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>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 262a707d899064aeddb9aa4e5ba398b990da9aa3..71b8a825c4479a13df38c5e3f8abeba75d885b0a 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2104,15 +2104,20 @@ static void btrfs_punch_hole_lock_range(struct inode *inode,
         * will always return true.
         * So here we need to do extra page alignment for
         * filemap_range_has_page().
+        *
+        * And do not decrease page_lockend right now, as it can be 0.
         */
        const u64 page_lockstart = round_up(lockstart, PAGE_SIZE);
-       const u64 page_lockend = round_down(lockend + 1, PAGE_SIZE) - 1;
+       const u64 page_lockend = round_down(lockend + 1, PAGE_SIZE);
 
        while (1) {
                truncate_pagecache_range(inode, lockstart, lockend);
 
                lock_extent(&BTRFS_I(inode)->io_tree, lockstart, lockend,
                            cached_state);
+               /* The same page or adjacent pages. */
+               if (page_lockend <= page_lockstart)
+                       break;
                /*
                 * We can't have ordered extents in the range, nor dirty/writeback
                 * pages, because we have locked the inode's VFS lock in exclusive
@@ -2124,7 +2129,7 @@ static void btrfs_punch_hole_lock_range(struct inode *inode,
                 * we do, unlock the range and retry.
                 */
                if (!filemap_range_has_page(inode->i_mapping, page_lockstart,
-                                           page_lockend))
+                                           page_lockend - 1))
                        break;
 
                unlock_extent(&BTRFS_I(inode)->io_tree, lockstart, lockend,