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(p200300cbc702de00711b76afb3359b70.dip0.t-ipconnect.de. [2003:cb:c702:de00:711b:76af:b335:9b70]) by smtp.gmail.com with ESMTPSA id r17-20020a0560001b9100b00207afaa8987sm2160122wru.27.2022.04.21.02.15.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 21 Apr 2022 02:15:40 -0700 (PDT) Message-ID: <31b37b7c-a969-eec2-bdff-a7e4dca9b770@redhat.com> Date: Thu, 21 Apr 2022 11:15:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH v3 16/16] mm/gup: sanity-check with CONFIG_DEBUG_VM that anonymous pages are exclusive when (un)pinning Content-Language: en-US To: Vlastimil Babka , linux-kernel@vger.kernel.org Cc: Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Liang Zhang , Pedro Gomes , Oded Gabbay , linux-mm@kvack.org References: <20220329160440.193848-1-david@redhat.com> <20220329160440.193848-17-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19.04.22 19:40, Vlastimil Babka wrote: > On 3/29/22 18:04, David Hildenbrand wrote: >> Let's verify when (un)pinning anonymous pages that we always deal with >> exclusive anonymous pages, which guarantees that we'll have a reliable >> PIN, meaning that we cannot end up with the GUP pin being inconsistent >> with he pages mapped into the page tables due to a COW triggered >> by a write fault. >> >> When pinning pages, after conditionally triggering GUP unsharing of >> possibly shared anonymous pages, we should always only see exclusive >> anonymous pages. Note that anonymous pages that are mapped writable >> must be marked exclusive, otherwise we'd have a BUG. >> >> When pinning during ordinary GUP, simply add a check after our >> conditional GUP-triggered unsharing checks. As we know exactly how the >> page is mapped, we know exactly in which page we have to check for >> PageAnonExclusive(). >> >> When pinning via GUP-fast we have to be careful, because we can race with >> fork(): verify only after we made sure via the seqcount that we didn't >> race with concurrent fork() that we didn't end up pinning a possibly >> shared anonymous page. >> >> Similarly, when unpinning, verify that the pages are still marked as >> exclusive: otherwise something turned the pages possibly shared, which >> can result in random memory corruptions, which we really want to catch. >> >> With only the pinned pages at hand and not the actual page table entries >> we have to be a bit careful: hugetlb pages are always mapped via a >> single logical page table entry referencing the head page and >> PG_anon_exclusive of the head page applies. Anon THP are a bit more >> complicated, because we might have obtained the page reference either via >> a PMD or a PTE -- depending on the mapping type we either have to check >> PageAnonExclusive of the head page (PMD-mapped THP) or the tail page >> (PTE-mapped THP) applies: as we don't know and to make our life easier, >> check that either is set. >> >> Take care to not verify in case we're unpinning during GUP-fast because >> we detected concurrent fork(): we might stumble over an anonymous page >> that is now shared. >> >> Signed-off-by: David Hildenbrand > > Acked-by: Vlastimil Babka > > Nits: > >> @@ -510,6 +563,10 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, >> page = ERR_PTR(-EMLINK); >> goto out; >> } >> + >> + VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) && >> + !PageAnonExclusive(page)); > > Do we rather want VM_BUG_ON_PAGE? Also for the same tests in mm/huge*.c below. Make sense, thanks: diff --git a/mm/gup.c b/mm/gup.c index 5c17d4816441..46ffd8c51c6e 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -564,8 +564,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, goto out; } - VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) && - !PageAnonExclusive(page)); + VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && + !PageAnonExclusive(page), page); /* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */ if (unlikely(!try_grab_page(page, flags))) { diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 390f22334ee9..a2f44d8d3d47 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1392,8 +1392,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma, if (!pmd_write(*pmd) && gup_must_unshare(flags, page)) return ERR_PTR(-EMLINK); - VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) && - !PageAnonExclusive(page)); + VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && + !PageAnonExclusive(page), page); if (!try_grab_page(page, flags)) return ERR_PTR(-ENOMEM); diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 8a635b5b5270..0ba2b1930b21 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -6100,8 +6100,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT; page = pte_page(huge_ptep_get(pte)); - VM_BUG_ON((flags & FOLL_PIN) && PageAnon(page) && - !PageAnonExclusive(page)); + VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && + !PageAnonExclusive(page), page); /* * If subpage information not requested, update counters -- Thanks, David / dhildenb