Re: [PATCH v2] block: flush queued bios when the process blocks

[Ming Lei]

On Fri, Oct 16, 2015 at 11:29 PM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Thu, Oct 15 2015 at 11:08pm -0400,
> Ming Lei <tom.leiming@xxxxxxxxx> wrote:
>
>> On Thu, Oct 15, 2015 at 4:06 PM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
>> > On Wed, Oct 14 2015 at 11:27pm -0400,
>> > Ming Lei <tom.leiming@xxxxxxxxx> wrote:
>> >
>> >> On Sat, Oct 10, 2015 at 3:52 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
>> >> >
>> >> > Turns out that this change:
>> >> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=2639638c77768a86216be456c2764e32a2bcd841
>> >> >
>> >> > needed to be reverted with:
>> >> > http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=ad3ccd760da7c05b90775372f9b39dc2964086fe
>> >> >
>> >> > Because nested plugs caused generic_make_request()'s onstack bio_list to
>> >> > go out of scope (blk_finish_plug() wouldn't actually flush the list
>> >> > within generic_make_request because XFS already added an outermost
>> >> > plug).
>> >>
>> >> Looks you should have defined bio_list in plug as
>> >>
>> >>                'struct bio_list bio_list'
>> >>
>> >> instead of one pointer.
>> >
>> > I realized that and fixed it (see commit ad3ccd760da7c05b90 referenced
>> > above that does exactly that).  That wasn't the problem.
>>
>> OK.
>>
>> >
>> >> >
>> >> > But even after fixing that I then hit issues with these changes now
>> >> > resulting in imperfect 'in_generic_make_request' accounting that happens
>> >> > lazily once the outermost plug completes blk_finish_plug.  manifested as
>> >> > dm-bufio.c:dm_bufio_prefetch's BUG_ON(dm_bufio_in_request()); hitting.
>> >>
>> >> Looks this problem should be related with above 'bio_list' definition too.
>> >
>> > No, as I explained it was due to the nested plug:
>> >
>> >> >
>> >> > Basically using the blk-core's onstack plugging isn't workable for
>> >> > fixing this deadlock and we're back to having to seriously consider
>> >> > this (with its additional hook in the scheduler)
>> >
>> > To elaborate, for the code in DM (and other subsystems like bcache) that
>> > rely on accurate accounting of whether we're actively _in_
>> > generic_make_request: using plug to store/manage the bio_list isn't
>>
>> That looks an interesting requirement, which means DM just need to know
>> if the current callsite is from generic_make_request(), so what you need
>> is just one per-task variable.
>>
>> With the stack variable of 'plug', it should be easier to do that for DM, for
>> example, you can introduce one flag in 'struct blk_plug', then set it in
>> the entry of generic_make_request(), and clear it in the exit of the
>> function.
>
> Yes, I mean we _could_ set/clear the 'in_generic_make_request' flag _in_
> generic_make_request() but then it just calls into question why the heck
> we're using the plug to begin with? (especially given plugging is for
> request-based devices at this point!).
>
> It really doesn't make _any_ sense to overload blk_plug by moving the
> bio_list into there and adding a 'in_generic_make_request'... when you
> consider the _only_ reason this was suggested is to (ab)use the existing
> hook in scheduler/core.c.

At the first glance, I mean it is doable to use blk_plug for the issue.

>From last year's discussion, looks Jens thought we have plug already which
should have covered this case, also Kent wanted to implement
plug for bio too.

>
> So I stand by my position that there is really no point in the exercise
> and that it actually hurts the code to try to make this a blk_plug
> "feature".
>
> We already have well established current->bio_list semantics that can be
> reused as a flag given it is a pointer.  The block callout in the
> scheduler is going to grow a conditional either way.  What I've proposed
> _seems_ the cleanest to me and others.  Hopefully you can see that
> aspect of things.
>
> So if you could review the v3 patch with a critical eye that'd be very
> much appreciated.

Will do.

>
> But I do look forward to Jens also having a look at this and providing
> his review feedback.
>
>> > workable because nested plugs change the lifetime of when the bio_list
>> > is processed (as I implemented it -- which was to respect nested plugs).
>> > I could've forced the issue by making the bio_list get processed
>> > regardless of nesting but that would've made the onstack plugging much
>> > more convoluted (duality between nested vs not just for bio_list's
>> > benefit and for what gain?  Simply to avoid an extra conditional
>> > immediately in the scheduler?  That conditional was still added anyway
>> > but just as part of blk_needs_flush_plug so in the end there wasn't any
>> > benefit!).
>> >
>> > Hopefully my middle-of-the-night reply is coherent and helped to clarify
>> > my position that (ab)using blk_plug for the bio_list management is
>> > _really_ awkward. ;)
>>
>> Hope it wan't my reply to cause the break of your sleep, :-)
>
> No, my dog woke me up to go outside at 4am.. I was up and couldn't
> resist looking at my phone.. the rest is history ;)



-- 
Ming Lei
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