On Thu, Jul 16, 2020 at 10:40:33AM -0400, Vivek Goyal wrote: > 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. I am assuming that file server will clear security.capability on host upon WRITE. Is it a fair assumption for all filesystems passthrough virtiofsd might be running? Vivek > > 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 >
