From 654255fa205cb2b010e9abb34b0c8afcca9c78c7 Mon Sep 17 00:00:00 2001
From: Jeffle Xu <jefflexu@linux.alibaba.com>
Date: Thu, 23 Apr 2020 19:06:55 +0800
Subject: [PATCH] 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>
---
fs/overlayfs/super.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index f57aa348dcd63c..af69f41f564dbe 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -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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