Ganesh Mahalingam reported that virtiofs is slow with small direct random writes when virtiofsd is run with cache=always. https://github.com/kata-containers/runtime/issues/2815 Little debugging showed that that file_remove_privs() is called in cached write path on every write. And everytime it calls security_inode_need_killpriv() which results in call to __vfs_getxattr(XATTR_NAME_CAPS). And this goes to file server to fetch xattr. This extra round trip for every write slows down writes a lot. Normally to avoid paying this penalty on every write, vfs has the notion of caching this information in inode (S_NOSEC). So vfs sets S_NOSEC, if filesystem opted for it using super block flag SB_NOSEC. And S_NOSEC is cleared when setuid/setgid bit is set or when security xattr is set on inode so that next time a write happens, we check inode again for clearing setuid/setgid bits as well clear any security.capability xattr. This seems to work well for local file systems but for remote file systems it is possible that VFS does not have full picture and a different client sets setuid/setgid bit or security.capability xattr on file and that means VFS information about S_NOSEC on another client will be stale. So for remote filesystems SB_NOSEC was disabled by default. commit 9e1f1de02c2275d7172e18dc4e7c2065777611bf Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Date: Fri Jun 3 18:24:58 2011 -0400 more conservative S_NOSEC handling That commit mentioned that these filesystems can still make use of SB_NOSEC as long as they clear S_NOSEC when they are refreshing inode attriutes from server. So this patch tries to enable SB_NOSEC on fuse (regular fuse as well as virtiofs). And clear SB_NOSEC when we are refreshing inode attributes. We need to clear SB_NOSEC either when inode has setuid/setgid bit set or security.capability xattr has been set. We have the first piece of information available in FUSE_GETATTR response. But we don't know if security.capability has been set on file or not. Question is, do we really need to know about security.capability. file_remove_privs() always removes security.capability if a file is being written to. That means when server writes to file, security.capability should be removed without guest having to tell anything to it. That means we don't have to worry about knowing if security.capability was set or not as long as writes by client don't get cached and go to server always. And server write should clear security.capability. Hence, I clear SB_NOSEC when writeback cache is enabled. This change improves random write performance very significantly. I am running virtiofsd with cache=auto and following fio command. fio --ioengine=libaio --direct=1 --name=test --filename=/mnt/virtiofs/random_read_write.fio --bs=4k --iodepth=64 --size=4G --readwrite=randwrite Before this patch I get around 40MB/s and after the patch I get around 300MB/s bandwidth. So improvement is very significant. Note: We probably could do this change for regular fuse filesystems as well. But I don't know all the possible configurations supported so I am limiting it to virtiofs. Reported-by: "Mahalingam, Ganesh" <ganesh.mahalingam@xxxxxxxxx> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> --- fs/fuse/inode.c | 7 +++++++ fs/fuse/virtio_fs.c | 4 ++++ 2 files changed, 11 insertions(+) diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c index 5b4aebf5821f..5e74c818b2aa 100644 --- a/fs/fuse/inode.c +++ b/fs/fuse/inode.c @@ -185,6 +185,13 @@ void fuse_change_attributes_common(struct inode *inode, struct fuse_attr *attr, inode->i_mode &= ~S_ISVTX; fi->orig_ino = attr->ino; + + /* + * File server see setuid/setgid bit set. Maybe another client did + * it. Reset S_NOSEC. + */ + if (IS_NOSEC(inode) && is_sxid(inode->i_mode)) + inode->i_flags &= ~S_NOSEC; } void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr, diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c index 4c4ef5d69298..e89628163ec4 100644 --- a/fs/fuse/virtio_fs.c +++ b/fs/fuse/virtio_fs.c @@ -1126,6 +1126,10 @@ static int virtio_fs_fill_super(struct super_block *sb) /* Previous unmount will stop all queues. Start these again */ virtio_fs_start_all_queues(fs); fuse_send_init(fc); + + if (!fc->writeback_cache) + sb->s_flags |= SB_NOSEC; + mutex_unlock(&virtio_fs_mutex); return 0; -- 2.25.4
