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

[Miklos Szeredi]

On Tue, Apr 6, 2021 at 4:07 PM Vivek Goyal <vgoyal at redhat.com> 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 at redhat.com>

Thanks, applied.

Miklos