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([2a09:80c0:192:0:20af:34be:985b:b6c8]) by smtp.gmail.com with ESMTPSA id p4-20020a05600c358400b00389f61bce7csm5416362wmq.32.2022.03.17.02.06.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 17 Mar 2022 02:06:51 -0700 (PDT) Message-ID: <2b280ac6-9d39-58c5-b255-f39b1dac607b@redhat.com> Date: Thu, 17 Mar 2022 10:06:50 +0100 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 v2 11/15] mm: remember exclusively mapped anonymous pages with PG_anon_exclusive To: Yang Shi Cc: linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , "Kirill A . Shutemov" , Matthew Wilcox , Vlastimil Babka , 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: <20220315104741.63071-1-david@redhat.com> <20220315104741.63071-12-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=byEga2fz; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf07.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 9217640011 X-Stat-Signature: zgkkk17emf3mizwrsqtk7nouscd7nnz3 X-HE-Tag: 1647508015-948229 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 16.03.22 22:23, Yang Shi wrote: > On Tue, Mar 15, 2022 at 3:52 AM David Hildenbrand wrote: >> >> Let's mark exclusively mapped anonymous pages with PG_anon_exclusive as >> exclusive, and use that information to make GUP pins reliable and stay >> consistent with the page mapped into the page table even if the >> page table entry gets write-protected. >> >> With that information at hand, we can extend our COW logic to always >> reuse anonymous pages that are exclusive. For anonymous pages that >> might be shared, the existing logic applies. >> >> As already documented, PG_anon_exclusive is usually only expressive in >> combination with a page table entry. Especially PTE vs. PMD-mapped >> anonymous pages require more thought, some examples: due to mremap() we >> can easily have a single compound page PTE-mapped into multiple page tables >> exclusively in a single process -- multiple page table locks apply. >> Further, due to MADV_WIPEONFORK we might not necessarily write-protect >> all PTEs, and only some subpages might be pinned. Long story short: once >> PTE-mapped, we have to track information about exclusivity per sub-page, >> but until then, we can just track it for the compound page in the head >> page and not having to update a whole bunch of subpages all of the time >> for a simple PMD mapping of a THP. >> >> For simplicity, this commit mostly talks about "anonymous pages", while >> it's for THP actually "the part of an anonymous folio referenced via >> a page table entry". >> >> To not spill PG_anon_exclusive code all over the mm code-base, we let >> the anon rmap code to handle all PG_anon_exclusive logic it can easily >> handle. >> >> If a writable, present page table entry points at an anonymous (sub)page, >> that (sub)page must be PG_anon_exclusive. If GUP wants to take a reliably >> pin (FOLL_PIN) on an anonymous page references via a present >> page table entry, it must only pin if PG_anon_exclusive is set for the >> mapped (sub)page. >> >> This commit doesn't adjust GUP, so this is only implicitly handled for >> FOLL_WRITE, follow-up commits will teach GUP to also respect it for >> FOLL_PIN without !FOLL_WRITE, to make all GUP pins of anonymous pages >> fully reliable. >> >> Whenever an anonymous page is to be shared (fork(), KSM), or when >> temporarily unmapping an anonymous page (swap, migration), the relevant >> PG_anon_exclusive bit has to be cleared to mark the anonymous page >> possibly shared. Clearing will fail if there are GUP pins on the page: >> * For fork(), this means having to copy the page and not being able to >> share it. fork() protects against concurrent GUP using the PT lock and >> the src_mm->write_protect_seq. >> * For KSM, this means sharing will fail. For swap this means, unmapping >> will fail, For migration this means, migration will fail early. All >> three cases protect against concurrent GUP using the PT lock and a >> proper clear/invalidate+flush of the relevant page table entry. >> >> This fixes memory corruptions reported for FOLL_PIN | FOLL_WRITE, when a >> pinned page gets mapped R/O and the successive write fault ends up >> replacing the page instead of reusing it. It improves the situation for >> O_DIRECT/vmsplice/... that