Re: [PATCH] virtiofs: Enable SB_NOSEC flag to improve small write performance

[Vivek Goyal <vgoyal@redhat.com>]

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@zeniv.linux.org.uk>
> 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@intel.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  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
>