Re: generic/418 regression seen on 5.12-rc3

[Matthew Wilcox <willy@infradead.org>]

On Thu, Mar 18, 2021 at 02:16:13PM -0400, Eric Whitney wrote:
> As mentioned in today's ext4 concall, I've seen generic/418 fail from time to
> time when run on 5.12-rc3 and 5.12-rc1 kernels.  This first occurred when
> running the 1k test case using kvm-xfstests.  I was then able to bisect the
> failure to a patch landed in the -rc1 merge window:
> 
> (bd8a1f3655a7) mm/filemap: support readpage splitting a page

Thanks for letting me know.  This failure is new to me.

I don't understand it; this patch changes the behaviour of buffered reads
from waiting on a page with a refcount held to waiting on a page without
the refcount held, then starting the lookup from scratch once the page
is unlocked.  I find it hard to believe this introduces a /new/ failure.
Either it makes an existing failure easier to hit, or there's a subtle
bug in the retry logic that I'm not seeing.

> Typical test output resulting from a failure looks like:
> 
>      QA output created by 418
>     +cmpbuf: offset 0: Expected: 0x1, got 0x0
>     +[6:0] FAIL - comparison failed, offset 3072
>     +diotest -w -b 512 -n 8 -i 4 failed at loop 0
>      Silence is golden
>     ...
> 
> I've also been able to reproduce the failure on -rc3 in the 4k test case as
> well.  The failure frequency there was 10 out of 100 runs.  It was anywhere
> from 2 to 8 failures out of 100 runs in the 1k case.
> 
> So, the failure isn't dependent upon block size less than page size.

That's a good data point.  I'll take a look at g/418 and see if i can
figure out what race we're hitting.  Nice that it happens so often.
I suppose I could get you to put some debugging in -- maybe dumping the
page if we hit a contended case, then again if we're retrying?

I presume it doesn't always happen at the same offset or anything
convenient like that.