[BUG]
When running the following fsx command (extracted from generic/127) on
subpage btrfs, it can create inline extent with regular extents:

  fsx -q -l 262144 -o 65536 -S 191110531 -N 9057 -R -W $mnt/file > /tmp/fsx

The offending extent would look like:

  item 9 key (257 INODE_REF 256) itemoff 15703 itemsize 14
    index 2 namelen 4 name: file
  item 10 key (257 EXTENT_DATA 0) itemoff 14975 itemsize 728
    generation 7 type 0 (inline)
    inline extent data size 707 ram_bytes 707 compression 0 (none)
  item 11 key (257 EXTENT_DATA 4096) itemoff 14922 itemsize 53
    generation 7 type 2 (prealloc)
    prealloc data disk byte 102346752 nr 4096
    prealloc data offset 0 nr 4096

[CAUSE]
For subpage btrfs, the writeback is triggered in page unit, which means,
even if we just want to writeback range [16K, 20K) for 64K page system,
we will still try to writeback any dirty sector of range [0, 64K).

This is never a problem if sectorsize == PAGE_SIZE, but for subpage,
this can cause unexpected problems.

For above test case, the last several operations from fsx are:

 9055 trunc      from 0x40000 to 0x2c3
 9057 falloc     from 0x164c to 0x19d2 (0x386 bytes)

In operation 9055, we dirtied sector [0, 4096), then in falloc, we call
btrfs_wait_ordered_range(inode, start=4096, len=4096), only expecting to
writeback any dirty data in [4096, 8192), but nothing else.

Unfortunately, in subpage case, above btrfs_wait_ordered_range() will
trigger writeback of the range [0, 64K), which includes the data at [0,
4096).

And since at the call site, we haven't yet increased i_size, which is
still 707, this means cow_file_range() can insert an inline extent.

Resulting above inline + regular extent.

[WORKAROUND]
I don't really have any good short-term solution yet, as this means all
operations that would trigger writeback need to be reviewed for any
isize change.

So here I choose to disable inline extent creation for subpage case as a
workaround.
We have done tons of work just to avoid such extent, so I don't to
create an exception just for subpage.

This only affects inline extent creation, btrfs subpage support has no
problem reading existing inline extents at all.

Signed-off-by: Qu Wenruo <redacted>
---
 fs/btrfs/inode.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e6eb20987351..25af1256dc74 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c

@@ -682,7 +682,11 @@ static noinline int compress_file_range(struct async_chunk *async_chunk)
 		}
 	}
 cont:
-	if (start == 0) {
+	/*
+	 * Check cow_file_range() for why we don't even try to create
+	 * inline extent for subpage case.
+	 */
+	if (start == 0 && fs_info->sectorsize == PAGE_SIZE) {
 		/* lets try to make an inline extent */
 		if (ret || total_in < actual_end) {
 			/* we didn't compress the entire range, try

@@ -1080,7 +1084,17 @@ static noinline int cow_file_range(struct btrfs_inode *inode,
 
 	inode_should_defrag(inode, start, end, num_bytes, SZ_64K);
 
-	if (start == 0) {
+	/*
+	 * Due to the page size limit, for subpage we can only trigger the
+	 * writeback for the dirty sectors of page, that means data writeback
+	 * is doing more writeback than what we want.
+	 *
+	 * This is especially unexpected for some call sites like fallocate,
+	 * where we only increase isize after everything is done.
+	 * This means we can trigger inline extent even we didn't want.
+	 * So here we skip inline extent creation completely.
+	 */
+	if (start == 0 && fs_info->sectorsize == PAGE_SIZE) {
 		/* lets try to make an inline extent */
 		ret = cow_file_range_inline(inode, start, end, 0,
 					    BTRFS_COMPRESS_NONE, NULL);

-- 
2.32.0