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

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

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.

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).

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.

-- 
Pavel Begunkov