From 69b4573296469fd3f70cf7044693074980517067 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Sat, 28 May 2011 08:25:51 -0700 Subject: Cache xattr security drop check for write v2 Some recent benchmarking on btrfs showed that a major scaling bottleneck on large systems on btrfs is currently the xattr lookup on every write. Why xattr lookup on every write I hear you ask? write wants to drop suid and security related xattrs that could set o capabilities for executables. To do that it currently looks up security.capability on EVERY write (even for non executables) to decide whether to drop it or not. In btrfs this causes an additional tree walk, hitting some per file system locks and quite bad scalability. In a simple read workload on a 8S system I saw over 90% CPU time in spinlocks related to that. Chris Mason tells me this is also a problem in ext4, where it hits the global mbcache lock. This patch adds a simple per inode to avoid this problem. We only do the lookup once per file and then if there is no xattr cache the decision. All xattr changes clear the flag. I also used the same flag to avoid the suid check, although that one is pretty cheap. A file system can also set this flag when it creates the inode, if it has a cheap way to do so. This is done for some common file systems in followon patches. With this patch a major part of the lock contention disappears for btrfs. Some testing on smaller systems didn't show significant performance changes, but at least it helps the larger systems and is generally more efficient. v2: Rename is_sgid. add file system helper. Cc: chris.mason@oracle.com Cc: josef@redhat.com Cc: viro@zeniv.linux.org.uk Cc: agruen@linbit.com Cc: Serge E. Hallyn Signed-off-by: Andi Kleen Signed-off-by: Al Viro --- fs/attr.c | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'fs/attr.c') diff --git a/fs/attr.c b/fs/attr.c index 91dbe2a107f..caf2aa521e2 100644 --- a/fs/attr.c +++ b/fs/attr.c @@ -175,6 +175,13 @@ int notify_change(struct dentry * dentry, struct iattr * attr) return -EPERM; } + if ((ia_valid & ATTR_MODE)) { + mode_t amode = attr->ia_mode; + /* Flag setting protected by i_mutex */ + if (is_sxid(amode)) + inode->i_flags &= ~S_NOSEC; + } + now = current_fs_time(inode->i_sb); attr->ia_ctime = now; -- cgit v1.2.3