From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6AD0B301493 for ; Wed, 18 Feb 2026 10:46:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771411594; cv=none; b=KNUHhfqOf7X0wkwlkK5Uwwo4qFbdUHTB2IHKeGHLSKuP+oudIB0XwoBIBqjH6DP1IslYQjyY/rz2y+pktPaN3bvfOcVpJz2BzYJLphIDGpxmP1bxskVjQOGqCwhOp6W3jeH91LgBjz7Pdb6MWeUsGKdwVaIZMR0N3XW2vIo9UWA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771411594; c=relaxed/simple; bh=7XhNNSvmExkbDNTO5Fje9LxDjaIAJ+98Vm+0ntU8Uic=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZT8IWlxfXvmlZ6WIzlKWcNWnnPPyaPCWXErLRkk8K7hkjZ4ijQzi/U4OszkrdaFHFtoRMObi3KH4PjpSt0PxJRsBehug2b+jU9IbDQDLZQkFQ1FQvt6zT/GSU3FfdDmvlNQNoAe23xibK7ET4dM4mZEEAhGmj0KNcvYoPNow2ao= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IG0GMRCf; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IG0GMRCf" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CB605C19425; Wed, 18 Feb 2026 10:46:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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