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[2003:cb:c706:2700:abf9:4eac:588c:adab]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4405b4c8294sm45007165e9.2.2025.04.17.01.09.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Apr 2025 01:09:56 -0700 (PDT) Message-ID: Date: Thu, 17 Apr 2025 10:09:55 +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 From: David Hildenbrand To: Hugh Dickins Cc: Gavin Guo , 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> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 17.04.25 10:07, 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). > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 2a47682d1ab77..754aa3103e8bf 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3075,22 +3075,11 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address, > pmd_t *pmd, bool freeze, struct folio *folio) > { > - VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio)); > VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE)); > - VM_WARN_ON_ONCE(folio && !folio_test_locked(folio)); > - VM_BUG_ON(freeze && !folio); > > - /* > - * When the caller requests to set up a migration entry, we > - * require a folio to check the PMD against. Otherwise, there > - * is a risk of replacing the wrong folio. > - */ > if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || > - is_pmd_migration_entry(*pmd)) { > - if (folio && folio != pmd_folio(*pmd)) > - return; > + is_pmd_migration_entry(*pmd)) > __split_huge_pmd_locked(vma, pmd, address, freeze); > - } > } > > void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, > diff --git a/mm/rmap.c b/mm/rmap.c > index 67bb273dfb80d..bf0320b03d615 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -2291,13 +2291,6 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, > if (flags & TTU_SYNC) > pvmw.flags = PVMW_SYNC; > > - /* > - * unmap_page() in mm/huge_memory.c is the only user of migration with > - * TTU_SPLIT_HUGE_PMD and it wants to freeze. > - */ > - if (flags & TTU_SPLIT_HUGE_PMD) > - split_huge_pmd_address(vma, address, true, folio); > - > /* > * For THP, we have to assume the worse case ie pmd for invalidation. > * For hugetlb, it could be much worse if we need to do pud > @@ -2326,6 +2319,14 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma, > #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION > /* PMD-mapped THP migration entry */ > if (!pvmw.pte) { > + if (flags & TTU_SPLIT_HUGE_PMD) { > + split_huge_pmd_locked(vma, pmvw.address, pvmw.pmd, > + true, NULL); > + ret = false; > + page_vma_mapped_walk_done(&pvmw); > + break; > + } > + > subpage = folio_page(folio, > pmd_pfn(*pvmw.pmd) - folio_pfn(folio)); > VM_BUG_ON_FOLIO(folio_test_hugetlb(folio) || > > Likely, we'd have to adjust the CONFIG_ARCH_ENABLE_THP_MIGRATION coverage here, for TTU_SPLIT_HUGE_PMD to get handled even without that. Just an idea. -- Cheers, David / dhildenb