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[2003:cb:c706:2700:abf9:4eac:588c:adab]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39eae97b249sm20040789f8f.58.2025.04.17.04.36.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Apr 2025 04:36:42 -0700 (PDT) Message-ID: <4828e28d-eb56-449b-83c3-b5b2dc2ac6e2@redhat.com> Date: Thu, 17 Apr 2025 13:36:41 +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] mm/huge_memory: fix dereferencing invalid pmd migration entry To: Gavin Guo , Hugh Dickins Cc: linux-mm@kvack.org, akpm@linux-foundation.org, willy@infradead.org, ziy@nvidia.com, linmiaohe@huawei.com, revest@google.com, kernel-dev@igalia.com, linux-kernel@vger.kernel.org References: <20250414072737.1698513-1-gavinguo@igalia.com> <27d13454-280f-4966-b694-d7e58d991547@redhat.com> <6787d0ea-a1b9-08cf-1f48-e361058eec20@google.com> <83f17b85-c9fa-43a0-bec1-22c8565b67ad@redhat.com> <98d1d195-7821-4627-b518-83103ade56c0@redhat.com> <7d0ef7b5-043b-beca-72a9-6ae98b0d55fb@google.com> <05a7d51e-f065-445a-af0e-481f3461a76e@redhat.com> Content-Language: en-US From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 17.04.25 13:21, Gavin Guo wrote: > On 4/17/25 17:04, David Hildenbrand wrote: >> On 17.04.25 10:55, Hugh Dickins wrote: >>> On Thu, 17 Apr 2025, David Hildenbrand wrote: >>>> On 17.04.25 09:18, David Hildenbrand wrote: >>>>> On 17.04.25 07:36, Hugh Dickins wrote: >>>>>> On Wed, 16 Apr 2025, David Hildenbrand wrote: >>>>>>> >>>>>>> Why not something like >>>>>>> >>>>>>> struct folio *entry_folio; >>>>>>> >>>>>>> if (folio) { >>>>>>>   if (is_pmd_migration_entry(*pmd)) >>>>>>>       entry_folio = pfn_swap_entry_folio(pmd_to_swp_entry(*pmd))); >>>>>>>   else >>>>>>>    entry_folio = pmd_folio(*pmd)); >>>>>>> >>>>>>>   if (folio != entry_folio) >>>>>>>         return; >>>>>>> } >>>>>> >>>>>> My own preference is to not add unnecessary code: >>>>>> if folio and pmd_migration entry, we're not interested in entry_folio. >>>>>> But yes it could be written in lots of other ways. >>>>> >>>>> While I don't disagree about "not adding unnecessary code" in general, >>>>> in this particular case just looking the folio up properly might be the >>>>> better alternative to reasoning about locking rules with conditional >>>>> input parameters :) >>>>> >>>> >>>> FWIW, I was wondering if we can rework that code, letting the caller >>>> to the >>>> checking and getting rid of the folio parameter. Something like this >>>> (incomplete, just to >>>> discuss if we could move the TTU_SPLIT_HUGE_PMD handling). >>> >>> Yes, I too dislike the folio parameter used for a single case, and agree >>> it's better for the caller who chose pmd to check that *pmd fits the >>> folio. >>> >>> I haven't checked your code below, but it looks like a much better way >>> to proceed, using the page_vma_mapped_walk() to get pmd lock and check; >>> and cutting out two or more layers of split_huge_pmd obscurity. >>> >>> Way to go.  However... what we want right now is a fix that can easily >>> go to stable: the rearrangements here in 6.15-rc mean, I think, that >>> whatever goes into the current tree will have to be placed differently >>> for stable, no seamless backports; but Gavin's patch (reworked if you >>> insist) can be adapted to stable (differently for different releases) >>> more more easily than the future direction you're proposing here. >> >> I'm fine with going with the current patch and looking into cleaning it >> up properly (if possible). >> >> So for this patch >> >> Acked-by: David Hildenbrand >> >> @Gavin, can you look into cleaning that up? > > Thank you for your review. Before I begin the cleanup, could you please > confirm the following action items: > > Zi Yan's suggestions for the patch are: > 1. Replace the page fault with an invalid address access in the commit > description. Yes, that makes sense. > > 2. Simplify the nested if-statements into a single if-statement to > reduce indentation. > > David, based on your comment, I understand that you are recommending the > entry_folio implementation. Also, from your discussion with Hugh, it > appears you agreed with my original approach of returning early when > encountering a PMD migration entry, thereby avoiding unnecessary checks. > Is that correct? If so, I will keep the current logic. Do you have any > additional cleanup suggestions? Yes, the current patch is okay for upstream+stable, but we should look into cleaning that up. See the cleanup RFC patch I posted where we remove the folio check completely. Please let me know if you need more information. -- Cheers, David / dhildenb