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ovl: inherit SB_NOSEC flag from upperdir

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Since the stacking of regular file operations [1], the overlayfs edition of
write_iter() is called when writing regular files.

Since then, xattr lookup is needed on every write since file_remove_privs()
is called from ovl_write_iter(), which would become the performance
bottleneck when writing small chunks of data. In my test case,
file_remove_privs() would consume ~15% CPU when running fstime of unixbench
(the workload is repeadly writing 1 KB to the same file) [2].

Inherit the SB_NOSEC flag from upperdir. Since then xattr lookup would be
done only once on the first write. Unixbench fstime gets a ~20% performance
gain with this patch.

[1] https://lore.kernel.org/lkml/20180606150905.GC9426@magnolia/T/
[2] https://www.spinics.net/lists/linux-unionfs/msg07153.html

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Jeffle Xu 2020-04-23 19:06:55 +08:00 committed by Miklos Szeredi
parent 32b1924b21
commit 654255fa20
1 changed files with 12 additions and 0 deletions
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12
fs/overlayfs/super.c
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@ -1100,6 +1100,18 @@ static int ovl_get_upper(struct super_block *sb, struct ovl_fs *ofs,
upper_mnt->mnt_flags &= ~(MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME);
ofs->upper_mnt = upper_mnt;
/*
* Inherit SB_NOSEC flag from upperdir.
*
* This optimization changes behavior when a security related attribute
* (suid/sgid/security.*) is changed on an underlying layer. This is
* okay because we don't yet have guarantees in that case, but it will
* need careful treatment once we want to honour changes to underlying
* filesystems.
*/
if (upper_mnt->mnt_sb->s_flags & SB_NOSEC)
sb->s_flags |= SB_NOSEC;
if (ovl_inuse_trylock(ofs->upper_mnt->mnt_root)) {
ofs->upperdir_locked = true;
} else {