From: Alexander Zhu alexlzhu@fb.com
commit b0c582233a8563f3c4228df838cdc67a8807ec78 upstream.
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 1854bc6e2420 ("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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/btrfs/file.c | 1 + 1 file changed, 1 insertion(+)
--- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -3810,6 +3810,7 @@ const struct file_operations btrfs_file_ .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,