Re: [Virtio-fs] [PATCH] fuse: Invalidate attrs when page writeback completes

[Vivek Goyal <vgoyal@redhat.com>]

Hi Miklos,

Ping.

Thanks
Vivek

On Tue, Apr 06, 2021 at 10:07:06AM -0400, Vivek Goyal wrote:
> In fuse when a direct/write-through write happens we invalidate attrs because
> that might have updated mtime/ctime on server and cached mtime/ctime
> will be stale.
> 
> What about page writeback path. Looks like we don't invalidate attrs there.
> To be consistent, invalidate attrs in writeback path as well. Only exception
> is when writeback_cache is enabled. In that case we strust local mtime/ctime
> and there is no need to invalidate attrs.
> 
> Recently users started experiencing failure of xfstests generic/080,
> geneirc/215 and generic/614 on virtiofs. This happened only newer
> "stat" utility and not older one. This patch fixes the issue.
> 
> So what's the root cause of the issue. Here is detailed explanation.
> 
> generic/080 test does mmap write to a file, closes the file and then
> checks if mtime has been updated or not. When file is closed, it
> leads to flushing of dirty pages (and that should update mtime/ctime
> on server). But we did not explicitly invalidate attrs after writeback
> finished. Still generic/080 passed so far and reason being that we
> invalidated atime in fuse_readpages_end(). This is called in fuse_readahead()
> path and always seems to trigger before mmaped write.
> 
> So after mmaped write when lstat() is called, it sees that atleast one
> of the fields being asked for is invalid (atime) and that results in
> generating GETATTR to server and mtime/ctime also get updated and test
> passes.
> 
> But newer /usr/bin/stat seems to have moved to using statx() syscall now
> (instead of using lstat()). And statx() allows it to query only ctime
> or mtime (and not rest of the basic stat fields). That means when
> querying for mtime, fuse_update_get_attr() sees that mtime is not
> invalid (only atime is invalid). So it does not generate a new GETATTR
> and fill stat with cached mtime/ctime. And that means updated mtime
> is not seen by xfstest and tests start failing.
> 
> Invalidating attrs after writeback completion should solve this problem
> in a generic manner.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  fs/fuse/file.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 8cccecb55fb8..482281bf170a 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1759,8 +1759,17 @@ static void fuse_writepage_end(struct fuse_mount *fm, struct fuse_args *args,
>  		container_of(args, typeof(*wpa), ia.ap.args);
>  	struct inode *inode = wpa->inode;
>  	struct fuse_inode *fi = get_fuse_inode(inode);
> +	struct fuse_conn *fc = get_fuse_conn(inode);
>  
>  	mapping_set_error(inode->i_mapping, error);
> +	/*
> +	 * A writeback finished and this might have updated mtime/ctime on
> +	 * server making local mtime/ctime stale. Hence invalidate attrs.
> +	 * Do this only if writeback_cache is not enabled. If writeback_cache
> +	 * is enabled, we trust local ctime/mtime.
> +	 */
> +	if (!fc->writeback_cache)
> +		fuse_invalidate_attr(inode);
>  	spin_lock(&fi->lock);
>  	rb_erase(&wpa->writepages_entry, &fi->writepages);
>  	while (wpa->next) {
> -- 
> 2.25.4
> 
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