Date:   Wed, 22 Nov 2017 06:44:07 +1100
From:   NeilBrown <neilb@...e.com>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        device-mapper development <dm-devel@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Zdenek Kabelac <zkabelac@...hat.com>
Subject: Re: [dm-devel] [PATCH 00/13] block: assorted cleanup for bio   splitting and cloning.

On Tue, Nov 21 2017, Mike Snitzer wrote:

> On Mon, Nov 20 2017 at  8:35pm -0500,
> Mike Snitzer <snitzer@...hat.com> wrote:
>
>> On Mon, Nov 20 2017 at  7:34pm -0500,
>> NeilBrown <neilb@...e.com> wrote:
>> 
>> > On Mon, Nov 20 2017, Mike Snitzer wrote:
>> > 
>> > >
>> > > But I've now queued this patch for once Linus gets back (reverts DM
>> > > changes from commit 47e0fb461f):
>> > > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c9fdc42ba23eabd1ba7aef199fb9bb4b4fe5c545
>> > 
>> > This patch does two things.
>> > 1/ It removes the BIOSET_NEED_RESCUER flag from biosets created by dm.
>> >   This a functional changed over the code from before my patches.
>> >   Previously, all biosets were given a rescuer thread.
>> >   After my patch set, biosets only got a rescuer thread if
>> >   BIOSET_NEED_RESCUER was passed, and it was passed for all biosets.
>> >   I then removed it from places were I was certain it wasn't needed.
>> >   I didn't remove it from dm because I wasn't certain.  Your
>> >   patch does remove the flags, which I think is incorrect - see below.
>
> Yeap, definitely was incorrect.  I've dropped the patch.
>
>> > 2/ It changes flush_current_bio_list() so that bios allocated from a
>> >    bioset that does not have a rescue_workqueue are now added to
>> >    the ->rescue_list for their bio_set, and ->rescue_work is queued
>> >    on the NULL ->rescue_workqueue, resulting in a NULL dereference.
>> >    I suspect you don't want this.
>
> Yes, I see that now.
>
>> > The patch description claims that the patch fixes something, but it
>> > isn't clear to me what it is meant to be fixing.
>> > 
>> > It makes reference to  dbba42d8 which is described as removing an unused
>> > bioset process, though what it actually does is remove an used bioset
>> > (and obvious the process disappears with it).  My patch doesn't change
>> > that behavior.
>> 
>> Well I looked at this because Zdenek reported that with more recent
>> kernels he is seeing the "bioset" per DM device again (whereas it was
>> thought to be removed with mikulas' commit dbba42d8 -- but that commit
>> removed "bioset" only in terms of q->bio_split.
>
> I think Zdenek triggered a false-positive that DM had magically sprouted
> a new "bioset" rescue_workqueue.  Reality is I cannot see how each
> bio-based DM device can avoid having one.  And the commit d67a5f4b59
> ("dm: flush queued bios when process blocks to avoid deadlock") I
> referenced earlier very much makes DM depend on it even more.
>
> So apologies for being so off-base (by looking to prematurely revert
> DM's use of BIOSET_NEED_RESCUER, etc).
>
>> > Please see
>> >    https://www.redhat.com/archives/dm-devel/2017-August/msg00310.html
>
> I'll very likely pick these up for 4.16 shortly.  But hope to work
> through complete removal of DM's use of BIOSET_NEED_RESCUER for 4.16 as
> well.
>
>> > and
>> >    https://www.redhat.com/archives/dm-devel/2017-August/msg00315.html
>
> This one [1] needs a lot of review and testing.  Particularly against this
> test case that Mikulas created to reproduce the snapshot deadlock (same
> deadlock that motivated commit dbba42d8):
> https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html

Thanks for that link.  I'll try to make time to experiment with the test
code and confirm my proposed approach doesn't break it.

>
>> > for which the thread continues:
>> >    https://www.redhat.com/archives/dm-devel/2017-September/msg00001.html
>
> Wish I could clone myself (or Kent, the world needs 2 Kents!) and pursue
> this: https://www.redhat.com/archives/dm-devel/2014-May/msg00100.html

In that email Kent mentions "punt off to a per request_queue workqueue".

That "per request_queue workqueue" is what I'm trying to get rid of.  I
don't think this is a good direction.

>
> Short of that, how would you like to proceed?

I'd like to confirm that my approach
1/ doesn't re-introduce a deadlock
2/ doesn't hurt performance
and then merge it.

Though to be honest, I don't recall exactly what "my approach" is.
Your next email picks out two important patches which probably cover
it.  If/when I get to do the testing I'll let you know how it goes.

Thanks,
NeilBrown


>
>> > That would then just leave bcache....  I find it a bit of a challenge to
>> > reason about the code in bcache, but if we can remove
>> > BIOSET_NEED_RESCUER from dm, that will be an extra incentive for me to learn :-)
>> 
>> I'm all for properly removing BIOSET_NEED_RESCUER from DM.
>
> Should we work to make [1] (above) sure it fixes Mikulas' test case?
>
> I'll set in on reviewing and playing with [1] now.
>
> Thanks,
> Mike
>
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