Commit 654255fa authored Apr 23, 2020 by Jeffle Xu Committed by Miklos Szeredi May 13, 2020

ovl: inherit SB_NOSEC flag from upperdir

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>

parent 32b1924b

fs/overlayfs/super.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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 {

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