Re: [PATCH] io_uring: don't free request to slab

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

On 8/25/21 4:38 PM, Hao Xu wrote:
> 在 2021/8/25 下午9:28, Pavel Begunkov 写道:
>> On 8/25/21 12:40 PM, Hao Xu wrote:
>>> It's not neccessary to free the request back to slab when we fail to
>>> get sqe, just update state->free_reqs pointer.
>>
>> It's a bit hackish because depends on the request being drawn
>> from the array in a particular way. How about returning it
> It seems a req is always allocated from state->reqs, so it should be
> ok? I actually didn't understand 'hackish' here, do you mean
> io_submit_sqes() shouldn't move state->free_reqs which is req caches'
> internal implementation?

I mean it uses implicit knowledge of how io_alloc_req() works, which
may and actually will change. It's just always too easy to forget
about that little one-off thing while changing another chunk.

To give an example, if one decides to remake it and first serve
requests from state->free_list and only when it's empty look into
state->reqs, it's too easy to forget opening a pretty severe
vulnerability.

If there is not much profit from it comparing to the risks, I'd
personally prefer to go with a safer way.  


>> into state->free_list. That thing is as cold as it can get,
>> only buggy apps can hit it.
>>

-- 
Pavel Begunkov