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Message-ID: <fc8664bb-7769-48a2-b470-71fb81828e26@kernel.dk> Date: Fri, 23 Jan 2026 07:50:53 -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 1/23/26 7:26 AM, Pavel Begunkov wrote: > On 1/22/26 21:51, Pavel Begunkov wrote: > ... >>>>> I already briefly touched on that earlier, for sure not going to be of >>>>> any practical concern. >>>> >>>> Modest 16 GB can give 1M entries. Assuming 50ns-100ns per entry for the >>>> xarray business, that's 50-100ms. It's all serialised, so multiply by >>>> the number of CPUs/threads, e.g. 10-100, that's 0.5-10s. Account sky >>>> high spinlock contention, and it jumps again, and there can be more >>>> memory / CPUs / numa nodes. Not saying that it's worse than the >>>> current O(n^2), I have a test program that borderline hangs the >>>> system. >>> >>> It's definitely not worse than the existing system, which is why I don't >>> think it's a big deal. Nobody has ever complained about time to register >>> buffers. It's inherently a slow path, and quite slow at that depending >>> on the use case. Out of curiosity, I ran some stilly testing on >>> registering 16GB of memory, with 1..32 threads. Each will do 16GB, so >>> 512GB registered in total for the 32 case. Before is the current kernel, >>> after is with per-user xarray accounting: >>> >>> before >>> >>> nthreads 1: 646 msec >>> nthreads 2: 888 msec >>> nthreads 4: 864 msec >>> nthreads 8: 1450 msec >>> nthreads 16: 2890 msec >>> nthreads 32: 4410 msec >>> >>> after >>> >>> nthreads 1: 650 msec >>> nthreads 2: 888 msec >>> nthreads 4: 892 msec >>> nthreads 8: 1270 msec >>> nthreads 16: 2430 msec >>> nthreads 32: 4160 msec >>> >>> This includes both registering buffers, cloning all of them to another >>> ring, and unregistering times, and nowhere is locking scalability an >>> issue for the xarray manipulation. The box has 32 nodes and 512 CPUs. So >>> no, I strongly believe this isn't an issue. >>> >>> IOW, accurate accounting is cheaper than the stuff we have now. None of >>> them are super cheap. Does it matter? I really don't think so, or people >>> would've complained already. The only complaint I got on these kinds of >>> things was for cloning, which did get fixed up some releases ago. >> >> You need compound pages >> >> always > /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled >> >> And use update() instead of register() as accounting dedup for >> registration is broken-disabled. For the current kernel: >> >> Single threaded: >> 1x1G: 7.5s >> 2x1G: 45s >> 4x1G: 190s >> >> 16x should be ~3000s, not going to run it. Uninterruptible and no >> cond_resched, so spawn NR_CPUS threads and the system is completely >> unresponsive (I guess it depends on the preemption mode). > The program is below for reference, but it's trivial. THP setting > is done inside for convenience. There are ways to make the runtime > even worse, but that should be enough. Thanks for sending that. Ran it on the same box, on current -git and with user_struct xarray accounting. Modified it so that 2nd arg is number of threads, for easy running: current -git axboe@...25 ~> cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled [always] inherit madvise never axboe@...25 ~> for i in 1 2 4 8 16; time ./ppage $i $i; end register 1 GB, num threads 1 ________________________________________________________ Executed in 178.91 millis fish external usr time 9.82 millis 313.00 micros 9.51 millis sys time 161.83 millis 149.00 micros 161.68 millis register 2 GB, num threads 2 ________________________________________________________ Executed in 638.49 millis fish external usr time 0.03 secs 285.00 micros 0.03 secs sys time 1.14 secs 135.00 micros 1.14 secs register 4 GB, num threads 4 ________________________________________________________ Executed in 2.17 secs fish external usr time 0.05 secs 314.00 micros 0.05 secs sys time 6.31 secs 150.00 micros 6.31 secs register 8 GB, num threads 8 ________________________________________________________ Executed in 4.97 secs fish external usr time 0.12 secs 299.00 micros 0.12 secs sys time 28.97 secs 142.00 micros 28.97 secs register 16 GB, num threads 16 ________________________________________________________ Executed in 10.34 secs fish external usr time 0.20 secs 294.00 micros 0.20 secs sys time 126.42 secs 140.00 micros 126.42 secs -git + user_struct xarray for accounting axboe@...25 ~> cat /sys/kernel/mm/transparent_hugepage/hugepages-16kB/enabled [always] inherit madvise never axboe@...25 ~> for i in 1 2 4 8 16; time ./ppage $i $i; end register 1 GB, num threads 1 ________________________________________________________ Executed in 54.05 millis fish external usr time 10.66 millis 327.00 micros 10.34 millis sys time 41.60 millis 259.00 micros 41.34 millis register 2 GB, num threads 2 ________________________________________________________ Executed in 105.70 millis fish external usr time 34.38 millis 206.00 micros 34.17 millis sys time 68.55 millis 206.00 micros 68.35 millis register 4 GB, num threads 4 ________________________________________________________ Executed in 214.72 millis fish external usr time 48.10 millis 193.00 micros 47.91 millis sys time 182.25 millis 193.00 micros 182.06 millis register 8 GB, num threads 8 ________________________________________________________ Executed in 441.96 millis fish external usr time 123.26 millis 195.00 micros 123.07 millis sys time 568.20 millis 195.00 micros 568.00 millis register 16 GB, num threads 16 ________________________________________________________ Executed in 917.70 millis fish external usr time 0.17 secs 202.00 micros 0.17 secs sys time 2.48 secs 202.00 micros 2.48 secs -- Jens Axboe
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