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From:   <alexlzhu@fb.com>
To:     <kernel-team@fb.com>, <linux-mm@kvack.org>, <clm@fb.com>,
        <josef@toxicpanda.com>, <dsterba@suse.com>,
        <linux-btrfs@vger.kernel.org>, <linux-kernel@vger.kernel.org>
CC:     alexlzhu <alexlzhu@fb.com>
Subject: [PATCH v2] mm: fix alginment of VMA for memory mapped files on THP
Date:   Mon, 1 Aug 2022 11:47:40 -0700
Message-ID: <20220801184740.2134364-1-alexlzhu@fb.com>
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From: alexlzhu <alexlzhu@fb.com>

With CONFIG_READ_ONLY_THP_FOR_FS, the Linux kernel supports using THPs fo=
r
read-only mmapped files, such as shared libraries. However, the
kernel makes no attempt to actually align those mappings on 2MB boundarie=
s,
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. 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 >=3D 2MB. The VMA alignment and
THPeligible field for anonymous memory is handled separately and
is thus not effected by this change.

Signed-off-by: alexlzhu <alexlzhu@fb.com>
---
 fs/btrfs/file.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 9dfde1af8a64..2423040db167 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3808,6 +3808,7 @@ const struct file_operations btrfs_file_operations =
=3D {
 	.mmap		=3D btrfs_file_mmap,
 	.open		=3D btrfs_file_open,
 	.release	=3D btrfs_release_file,
+	.get_unmapped_area =3D thp_get_unmapped_area,
 	.fsync		=3D btrfs_sync_file,
 	.fallocate	=3D btrfs_fallocate,
 	.unlocked_ioctl	=3D btrfs_ioctl,
--=20
2.30.2