Re: Re: [TEST PATCH] xen/blkfront: use blk_rq_map_sg togenerate ring entries

[Jeremy Fitzhardinge <jeremy@goop.org>]

Jan Beulich wrote:
> But isn't this just reducing the likelihood of hitting the problem? Without
> understanding why the entry limit gets exceeded (and fixing that), the
> potential for this to happen again is still there.
>   

I had the same thought, but Jens says it fixes the real problem, so that 
satisfies me ;)

Greg reported that simply decreasing the max segments to one less than 
the ring size was sufficient to clear up the symptoms, which indicates 
it was never submitting more than one extra segment.  Jens writes:
> The second problem is
> that the block layer then appears to create one too many segments, but
> from the dump it has rq->nr_phys_segments ==
> BLKIF_MAX_SEGMENTS_PER_REQUEST. I suspect the latter is due to
> xen-blkfront not handling the merging on its own. It should check that
> the new page doesn't form part of the previous page.
which suggests that blkfront was omitting a merging check that driver 
should perform, results in this problem.  I don't really understand why 
the upper layers can't do this merge check, but I'm sure Jens has a good 
reason.

> Of course, the change made here has its own benefits, so I fully agree
> it should be applied (though I wonder whether it's indeed a -stable
> candidate).

Well, it appears to be the proper fix to a real bug observed in the 
field, which makes it an ideal -stable candidate.

    J