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t=1771411594; bh=7XhNNSvmExkbDNTO5Fje9LxDjaIAJ+98Vm+0ntU8Uic=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=IG0GMRCf3UsdcF1GtjK7G6hfwJr58csKlNTW1jawJzOIXYSAFxkwn9maOi3PXV4CQ il+QHs1pWFt/17uApeDdE13QCpOsQT5jA3CjyRXl4Tl9D3wnefC95j+m0ERcsHEX79 GAdY8LDp1untUC5GhrkN8+OHtvFscC36X+wX0hHqPqaqaVtefM+d4BdnThvk8uN9xc 4i2vqNPGn8uhqalR3U4f1CzJegHZ5UVn+99cP+Ba9MmRvX3Xl0mOZ0n744PcXTR5Zv r9TWyIVStgNJf6Wi5iFq92CHz1qBPTAlTvxIIvMiUCFddSkKCCrYOdH0L/rmD3BuNs C4Fte9qINIUlg== Message-ID: <340be2bc-cf9b-4e22-b557-dfde6efa9de8@kernel.org> Date: Wed, 18 Feb 2026 11:46:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [REGRESSION] mm/mprotect: 2x+ slowdown for >=400KiB regions since PTE batching (cac1db8c3aad) To: Dev Jain , Pedro Falcato Cc: Luke Yang , surenb@google.com, jhladky@redhat.com, akpm@linux-foundation.org, Liam.Howlett@oracle.com, willy@infradead.org, vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <764792ea-6029-41d8-b079-5297ca62505a@kernel.org> <71fbee21-f1b4-4202-a790-5076850d8d00@arm.com> <8315cbde-389c-40c5-ac72-92074625489a@arm.com> <5dso4ctke4baz7hky62zyfdzyg27tcikdbg5ecnrqmnluvmxzo@sciiqgatpqqv> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 2AF8440002 X-Stat-Signature: rb3pw43ongbmuc8dto757yq67j11yn13 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1771411595-243722 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2/18/26 11:38, Dev Jain wrote: > > On 18/02/26 3:36 pm, Pedro Falcato wrote: >> On Wed, Feb 18, 2026 at 10:31:19AM +0530, Dev Jain wrote: >>> Thanks for working on this. Some comments - >>> >>> 1. Rejecting batching with pte_batch_hint() means that we also don't batch 16K and 32K large >>> folios on arm64, since the cont bit is on starting only at 64K. Not sure how imp this is. >> I don't understand what you mean. Is ARM64 doing large folio optimization, >> even when there's no special MMU support for it (the aforementioned 16K and >> 32K cases)? If so, perhaps it's time for a ARCH_SUPPORTS_PTE_BATCHING flag. >> Though if you could provide numbers in that case it would be much appreciated. > > There are two things at play here: > > 1. All arches are expected to benefit from pte batching on large folios, because > of doing similar operations together in one shot. For code paths except mprotect > and mremap, that benefit is far more clear due to: > > a) batching across atomic operations etc. For example, see copy_present_ptes -> folio_ref_add. > Instead of bumping the reference by 1 nr times, we bump it by nr in one shot. > > b) vm_normal_folio was already being invoked. So, all in all the only new overhead > we introduce is of folio_pte_batch(_flags). In fact, since we already have the > folio, I recall that we even just special case the large folio case, out from > the small folio case. Thus 4K folio processing will have no overhead. > > 2. Due to the requirements of contpte, ptep_get() on arm64 needs to fetch a/d bits > across a cont block. Thus, for each ptep_get, it does 16 pte accesses. To avoid this, > it becomes critical to batch on arm64. > > >> >>> 2. Did you measure if there is an optimization due to just the first commit ("prefetch the next pte")? >> Yes, I could measure a sizeable improvement (perhaps some 5%). I tested on >> zen5 (which is a pretty beefy uarch) and the loop is so full of ~~crap~~ >> features that the prefetcher seems to be doing a poor job, at least per my >> results. > > Nice. > >> >>> I actually had prefetch in mind - is it possible to do some kind of prefetch(pfn_to_page(pte_pfn(pte))) >>> to optimize the call to vm_normal_folio()? >> Certainly possible, but I suspect it doesn't make too much sense. You want to >> avoid bringing in the cacheline if possible. In the pte's case, I know we're >> probably going to look at it and modify it, and if I'm wrong it's just one >> cacheline we misprefetched (though I had some parallel convos and it might >> be that we need a branch there to avoid prefetching out of the PTE table). >> We would like to avoid bringing in the folio cacheline at all, even if we >> don't stall through some fancy prefetching or sheer CPU magic. > > I dunno, need other opinions. Let's repeat my question: what, besides the micro-benchmark in some cases with all small-folios, are we trying to optimize here. No hand waving (Androids does this or that) please. -- Cheers, David