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

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Message-ID: <32b884bc-929b-4b27-ae74-5754fa2473de@kernel.dk>
Date: Sat, 24 Jan 2026 08:55:43 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Pavel Begunkov <asml.silence@...il.com>,
 Yuhao Jiang <danisjiang@...il.com>
Cc: io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v2] io_uring/rsrc: fix RLIMIT_MEMLOCK bypass by removing
 cross-buffer accounting

On 1/24/26 8:14 AM, Jens Axboe wrote:
>>> ________________________________________________________
>>> Executed in    2.81 secs    fish           external
>>>     usr time    0.71 secs  497.00 micros    0.71 secs
>>>     sys time   19.57 secs  183.00 micros   19.57 secs
>>>
>>> which isn't insane. Obviously also needs conditional rescheduling in the
>>> page loops, as those can take a loooong time for large amounts of
>>> memory.
>>
>> 2.8 sec sounds like a lot as well, makes me wonder which part of
>> that is mm, but it mm should scale fine-ish. Surely there will be
>> contention on page refcounts but at least the table walk is
>> lockless in the best case scenario and otherwise seems to be read
>> protected by an rw lock.
> 
> Well a lot of that is also just faulting in the memory on clear, test
> case should probably be modified to do its own timing. And iterating
> page arrays is a huge part of it too. There's no real contention in that
> 2.8 seconds.

I checked and the faulting part is 2.0s of that runtime. On a re-run:

axboe@...25 ~> time ./ppage 32 32
register 32 GB, num threads 32
clear msec 2011

________________________________________________________
Executed in    3.13 secs    fish           external
   usr time    0.78 secs  193.00 micros    0.78 secs
   sys time   27.46 secs  271.00 micros   27.46 secs

Or just a single thread:

axboe@...25 ~> time ./ppage 32 1
register 32 GB, num threads 1
clear msec 2081

________________________________________________________
Executed in    2.29 secs    fish           external
   usr time    0.58 secs  750.00 micros    0.58 secs
   sys time    1.71 secs    0.00 micros    1.71 secs

axboe@...25 ~ [1]> time ./ppage 64 1
register 64 GB, num threads 1
clear msec 5380

________________________________________________________
Executed in    6.24 secs    fish           external
   usr time    1.42 secs  328.00 micros    1.42 secs
   sys time    4.82 secs  375.00 micros    4.82 secs

-- 
Jens Axboe