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(p200300d82f249200124ef0bf6f8ccbd8.dip0.t-ipconnect.de. [2003:d8:2f24:9200:124e:f0bf:6f8c:cbd8]) by smtp.gmail.com with ESMTPSA id z5-20020a05600c0a0500b0037bb8df81a2sm14816035wmp.13.2022.03.19.03.21.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 19 Mar 2022 03:21:15 -0700 (PDT) Message-ID: <2505408d-6cc7-f14e-79a4-c5a1c716f737@redhat.com> Date: Sat, 19 Mar 2022 11:21:14 +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> <2b280ac6-9d39-58c5-b255-f39b1dac607b@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-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 0B71020030 Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=SN1SGvE5; spf=none (imf03.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspam-User: X-Stat-Signature: ttuauynu3pbbwj93enwy7ce4wewb6hou X-HE-Tag: 1647685279-263343 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 18.03.22 21:29, Yang Shi wrote: > On Thu, Mar 17, 2022 at 2:06 AM David Hildenbrand wrote: >> >> 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, I missed that. Yes, the page will still be mapped. This should > trigger the VM_WARN_ON_ONCE in unmap_page(), if this change will make > this happen more often, we may consider removing that warning even > though it is "once" since seeing a mapped page may become a normal > case (once DIO is switched to FOLL_PIN, it may be more often). Anyway > we don't have to remove it right now. Oh, very good catch! I wasn't able to trigger that warning in my testing so far. Interestingly, arch_unmap_one() could theoretically make this fail already and trigger the warning. Apart from that warning, split_huge_page_to_list() should work as expected: freezing the refcount will fail if still mapped and we'll remap. I'll include a separate patch to just remove that VM_WARN_ON_ONCE -- thanks! -- Thanks, David / dhildenb