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References: <20190314111637.20236-1-hans@owltronix.com> <103c19f4-02a2-f54c-13f8-30bc5447426e@intel.com>
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From:   Heiner Litz <hlitz@ucsc.edu>
Date:   Thu, 14 Mar 2019 09:14:58 -0700
Message-ID: <CAJbgVnU2RXK4ci79n0szwTpZ+UB2zx977PHo1ie7h5y5kJrbhQ@mail.gmail.com>
Subject: Re: [PATCH RFC] lightnvm: pblk: fix crash in pblk_end_partial_read
 due to multipage bvecs
To:     =?UTF-8?Q?Javier_Gonz=C3=A1lez?= <javier@javigon.com>
Cc:     "Konopko, Igor J" <igor.j.konopko@intel.com>,
        Hans Holmberg <hans@owltronix.com>,
        =?UTF-8?Q?Matias_Bj=C3=B8rling?= <mb@lightnvm.io>,
        Klaus Birkelund Jensen <klaus.jensen@cnexlabs.com>,
        linux-block@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hans Holmberg <hans.holmberg@cnexlabs.com>
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On Thu, Mar 14, 2019 at 7:16 AM Javier Gonz=C3=A1lez <javier@javigon.com> w=
rote:
>
> > On 14 Mar 2019, at 06.49, Igor Konopko <igor.j.konopko@intel.com> wrote=
:
> >
> > While reading this patch, idea came to my mind - maybe it would be simp=
ly better to get rid of partial read handling from pblk in current form at =
all and use bio_split() & bio_chain() combination instead?
> >
> > This would definitely reduce a number of this 'kind of complicated' cod=
e inside pblk and let block layer help us with that. Even that the performa=
nce of such a requests could be a little worse (few smaller IOs instead of =
single vector IO), I believe that partial read is more a corner case, then =
a typical use case, so maybe this would be a path to go.
> >
> > Let me know what you think about such an approach - I can make a patch =
with that if you want.

I like this idea as it will clean up a lot of code and get rid of the
read_bitmap.
Note that a single request can be turned into up to 32 requests if
cached and non-cached sectors alternate, each one probably requiring
an l2p lookup. I still think it's worth doing though.
When you split, note that you have to release all line_refs which were
acquired during l2p lookup.

>
> I agree with Igor.
>
> As I mentioned offline, we should fix this in a way that survives
> further changes in struct bio; either making pblk handling visible to
> the outside or rethinking the whole thing.
>
> Igor: If you can send a patch you mention, it would be great. I have
> been trying the helper approach for some time, but it is too specific,
> as fixing holes in the bvec breaks the bio advance-only semantics. Your
> approach seems much better.
>
> Thanks,
> Javier