Re: [linux-next][XFS][trinity] WARNING: CPU: 32 PID: 31369 at fs/iomap.c:993

[Dave Chinner <david@fromorbit.com>]

On Mon, Sep 18, 2017 at 09:51:29AM -0600, Jens Axboe wrote:
> On 09/18/2017 09:43 AM, Al Viro wrote:
> > On Mon, Sep 18, 2017 at 05:39:47PM +0200, Christoph Hellwig wrote:
> >> On Mon, Sep 18, 2017 at 09:28:55AM -0600, Jens Axboe wrote:
> >>> If it's expected, why don't we kill the WARN_ON_ONCE()? I get it all
> >>> the time running xfstests as well.
> >>
> >> Dave insisted on it to decourage users/applications from mixing
> >> mmap and direct I/O.
> >>
> >> In many ways a tracepoint might be the better way to diagnose these.
> > 
> > sysctl suppressing those two, perhaps?
> 
> I'd rather just make it a trace point, but don't care too much.
> 
> The code doesn't even have a comment as to why that WARN_ON() is
> there or expected.

The big comment about how bad cache invalidation failures are is on
the second, post-io invocation of the page cache flush. That's the
failure that exposes the data coherency problem to userspace:

        /*
         * Try again to invalidate clean pages which might have been cached by
         * non-direct readahead, or faulted in by get_user_pages() if the source
         * of the write was an mmap'ed region of the file we're writing.  Either
         * one is a pretty crazy thing to do, so we don't support it 100%.  If
         * this invalidation fails, tough, the write still worked...
         */
        if (iov_iter_rw(iter) == WRITE) {
                int err = invalidate_inode_pages2_range(mapping,
                                start >> PAGE_SHIFT, end >> PAGE_SHIFT);
                WARN_ON_ONCE(err);
        }

IOWs, the first warning is a "bad things might be about to
happen" warning, the second is "bad things have happened".

> Seems pretty sloppy to me, not a great way
> to "discourage" users to mix mmap/dio.

Again, it has nothing to do with "discouraging users" and everything
about post-bug report problem triage.

Yes, the first invalidation should also have a comment like the post
IO invalidation - the comment probably got dropped and not noticed
when the changeover from internal XFS code to generic iomap code was
made...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com