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

[Ming Lei]

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.

> 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, :-)

Thanks,
Ming Lei
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