Date:   Tue, 21 Nov 2017 11:34:58 +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>
Subject: Re: [dm-devel] [PATCH 00/13] block: assorted cleanup for bio   splitting and cloning.

On Mon, Nov 20 2017, Mike Snitzer wrote:

> On Sun, Jun 18, 2017 at 5:36 PM, NeilBrown <neilb@...e.com> wrote:
>> On Sun, Jun 18 2017, Jens Axboe wrote:
>>
>>> On Sun, Jun 18 2017, NeilBrown wrote:
>>>> This is a resend of my series of patches working
>>>> towards removing the bioset work queues.
>>>>
>>>> This set is based on for-4.13/block.
>>>>
>>>> It incorporates the revised versions of all the patches that were
>>>> resent following feedback on the last set.
>>>>
>>>> It also includes a minor grammatic improvement to a comment, and
>>>> simple changes to compensate for a couple of changes to the block tree
>>>> since the last posting.
>>>>
>>>> I hope to eventually get rid of the new BIOSET_NEED_RESCUER flag,
>>>> but that needs work in dm and probably bcache first.
>>>
>>> Thanks Neil, applied.
>>
>> Thanks a lot Jens.
>
> I missed this line of work until now.  Not quite sure why I wasn't
> cc'd or my review/ack required for the DM changes in this patchset.

Hi Mike,
 I'm sorry you weren't included on those.  My thinking at the time was
 probably that they were purely cosmetic changes which made no
 functional difference to dm.  That is no excuse though and I do
 apologize.

>
> 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.

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.

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.

>
> As is, it is my understanding that DM has no need for a bio_set's
> rescue_workqueue.  So its removal would appear to only be gated by
> bcache?
>
> But I could be mistaken, moving forward please let me know what you
> feel needs doing in DM to make it a better citizen.

I think you are mistaken.
Please see
   https://www.redhat.com/archives/dm-devel/2017-August/msg00310.html
and
   https://www.redhat.com/archives/dm-devel/2017-August/msg00315.html
for which the thread continues:
   https://www.redhat.com/archives/dm-devel/2017-September/msg00001.html

These were sent to you, though most of the conversation happened with
Mikulas.

I think that the patches in those threads explain why dm currently needs
rescuer threads, and shows how dm can be changed to no longer need the
rescuer.  I would appreciate your thoughts on these patches.  I can
resend them if that would help.

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 :-)

Thanks,
NeilBrown


>
> Thanks,
> Mike
>
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