btrfs: fix alignment of VMA for memory mapped files on THP

With CONFIG_READ_ONLY_THP_FOR_FS, the Linux kernel supports using THPs for
read-only mmapped files, such as shared libraries. However, the kernel
makes no attempt to actually align those mappings on 2MB boundaries,
which makes it impossible to use those THPs most of the time. This issue
applies to general file mapping THP as well as existing setups using
CONFIG_READ_ONLY_THP_FOR_FS. This is easily fixed by using
thp_get_unmapped_area for the unmapped_area function in btrfs, which
is what ext2, ext4, fuse, and xfs all use.

Initially btrfs had been left out in commit 8c07fc452ac0 ("btrfs: fix
alignment of VMA for memory mapped files on THP") as btrfs does not support
DAX. However, commit 1854bc6e ("mm/readahead: Align file mappings
for non-DAX") removed the DAX requirement. We should now be able to call
thp_get_unmapped_area() for btrfs.

The problem can be seen in /proc/PID/smaps where THPeligible is set to 0
on mappings to eligible shared object files as shown below.

Before this patch:

  7fc6a7e18000-7fc6a80cc000 r-xp 00000000 00:1e 199856
  /usr/lib64/libcrypto.so.1.1.1k
  Size:               2768 kB
  THPeligible:    0
  VmFlags: rd ex mr mw me

With this patch the library is mapped at a 2MB aligned address:

  fbdfe200000-7fbdfe4b4000 r-xp 00000000 00:1e 199856
  /usr/lib64/libcrypto.so.1.1.1k
  Size:               2768 kB
  THPeligible:    1
  VmFlags: rd ex mr mw me

This fixes the alignment of VMAs for any mmap of a file that has the
rd and ex permissions and size >= 2MB. The VMA alignment and
THPeligible field for anonymous memory is handled separately and
is thus not effected by this change.

CC: stable@vger.kernel.org # 5.18+
Signed-off-by: Alexander Zhu <alexlzhu@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -3810,6 +3810,7 @@ const struct file_operations btrfs_file_operations = {
 	.mmap		= btrfs_file_mmap,
 	.open		= btrfs_file_open,
 	.release	= btrfs_release_file,
+	.get_unmapped_area = thp_get_unmapped_area,
 	.fsync		= btrfs_sync_file,
 	.fallocate	= btrfs_fallocate,
 	.unlocked_ioctl	= btrfs_ioctl,