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[2003:d8:2f35:8a00:42f7:2657:34cc:a51f]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b79c3ac036sm22401606f8f.15.2025.08.06.03.28.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Aug 2025 03:28:56 -0700 (PDT) Message-ID: Date: Wed, 6 Aug 2025 12:28:54 +0200 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: [PATCH v5 6/7] mm: Optimize mprotect() by PTE batching To: Dev Jain , Lorenzo Stoakes Cc: akpm@linux-foundation.org, ryan.roberts@arm.com, willy@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, anshuman.khandual@arm.com, peterx@redhat.com, joey.gouly@arm.com, ioworker0@gmail.com, baohua@kernel.org, kevin.brodsky@arm.com, quic_zhenhuah@quicinc.com, christophe.leroy@csgroup.eu, yangyicong@hisilicon.com, linux-arm-kernel@lists.infradead.org, hughd@google.com, yang@os.amperecomputing.com, ziy@nvidia.com References: <20250718090244.21092-1-dev.jain@arm.com> <20250718090244.21092-7-dev.jain@arm.com> <7567c594-7588-49e0-8b09-2a591181b24d@redhat.com> <9552ddff-d778-4934-9349-37c7237cbb78@lucifer.local> <54ee1971-d91a-4fe6-90b8-16212c8568d1@redhat.com> <0d52d680-f3d3-454f-8c12-602f650469ab@arm.com> <9cbe9c6a-0013-4239-9347-bf5d43021fe3@lucifer.local> <2d0d58a4-bd27-41ac-9b25-1cd989c02383@arm.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 06.08.25 12:20, Dev Jain wrote: > > On 06/08/25 3:41 pm, David Hildenbrand wrote: >> On 06.08.25 11:50, Lorenzo Stoakes wrote: >>> On Wed, Aug 06, 2025 at 03:07:49PM +0530, Dev Jain wrote: >>>>>> >>>>>> You mean in _this_ PTE of the batch right? As we're invoking these >>>>>> on each part >>>>>> of the PTE table. >>>>>> >>>>>> I mean I guess we can simply do: >>>>>> >>>>>>      struct page *first_page = pte_page(ptent); >>>>>> >>>>>> Right? >>>>> >>>>> Yes, but we should forward the result from vm_normal_page(), which >>>>> does >>>>> exactly that for you, and increment the page accordingly as required, >>>>> just like with the pte we are processing. >>>> >>>> Makes sense, so I guess I will have to change the signature of >>>> prot_numa_skip() >>>> >>>> to pass a double ptr to a page instead of folio and derive the folio >>>> in the >>>> caller, >>>> >>>> and pass down both the folio and the page to >>>> set_write_prot_commit_flush_ptes. >>> >>> I already don't love how we psas the folio back from there for very >>> dubious >>> benefit. I really hate the idea of having a struct **page parameter... >>> >>> I wonder if we should just have a quick fixup for hotfix, and refine >>> this more >>> later? >> >> This is not an issue in any released kernel, so we can do this properly. >> >> We should just remove that nested vm_normal_folio(). >> >> Untested, but should give an idea what we can do. > > This puts the overhead of vm_normal_folio() unconditionally into the > pte_present path. > > Although I am guessing that is already happening assuming prot_numa case > is not the > > hot path. This is fine by me. So I guess I shouldn't have done that > "reuse the folio > > from prot_numa case if possible" thingy at all :) I mean, it only applies when trying to NUMA-protect something that is already protected. Not sure how relevant that is in practice. As we don't even batch these today, we could just do: diff --git a/mm/mprotect.c b/mm/mprotect.c index 4e0a22f7db495..2154a1a3c6656 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -127,10 +127,6 @@ static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr, bool toptier; int nid; - /* Avoid TLB flush if possible */ - if (pte_protnone(oldpte)) - goto skip; - if (!folio) goto skip; @@ -304,6 +300,9 @@ static long change_pte_range(struct mmu_gather *tlb, struct page *page; pte_t ptent; + if (prot_numa && pte_protnone(oldpte)) + continue; + page = vm_normal_folio(vma, addr, oldpte); if (page) folio = page_folio(page); But with my change, we could actually batch-skip such large folios, because mprotect_folio_pte_batch() would see the folio. -- Cheers, David / dhildenb