (PageWriteback(page))
                                        flush_write_bio(epd);
                                wait_on_page_writeback(page);
                        }

I don't see any problems with the solution you picked instead, but if we run into more complex variations of this we can just pass a callback and a flushing cookie to generic_writepages().

Open question on the perf benefits of staring IO early while we wait for writeback on page X or letting our bio build bigger.

Open question #2, at least for btrfs we're building a single fat bio by adding pages to it along the way. This is a small variation on plugging...we could just pull the last bio off the plug stack and stuff the pages in. Then we'd get all the deadlock prevent implicit in plugging for free.

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

Quoted 6 lines · Dave Chinner

From: Dave Chinner <[email protected]>

We've recently seen a workload on XFS filesystems with a repeatable deadlock between background writeback and a multi-process application doing concurrent writes and fsyncs to a small range of a file.

Going through some old email and this didn't seem to be merged yet? FWIW I agree with your solution and the patch looks good to me. Feel free to add:

Reviewed-by Jan Kara <[email protected]>

and then send it to AKPM for inclusion.