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[2003:d8:2f35:8a00:42f7:2657:34cc:a51f]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-459dbba5210sm109716695e9.2.2025.08.06.02.21.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Aug 2025 02:21:49 -0700 (PDT) Message-ID: <54ee1971-d91a-4fe6-90b8-16212c8568d1@redhat.com> Date: Wed, 6 Aug 2025 11:21:47 +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: Lorenzo Stoakes Cc: Dev Jain , 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.08.25 11:12, Lorenzo Stoakes wrote: > On Wed, Aug 06, 2025 at 10:08:33AM +0200, David Hildenbrand wrote: >> On 18.07.25 11:02, Dev Jain wrote: >>> Signed-off-by: Dev Jain >> >> >> I wanted to review this, but looks like it's already upstream and I suspect >> it's buggy (see the upstream report I cc'ed you on) >> >> [...] >> >>> + >>> +/* >>> + * This function is a result of trying our very best to retain the >>> + * "avoid the write-fault handler" optimization. In can_change_pte_writable(), >>> + * if the vma is a private vma, and we cannot determine whether to change >>> + * the pte to writable just from the vma and the pte, we then need to look >>> + * at the actual page pointed to by the pte. Unfortunately, if we have a >>> + * batch of ptes pointing to consecutive pages of the same anon large folio, >>> + * the anon-exclusivity (or the negation) of the first page does not guarantee >>> + * the anon-exclusivity (or the negation) of the other pages corresponding to >>> + * the pte batch; hence in this case it is incorrect to decide to change or >>> + * not change the ptes to writable just by using information from the first >>> + * pte of the batch. Therefore, we must individually check all pages and >>> + * retrieve sub-batches. >>> + */ >>> +static void commit_anon_folio_batch(struct vm_area_struct *vma, >>> + struct folio *folio, unsigned long addr, pte_t *ptep, >>> + pte_t oldpte, pte_t ptent, int nr_ptes, struct mmu_gather *tlb) >>> +{ >>> + struct page *first_page = folio_page(folio, 0); >> >> Who says that we have the first page of the folio mapped into the first PTE >> of the batch? > > Yikes, missed this sorry. Got too tied up in alogrithm here. > > 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. ... >> >>> + else >>> + prot_commit_flush_ptes(vma, addr, pte, oldpte, ptent, >>> + nr_ptes, /* idx = */ 0, /* set_write = */ false, tlb); >> >> Semi-broken intendation. > > Because of else then 2 lines after? prot_commit_flush_ptes(vma, addr, pte, oldpte, ptent, nr_ptes, /* idx = */ 0, /* set_write = */ false, tlb); Is what I would have expected. I think a smart man once said, that if you need more than one line per statement in an if/else clause, a set of {} can aid readability. But I don't particularly care :) -- Cheers, David / dhildenb