btrfs: avoid page_lockend underflow in btrfs_punch_hole_lock_range()

[BUG]
When running btrfs/004 with 4K fs block size and 64K page size,
sometimes fsstress workload can take 100% CPU for a while, but not long
enough to trigger a 120s hang warning.

[CAUSE]
When such 100% CPU usage happens, btrfs_punch_hole_lock_range() is
always in the call trace.

One example when this problem happens, the function
btrfs_punch_hole_lock_range() got the following parameters:

  lock_start = 4096, lockend = 20469

Then we calculate @page_lockstart by rounding up lock_start to page
boundary, which is 64K (page size is 64K).

For @page_lockend, we round down the value towards page boundary, which
result 0.  Then since we need to pass an inclusive end to
filemap_range_has_page(), we subtract 1 from the rounded down value,
resulting in (u64)-1.

In the above case, the range is inside the same page, and we do not even
need to call filemap_range_has_page(), not to mention to call it with
(u64)-1 at the end.

This behavior will cause btrfs_punch_hole_lock_range() to busy loop
waiting for irrelevant range to have its pages dropped.

[FIX]
Calculate @page_lockend by just rounding down @lockend, without
decreasing the value by one.  So @page_lockend will no longer overflow.

Then exit early if @page_lockend is no larger than @page_lockstart.
As it means either the range is inside the same page, or the two pages
are adjacent already.

Finally only decrease @page_lockend when calling filemap_range_has_page().

Fixes: 0528476b6ac7 ("btrfs: fix the filemap_range_has_page() call in btrfs_punch_hole_lock_range()")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions