Re: [PATCH v2] io_uring/rsrc: fix RLIMIT_MEMLOCK bypass by removing cross-buffer accounting

[Jens Axboe <axboe@kernel.dk>]

On 1/20/26 5:05 AM, Pavel Begunkov wrote:
> On 1/20/26 07:05, Yuhao Jiang wrote:
>> Hi Jens,
>>
>> On Mon, Jan 19, 2026 at 5:40 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 1/19/26 4:34 PM, Yuhao Jiang wrote:
>>>> On Mon, Jan 19, 2026 at 11:03 AM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>
>>>>> On 1/19/26 12:10 AM, Yuhao Jiang wrote:
>>>>>> The trade-off is that memory accounting may be overestimated when
>>>>>> multiple buffers share compound pages, but this is safe and prevents
>>>>>> the security issue.
>>>>>
>>>>> I'd be worried that this would break existing setups. We obviously need
>>>>> to get the unmap accounting correct, but in terms of practicality, any
>>>>> user of registered buffers will have had to bump distro limits manually
>>>>> anyway, and in that case it's usually just set very high. Otherwise
>>>>> there's very little you can do with it.
>>>>>
>>>>> How about something else entirely - just track the accounted pages on
>>>>> the side. If we ref those, then we can ensure that if a huge page is
>>>>> accounted, it's only unaccounted when all existing "users" of it have
>>>>> gone away. That means if you drop parts of it, it'll remain accounted.
>>>>>
>>>>> Something totally untested like the below... Yes it's not a trivial
>>>>> amount of code, but it is actually fairly trivial code.
>>>>
>>>> Thanks, this approach makes sense. I'll send a v3 based on this.
>>>
>>> Great, thanks! I think the key is tracking this on the side, and then
>>> a ref to tell when it's safe to unaccount it. The rest is just
>>> implementation details.
>>>
>>> -- 
>>> Jens Axboe
>>>
>>
>> I've been implementing the xarray-based ref tracking approach for v3.
>> While working on it, I discovered an issue with buffer cloning.
>>
>> If ctx1 has two buffers sharing a huge page, ctx1->hpage_acct[page] = 2.
>> Clone to ctx2, now both have a refcount of 2. On cleanup both hit zero
>> and unaccount, so we double-unaccount and user->locked_vm goes negative.
>>
>> The per-context xarray can't coordinate across clones - each context
>> tracks its own refcount independently. I think we either need a global
>> xarray (shared across all contexts), or just go back to v2. What do
>> you think?
> 
> The Jens' diff is functionally equivalent to your v1 and has
> exactly same problems. Global tracking won't work well.

Why not? My thinking was that we just use xa_lock() for this, with
a global xarray. It's not like register+unregister is a high frequency
thing. And if they are, then we've got much bigger problems than the
single lock as the runtime complexity isn't ideal.

-- 
Jens Axboe