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

[Jens Axboe]

On 1/20/26 2:45 PM, Pavel Begunkov wrote:
> On 1/20/26 17:03, Jens Axboe wrote:
>> On 1/20/26 5:05 AM, Pavel Begunkov wrote:
>>> On 1/20/26 07:05, Yuhao Jiang wrote:
> ...
>>>>
>>>> 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.
> 
> 1. There could be quite a lot of entries even for a single ring
> with realistic amount of memory. If lots of threads start up
> at the same time taking it in a loop, it might become a chocking
> point for large systems. Should be even more spectacular for
> some numa setups.

I already briefly touched on that earlier, for sure not going to be of
any practical concern.

> 2. Most likely it'll further relax accounting (i.e. one way
> road), and I don't believe that's the right thing. Could even
> be unexpected if consolidated w/o any explicit communication
> b/w rings (like buffer cloning).

Well the aim is to make the accounting actually correct.

> 3. Map keys will need to be {page, user, mm}, so I suspect
> impl is not going to be exactly trivial either way. Maybe some
> nested xarrays + something for counting middle layer entries.

Honestly I think the xarray just needs to go into struct user_struct.

-- 
Jens Axboe