From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 65BB4C433EF for ; Mon, 21 Mar 2022 20:51:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235903AbiCUUxJ (ORCPT ); Mon, 21 Mar 2022 16:53:09 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54476 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229475AbiCUUxG (ORCPT ); Mon, 21 Mar 2022 16:53:06 -0400 Received: from mail-pf1-x42a.google.com (mail-pf1-x42a.google.com [IPv6:2607:f8b0:4864:20::42a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 88ECD18DAA2 for ; Mon, 21 Mar 2022 13:51:38 -0700 (PDT) Received: by mail-pf1-x42a.google.com with SMTP id u17so16482254pfk.11 for ; Mon, 21 Mar 2022 13:51:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=kr+MPjKk9FHTdVQFfmD1MoRxJl2rXg4nYCL6ZCl6m1c=; b=ZybJEpuN7tTQoz7ty/f1WxUYb7Wn/qyGsaMHq4AZ0iw82jKoynCE/JgmZ+mXD0gAfv w7pu/F/Z/kmM3otqtyn4oeAcs0wB+Eq2Jt0+p78r8S9SBZMyqoVdvi9y9PRUbJ78cSkr CAakf5C4H4ZsANG8M+4m5NuqFvZdQmtHELsbuZ0gDg1KN4JI67THn4yyIpbTmU3kUHQk 4nUWW8QgGVca9rb67+vjQqCXqijesecYb8TGlzW4rEl33SddVOQt3X8gM2WHWypHBPS6 Rb49f0e2tmt6jdansz0lcsZuq6pTRdC4dvcANZweXhCViEcv/5nuiQDzl0g2Qa8vHKYD rXYg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=kr+MPjKk9FHTdVQFfmD1MoRxJl2rXg4nYCL6ZCl6m1c=; b=bAToTN9pEKkjZqIXMMojfaHgKsRaP18G3/rm3w6xLT0DxjAFQrafQcUXHtv8+aUUpT Vusma1GUJwoK55q5kz9yQw0NHFX2sMqtRBTWSinnpH9E4/wUX5KECVnZBWqjj7qQeRBU rMc4BmVkn/RYsLMx7M2wpQNMFHAOOsVsa0gY6vPS9v8wRqlpSgr0TYC/cZNdvVB7BtJX WQaCs98v2Xw37WWODRR3xuxc2YPQ0W/V34hoE+qsO2UDPN749UdJyXiKf9ctrUEJDjnm /mOHZhByLlP7ugFlhf6fVTe5N3uyLYGvOhLwJJESPQ8EacCxIgvU3JrAeo0/Gf2oKljP +9uQ== X-Gm-Message-State: AOAM532Ae3j3LMK3ST+XlKs8i6FFsWEec6IPLUJ4tdRjq84yihOzQu/F aQxaXdB0rLyX0i1cpmwUjREUh8GCDyaQnu084oo= X-Google-Smtp-Source: ABdhPJxs3PYqtE6hkSraE6d17BsSsiRuORtSf0nSSgzBHav2F7OMyB6ad7W/El52C+0Am0VfQidUQNHM//Q5Bdzit/s= X-Received: by 2002:a65:6951:0:b0:381:f10:ccaa with SMTP id w17-20020a656951000000b003810f10ccaamr18820374pgq.587.1647895897955; Mon, 21 Mar 2022 13:51:37 -0700 (PDT) MIME-Version: 1.0 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> In-Reply-To: <2505408d-6cc7-f14e-79a4-c5a1c716f737@redhat.com> From: Yang Shi Date: Mon, 21 Mar 2022 13:51:26 -0700 Message-ID: Subject: Re: [PATCH v2 11/15] mm: remember exclusively mapped anonymous pages with PG_anon_exclusive To: David Hildenbrand 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 19, 2022 at 3:21 AM 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. It should be very rare to trigger that warning. I just saw one report before. And arch_unmap_one() actually can't fail except for Sparc. So that warning was intended to catch some unusual issues, but this COW patchset may make it much more usual than before. > > 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. Yes, that was why it was changed to a WARN from a BUG, please see commit 504e070dc08f ("mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split") > > I'll include a separate patch to just remove that VM_WARN_ON_ONCE -- thanks! Thanks, I'm fine to remove it. > > -- > Thanks, > > David / dhildenb >