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Subject[PATCH AUTOSEL 5.12 72/80] btrfs: fix the filemap_range_has_page() call in btrfs_punch_hole_lock_range()
Date
From: Qu Wenruo <wqu@suse.com>

[ Upstream commit 0528476b6ac7832f31e2ed740a57ae31316b124e ]

[BUG]
With current subpage RW support, the following script can hang the fs
with 64K page size.

# mkfs.btrfs -f -s 4k $dev
# mount $dev -o nospace_cache $mnt
# fsstress -w -n 50 -p 1 -s 1607749395 -d $mnt

The kernel will do an infinite loop in btrfs_punch_hole_lock_range().

[CAUSE]
In btrfs_punch_hole_lock_range() we:

- Truncate page cache range
- Lock extent io tree
- Wait any ordered extents in the range.

We exit the loop until we meet all the following conditions:

- No ordered extent in the lock range
- No page is in the lock range

The latter condition has a pitfall, it only works for sector size ==
PAGE_SIZE case.

While can't handle the following subpage case:

0 32K 64K 96K 128K
| |///////||//////| ||

lockstart=32K
lockend=96K - 1

In this case, although the range crosses 2 pages,
truncate_pagecache_range() will invalidate no page at all, but only zero
the [32K, 96K) range of the two pages.

Thus filemap_range_has_page(32K, 96K-1) will always return true, thus we
will never meet the loop exit condition.

[FIX]
Fix the problem by doing page alignment for the lock range.

Function filemap_range_has_page() has already handled lend < lstart
case, we only need to round up @lockstart, and round_down @lockend for
truncate_pagecache_range().

This modification should not change any thing for sector size ==
PAGE_SIZE case, as in that case our range is already page aligned.

Tested-by: Ritesh Harjani <riteshh@linux.ibm.com> # [ppc64]
Tested-by: Anand Jain <anand.jain@oracle.com> # [aarch64]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/file.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index f21d98c73878..9c00cabcf0da 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2483,6 +2483,17 @@ static int btrfs_punch_hole_lock_range(struct inode *inode,
const u64 lockend,
struct extent_state **cached_state)
{
+ /*
+ * For subpage case, if the range is not at page boundary, we could
+ * have pages at the leading/tailing part of the range.
+ * This could lead to dead loop since filemap_range_has_page()
+ * will always return true.
+ * So here we need to do extra page alignment for
+ * filemap_range_has_page().
+ */
+ const u64 page_lockstart = round_up(lockstart, PAGE_SIZE);
+ const u64 page_lockend = round_down(lockend + 1, PAGE_SIZE) - 1;
+
while (1) {
struct btrfs_ordered_extent *ordered;
int ret;
@@ -2503,7 +2514,7 @@ static int btrfs_punch_hole_lock_range(struct inode *inode,
(ordered->file_offset + ordered->num_bytes <= lockstart ||
ordered->file_offset > lockend)) &&
!filemap_range_has_page(inode->i_mapping,
- lockstart, lockend)) {
+ page_lockstart, page_lockend)) {
if (ordered)
btrfs_put_ordered_extent(ordered);
break;
--
2.30.2
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