still use FOLL_GET instead of FOLL_PIN, >> if fork() is *not* involved, however swapout and fork() are still >> problematic. Properly using FOLL_PIN instead of FOLL_GET for these >> GUP users will fix the issue for them. >> >> I. Details about basic handling >> >> I.1. Fresh anonymous pages >> >> page_add_new_anon_rmap() and hugepage_add_new_anon_rmap() will mark the >> given page exclusive via __page_set_anon_rmap(exclusive=1). As that is >> the mechanism fresh anonymous pages come into life (besides migration >> code where we copy the page->mapping), all fresh anonymous pages will >> start out as exclusive. >> >> I.2. COW reuse handling of anonymous pages >> >> When a COW handler stumbles over a (sub)page that's marked exclusive, it >> simply reuses it. Otherwise, the handler tries harder under page lock to >> detect if the (sub)page is exclusive and can be reused. If exclusive, >> page_move_anon_rmap() will mark the given (sub)page exclusive. >> >> Note that hugetlb code does not yet check for PageAnonExclusive(), as it >> still uses the old COW logic that is prone to the COW security issue >> because hugetlb code cannot really tolerate unnecessary/wrong COW as >> huge pages are a scarce resource. >> >> I.3. Migration handling >> >> try_to_migrate() has to try marking an exclusive anonymous page shared >> via page_try_share_anon_rmap(). If it fails because there are GUP pins >> on the page, unmap fails. migrate_vma_collect_pmd() and >> __split_huge_pmd_locked() are handled similarly. >> >> Writable migration entries implicitly point at shared anonymous pages. >> For readable migration entries that information is stored via a new >> "readable-exclusive" migration entry, specific to anonymous pages. >> >> When restoring a migration entry in remove_migration_pte(), information >> about exlusivity is detected via the migration entry type, and >> RMAP_EXCLUSIVE is set accordingly for >> page_add_anon_rmap()/hugepage_add_anon_rmap() to restore that >> information. >> >> I.4. Swapout handling >> >> try_to_unmap() has to try marking the mapped page possibly shared via >> page_try_share_anon_rmap(). If it fails because there are GUP pins on the >> page, unmap fails. For now, information about exclusivity is lost. In the >> future, we might want to remember that information in the swap entry in >> some cases, however, it requires more thought, care, and a way to store >> that information in swap entries. >> >> I.5. Swapin handling >> >> do_swap_page() will never stumble over exclusive anonymous pages in the >> swap cache, as try_to_migrate() prohibits that. do_swap_page() always has >> to detect manually if an anonymous page is exclusive and has to set >> RMAP_EXCLUSIVE for page_add_anon_rmap() accordingly. >> >> I.6. THP handling >> >> __split_huge_pmd_locked() has to move the information about exclusivity >> from the PMD to the PTEs. >> >> a) In case we have a readable-exclusive PMD migration entry, simply insert >> readable-exclusive PTE migration entries. >> >> b) In case we have a present PMD entry and we don't want to freeze >> ("convert to migration entries"), simply forward PG_anon_exclusive to >> all sub-pages, no need to temporarily clear the bit. >> >> c) In case we have a present PMD entry and want to freeze, handle it >> similar to try_to_migrate(): try marking the page shared first. In case >> we fail, we ignore the "freeze" instruction and simply split ordinarily. >> try_to_migrate() will properly fail because the THP is still mapped via >> PTEs. Hi, thanks for the review! > > How come will try_to_migrate() fail? The afterward pvmw will find > those PTEs then convert them to migration entries anyway IIUC. > It will run into that code: >> @@ -1903,6 +1938,15 @@ static bool try_to_migrate_one(struct page *page, struct vm_area_struct *vma, >> page_vma_mapped_walk_done(&pvmw); >> break; >> } >> + VM_BUG_ON_PAGE(pte_write(pteval) && PageAnon(page) && >> + !anon_exclusive, page); >> + if (anon_exclusive && >> + page_try_share_anon_rmap(subpage)) { >> + set_pte_at(mm, address, pvmw.pte, pteval); >> + ret = false; >> + page_vma_mapped_walk_done(&pvmw); >> + break; >> + } and similarly fail the page_try_share_anon_rmap(), at which point try_to_migrate() stops and the caller will still observe a "page_mapped() == true". -- Thanks, David / dhildenb