Re: [PATCH 6/8] io_uring/zcrx: notify user when out of buffers

[Jens Axboe <axboe@kernel.dk>]

On 5/19/26 9:30 AM, Pavel Begunkov wrote:
> On 5/19/26 16:26, Jens Axboe wrote:
>> On 5/19/26 5:44 AM, Pavel Begunkov wrote:
>>> @@ -1126,6 +1142,48 @@ static unsigned io_zcrx_refill_slow(struct page_pool *pp, struct io_zcrx_ifq *if
>>>       return allocated;
>>>   }
>>>   +static void zcrx_notif_tw(struct io_tw_req tw_req, io_tw_token_t tw)
>>> +{
>>> +    struct io_kiocb *req = tw_req.req;
>>> +    struct io_ring_ctx *ctx = req->ctx;
>>> +
>>> +    io_post_aux_cqe(ctx, req->cqe.user_data, req->cqe.res, 0);
>>> +    percpu_ref_put(&ctx->refs);
>>> +    io_poison_req(req);
>>> +    kmem_cache_free(req_cachep, req);
>>> +}
>>> +
>>> +static void zcrx_send_notif(struct io_zcrx_ifq *ifq, unsigned type)
>>> +{
>>> +    gfp_t gfp = GFP_ATOMIC | __GFP_NOWARN | __GFP_ZERO;
>>> +    u32 type_mask = 1 << type;
>>> +    struct io_kiocb *req;
>>> +
>>> +    if (!(type_mask & ifq->allowed_notif_mask))
>>> +        return;
>>> +
>>> +    guard(spinlock_bh)(&ifq->ctx_lock);
>>> +    if (!ifq->master_ctx)
>>> +        return;
>>> +    if (type_mask & ifq->fired_notifs)
>>> +        return;
>>> +
>>> +    req = kmem_cache_alloc(req_cachep, gfp);
>>> +    if (unlikely(!req))
>>> +        return;
>>
>> It'd be nice to avoid an allocation here inside ctx_lock and with bh's
>> disabled, which looks like is also the only reason why GFP_ATOMIC is
>> being used here.
> 
> I thought about it, but it's already bh, it'd need to do pre
> allocations + caching to be reliable, but that's left out for now.

Not sure I follow - GFP_KERNEL would be more reliable than GFP_ATOMIC.
What's the contract in terms of the notification? If we fail the alloc,
then userspace can't rely on the notification on the refill failure.

Are we under bh save already here, before doing it ourselves? If so,
then how does the guard work?

>> Maybe opportunistically check ->fired_notifs early? Might also avoid the
>> lock in the first place if we get back-to-back of these.
> 
> Slow path, doesn't matter

Agree, not a huge deal as we hope to not hit the notif path.

-- 
Jens Axboe