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(p200300cbc708de00549ee4e498dfff72.dip0.t-ipconnect.de. [2003:cb:c708:de00:549e:e4e4:98df:ff72]) by smtp.gmail.com with ESMTPSA id 20-20020a05600c22d400b0038c8dbdc1a3sm1432745wmg.38.2022.03.22.02.41.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 22 Mar 2022 02:41:53 -0700 (PDT) Message-ID: <9015e3af-8c6d-bab2-27ae-ad201e499c24@redhat.com> Date: Tue, 22 Mar 2022 10:41:51 +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 Mailing List , 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 References: <20220315104741.63071-1-david@redhat.com> <20220315104741.63071-12-david@redhat.com> <2b280ac6-9d39-58c5-b255-f39b1dac607b@redhat.com> <2505408d-6cc7-f14e-79a4-c5a1c716f737@redhat.com> <787e7ac7-917c-71eb-8050-a01f6a96a4cc@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 Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=IAxj4Nq+; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf23.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: DEDA714002C X-Stat-Signature: npg6ib7wqhrdw33uastc86rk6mu3iqgj X-HE-Tag: 1647942117-860551 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 21.03.22 21:56, Yang Shi wrote: > On Sat, Mar 19, 2022 at 3:50 AM David Hildenbrand wrote: >> >> On 19.03.22 11:21, David Hildenbrand wrote: >>> 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! >>> >> >> From e6e983d841cd2aa2a9c8dc71779211881cf0d96f Mon Sep 17 00:00:00 2001 >> From: David Hildenbrand >> Date: Sat, 19 Mar 2022 11:49:39 +0100 >> Subject: [PATCH] mm/huge_memory: remove outdated VM_WARN_ON_ONCE_PAGE from >> unmap_page() >> >> We can already theoretically fail to unmap (still having page_mapped()) in >> case arch_unmap_one() fails. Although this applies only to anonymous pages >> for now, get rid of the VM_WARN_ON_ONCE_PAGE() completely: the caller -- >> split_huge_page_to_list() -- will fail to freeze the refcount and >> remap the page via remap_page(). So the caller can handle unmap errors >> just fine. >> >> This is a preparation for making try_to_migrate() fail on anonymous pages >> with GUP pins. > > Better to mention "making try_to_migrate() fail on pinned anonymous > pages may make this happen more usual". > >> >> Reported-by: Yang Shi >> Signed-off-by: David Hildenbrand > > Fine to me. Reviewed-by: Yang Shi Thanks, updated patch: >From 4f68aa1067e43caa370a62b930094a4d07492f92 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Sat, 19 Mar 2022 11:49:39 +0100 Subject: [PATCH] mm/huge_memory: remove outdated VM_WARN_ON_ONCE_PAGE from unmap_page() We can already theoretically fail to unmap (still having page_mapped()) in case arch_unmap_one() fails, which can happen on sparc. Failures to unmap are handled gracefully, just as if there are other references on the target page: freezing the refcount in split_huge_page_to_list() will fail if still mapped and we'll simply remap. In commit 504e070dc08f ("mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split") we already converted to VM_WARN_ON_ONCE_PAGE, let's get rid of it completely now. This is a preparation for making try_to_migrate() fail on anonymous pages with GUP pins, which will make this VM_WARN_ON_ONCE_PAGE trigger more frequently. Reported-by: Yang Shi Reviewed-by: Yang Shi Signed-off-by: David Hildenbrand --- mm/huge_memory.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 0b6fb409b9e4..0fe0ab3ec3fc 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2263,8 +2263,6 @@ static void unmap_page(struct page *page) try_to_migrate(page, ttu_flags); else try_to_unmap(page, ttu_flags | TTU_IGNORE_MLOCK); - - VM_WARN_ON_ONCE_PAGE(page_mapped(page), page); } static void remap_page(struct page *page, unsigned int nr) -- 2.35.1 -- Thanks, David / dhildenb