Re: [PATCH v3] btrfs: fix alginment of VMA for memory mapped files on THP

[David Sterba]

On Tue, Aug 02, 2022 at 01:32:46PM -0700, alexlzhu@xxxxxx wrote:
> From: Alexander Zhu <alexlzhu@xxxxxx>
> 
> 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
> alginment 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.
> 
> Signed-off-by: Alexander Zhu <alexlzhu@xxxxxx>

Added to misc-next, thanks.