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b=kmuzlFglRmkvwwjFCnM+UN+/0i AMXAhQnC+MpXEUzEWy7310zMVAtdM17ezjvMq6vrbkafMxqoIWLa8TGSk55xx2RFoDGlT3fgVpmDL gR0ULX8zekNaFw48xR4Oy8VRf200hG48vd2/K5u7i6mqzMkagmNXifeVmZzB7iTUyDgxHhsjE2q/U yrQS1087fYAIDtMxCopRT+9CVysIAzON+joB5ljxDXl2mO4qHA+3gUt3k/NKtGpRhMmYjtt7GTLf0 +FjM2c61Lxwe+h2wgTEFL0Xk7uwnr0FsS1cTiXszfwV4+C+nVsDN4jGiXoHc/pfDA/AnUBnA5Erqw wlzSZTzA==; Received: from 39-14-49-133.adsl.fetnet.net ([39.14.49.133] helo=[192.168.220.43]) by fanzine2.igalia.com with esmtpsa (Cipher TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_128_GCM:128) (Exim) id 1u5NJb-000ma4-V0; Thu, 17 Apr 2025 13:21:56 +0200 Message-ID: Date: Thu, 17 Apr 2025 19:21:48 +0800 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: David Hildenbrand , 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: Gavin Guo In-Reply-To: <05a7d51e-f065-445a-af0e-481f3461a76e@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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. 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? I will start the cleanup work after confirmation. > >> >> (Hmm, that may be another reason for preferring the reasoning by >> folio lock: forgive me if I'm misremembering, but didn't those >> page migration swapops get renamed, some time around 5.11?) > > I remember that we did something to PTE handling stuff in the context of > PTE markers. But things keep changing all of the time .. :) >