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Date: Tue, 19 Sep 2017 07:53:29 +1000 From: Dave Chinner <david@...morbit.com> To: Jens Axboe <axboe@...nel.dk> Cc: Al Viro <viro@...IV.linux.org.uk>, Christoph Hellwig <hch@....de>, Abdul Haleem <abdhalee@...ux.vnet.ibm.com>, linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>, linux-xfs <linux-xfs@...r.kernel.org>, linux-next <linux-next@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>, chandan <chandan@...ux.vnet.ibm.com> Subject: Re: [linux-next][XFS][trinity] WARNING: CPU: 32 PID: 31369 at fs/iomap.c:993 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@...morbit.com
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