Re: [PATCH for-6.1 0/2] iopoll bio pcpu cache fix

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/28/22 00:55, Jens Axboe wrote:
> On 10/27/22 5:27 PM, Jens Axboe wrote:
>> On 10/27/22 4:14 PM, Pavel Begunkov wrote:
>>> There are ways to deprive bioset mempool of requests using pcpu caches
>>> and never return them back, which breaks forward progress guarantees bioset
>>> tried to provide. Fix it.
>>>
>>> Pavel Begunkov (2):
>>>    mempool: introduce mempool_is_saturated
>>>    bio: don't rob bios from starving bioset
>>>
>>>   block/bio.c             | 2 ++
>>>   include/linux/mempool.h | 5 +++++
>>>   2 files changed, 7 insertions(+)
>>>
>>
>> This isn't really a concern for 6.1 and earlier, because the caching is
>> just for polled IO. Polled IO will not be grabbing any of the reserved
>> inflight units on the mempool side, which is what guarantees the forward
>> progress.

And after looking it up, apparently it can allocate from reserves.


>> It'll be a concern for the 6.2 irq based general caching, so it would
>> need to be handled there. So perhaps this can be a pre-series for a
>> reposting of that patchset.
> 
> Just a followup, since we had some out-of-band discussion. This is
> a potential concern on the bio side, though not on the request side.
> But this approach is racy, we'll figure something else out.

I agree that it _may_ be, but I can't think of an counter example
for arches w/ strong ordering and would need to think more if there
is an issue in the looser ephemeral world.

-- 
Pavel Begunkov