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btrfs: avoid blocking on page locks with nowait dio on compressed range

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If we are doing NOWAIT direct IO read/write and our inode has compressed
extents, we call filemap_fdatawrite_range() against the range in order
to wait for compressed writeback to complete, since the generic code at
iomap_dio_rw() calls filemap_write_and_wait_range() once, which is not
enough to wait for compressed writeback to complete.

This call to filemap_fdatawrite_range() can block on page locks, since
the first writepages() on a range that we will try to compress results
only in queuing a work to compress the data while holding the pages
locked.

Even though the generic code at iomap_dio_rw() will do the right thing
and return -EAGAIN for NOWAIT requests in case there are pages in the
range, we can still end up at btrfs_dio_iomap_begin() with pages in the
range because either of the following can happen:

1) Memory mapped writes, as we haven't locked the range yet;

2) Buffered reads might have started, which lock the pages, and we do
   the filemap_fdatawrite_range() call before locking the file range.

So don't call filemap_fdatawrite_range() at btrfs_dio_iomap_begin() if we
are doing a NOWAIT read/write. Instead call filemap_range_needs_writeback()
to check if there are any locked, dirty, or under writeback pages, and
return -EAGAIN if that's the case.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2022-03-23 16:19:23 +00:00 committed by David Sterba
parent b0a66a3137
commit b023e67512
1 changed files with 26 additions and 8 deletions
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34
fs/btrfs/inode.c
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@ -7532,17 +7532,35 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
lockend = start + len - 1;
/*
* The generic stuff only does filemap_write_and_wait_range, which
* isn't enough if we've written compressed pages to this area, so we
* need to flush the dirty pages again to make absolutely sure that any
* outstanding dirty pages are on disk.
* iomap_dio_rw() only does filemap_write_and_wait_range(), which isn't
* enough if we've written compressed pages to this area, so we need to
* flush the dirty pages again to make absolutely sure that any
* outstanding dirty pages are on disk - the first flush only starts
* compression on the data, while keeping the pages locked, so by the
* time the second flush returns we know bios for the compressed pages
* were submitted and finished, and the pages no longer under writeback.
*
* If we have a NOWAIT request and we have any pages in the range that
* are locked, likely due to compression still in progress, we don't want
* to block on page locks. We also don't want to block on pages marked as
* dirty or under writeback (same as for the non-compression case).
* iomap_dio_rw() did the same check, but after that and before we got
* here, mmap'ed writes may have happened or buffered reads started
* (readpage() and readahead(), which lock pages), as we haven't locked
* the file range yet.
*/
if (test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT,
&BTRFS_I(inode)->runtime_flags)) {
ret = filemap_fdatawrite_range(inode->i_mapping, start,
start + length - 1);
if (ret)
return ret;
if (flags & IOMAP_NOWAIT) {
if (filemap_range_needs_writeback(inode->i_mapping,
lockstart, lockend))
return -EAGAIN;
} else {
ret = filemap_fdatawrite_range(inode->i_mapping, start,
start + length - 1);
if (ret)
return ret;
}
}
dio_data = kzalloc(sizeof(*dio_data), GFP_NOFS);