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[2003:d8:2f23:5600:727b:b2f6:94a1:a51b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3f1549285c9sm1179821f8f.52.2025.09.19.06.09.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 19 Sep 2025 06:09:15 -0700 (PDT) Message-ID: Date: Fri, 19 Sep 2025 15:09:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when splitting isolated thp To: Lance Yang Cc: =?UTF-8?B?UXVuLXdlaSBMaW4gKOael+e+pOW0tCk=?= , "catalin.marinas@arm.com" , "usamaarif642@gmail.com" , "linux-mm@kvack.org" , "yuzhao@google.com" , "akpm@linux-foundation.org" , "corbet@lwn.net" , =?UTF-8?B?QW5kcmV3IFlhbmcgKOaliuaZuuW8tyk=?= , "npache@redhat.com" , "rppt@kernel.org" , "willy@infradead.org" , "kernel-team@meta.com" , "roman.gushchin@linux.dev" , "hannes@cmpxchg.org" , "cerasuolodomenico@gmail.com" , "linux-kernel@vger.kernel.org" , "ryncsn@gmail.com" , "surenb@google.com" , "riel@surriel.com" , "shakeel.butt@linux.dev" , =?UTF-8?B?Q2hpbndlbiBDaGFuZyAo5by16Yym5paHKQ==?= , "linux-doc@vger.kernel.org" , =?UTF-8?B?Q2FzcGVyIExpICjmnY7kuK3mpq4p?= , "ryan.roberts@arm.com" , "linux-mediatek@lists.infradead.org" , "baohua@kernel.org" , "kaleshsingh@google.com" , "zhais@google.com" , "linux-arm-kernel@lists.infradead.org" References: <20240830100438.3623486-1-usamaarif642@gmail.com> <20240830100438.3623486-3-usamaarif642@gmail.com> <434c092b-0f19-47bf-a5fa-ea5b4b36c35e@redhat.com> <120445c8-7250-42e0-ad6a-978020c8fad3@linux.dev> <9d2c3e3e-439d-4695-b7c9-21fa52f48ced@redhat.com> <4cf41cd5-e93a-412b-b209-4180bd2d4015@linux.dev> <9395a9ca-d865-42d7-9ea1-8e693e4e38e0@linux.dev> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250919_060921_723762_04774D51 X-CRM114-Status: GOOD ( 23.11 ) X-BeenThere: linux-mediatek@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-mediatek" Errors-To: linux-mediatek-bounces+linux-mediatek=archiver.kernel.org@lists.infradead.org On 19.09.25 14:19, Lance Yang wrote: > Hey David, > > I believe I've found the exact reason why KSM skips MTE-tagged pages ;p > >> >> >> On 2025/9/19 16:14, Lance Yang wrote: >>> >>> >>> On 2025/9/19 15:55, David Hildenbrand wrote: >>>>>> I think where possible we really only want to identify problematic >>>>>> (tagged) pages and skip them. And we should either look into fixing >>>>>> KSM >>>>>> as well or finding out why KSM is not affected. >>>>> >>>>> Yeah. Seems like we could introduce a new helper, >>>>> folio_test_mte_tagged(struct >>>>> folio *folio). By default, it would return false, and architectures >>>>> like >>>>> arm64 >>>>> can override it. >>>> >>>> If we add a new helper it should instead express the semantics that >>>> we cannot deduplicate. >>> >>> Agreed. >>> >>>> >>>> For THP, I recall that only some pages might be tagged. So likely we >>>> want to check per page. >>> >>> Yes, a per-page check would be simpler. >>> >>>> >>>>> >>>>> Looking at the code, the PG_mte_tagged flag is not set for regular THP. >>>> >>>> I think it's supported for THP per page. Only for hugetlb we tag the >>>> whole thing through the head page instead of individual pages. >>> >>> Right. That's exactly what I meant. >>> >>>> >>>>> The MTE >>>>> status actually comes from the VM_MTE flag in the VMA that maps it. >>>>> >>>> >>>> During the rmap walk we could check the VMA flag, but there would be >>>> no way to just stop the THP shrinker scanning this page early. >>>> >>>>> static inline bool folio_test_hugetlb_mte_tagged(struct folio *folio) >>>>> { >>>>>     bool ret = test_bit(PG_mte_tagged, &folio->flags.f); >>>>> >>>>>     VM_WARN_ON_ONCE(!folio_test_hugetlb(folio)); >>>>> >>>>>     /* >>>>>      * If the folio is tagged, ensure ordering with a likely subsequent >>>>>      * read of the tags. >>>>>      */ >>>>>     if (ret) >>>>>         smp_rmb(); >>>>>     return ret; >>>>> } >>>>> >>>>> static inline bool page_mte_tagged(struct page *page) >>>>> { >>>>>     bool ret = test_bit(PG_mte_tagged, &page->flags.f); >>>>> >>>>>     VM_WARN_ON_ONCE(folio_test_hugetlb(page_folio(page))); >>>>> >>>>>     /* >>>>>      * If the page is tagged, ensure ordering with a likely subsequent >>>>>      * read of the tags. >>>>>      */ >>>>>     if (ret) >>>>>         smp_rmb(); >>>>>     return ret; >>>>> } >>>>> >>>>> contpte_set_ptes() >>>>>     __set_ptes() >>>>>         __set_ptes_anysz() >>>>>             __sync_cache_and_tags() >>>>>                 mte_sync_tags() >>>>>                     set_page_mte_tagged() >>>>> >>>>> Then, having the THP shrinker skip any folios that are identified as >>>>> MTE-tagged. >>>> >>>> Likely we should just do something like (maybe we want better naming) >>>> >>>> #ifndef page_is_mergable >>>> #define page_is_mergable(page) (true) >>>> #endif >>> >>> >>> Maybe something like page_is_optimizable()? Just a thought ;p >>> >>>> >>>> And for arm64 have it be >>>> >>>> #define page_is_mergable(page) (!page_mte_tagged(page)) >>>> >>>> >>>> And then do >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 1f0813b956436..1cac9093918d6 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -4251,7 +4251,8 @@ static bool thp_underused(struct folio *folio) >>>> >>>>          for (i = 0; i < folio_nr_pages(folio); i++) { >>>>                  kaddr = kmap_local_folio(folio, i * PAGE_SIZE); >>>> -               if (!memchr_inv(kaddr, 0, PAGE_SIZE)) { >>>> +               if (page_is_mergable(folio_page(folio, i)) && >>>> +                   !memchr_inv(kaddr, 0, PAGE_SIZE)) { >>>>                          num_zero_pages++; >>>>                          if (num_zero_pages > >>>> khugepaged_max_ptes_none) { >>>>                                  kunmap_local(kaddr); >>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>> index 946253c398072..476a9a9091bd3 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -306,6 +306,8 @@ static bool try_to_map_unused_to_zeropage(struct >>>> page_vma_mapped_walk *pvmw, >>>> >>>>          if (PageCompound(page)) >>>>                  return false; >>>> +       if (!page_is_mergable(page)) >>>> +               return false; >>>>          VM_BUG_ON_PAGE(!PageAnon(page), page); >>>>          VM_BUG_ON_PAGE(!PageLocked(page), page); >>>>          VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page); >>> >>> Looks good to me! >>> >>>> >>>> >>>> For KSM, similarly just bail out early. But still wondering if this >>>> is already checked >>>> somehow for KSM. >>> >>> +1 I'm looking for a machine to test it on. >> >> Interestingly, it seems KSM is already skipping MTE-tagged pages. My test, >> running on a v6.8.0 kernel inside QEMU (with MTE enabled), shows no merging >> activity for those pages ... > > KSM's call to pages_identical() ultimately leads to memcmp_pages(). The > arm64 implementation of memcmp_pages() in arch/arm64/kernel/mte.c contains > a specific check that prevents merging in this case. > > try_to_merge_one_page() > -> pages_identical() > -> !memcmp_pages() Fails! > -> replace_page() > > > int memcmp_pages(struct page *page1, struct page *page2) > { > char *addr1, *addr2; > int ret; > > addr1 = page_address(page1); > addr2 = page_address(page2); > ret = memcmp(addr1, addr2, PAGE_SIZE); > > if (!system_supports_mte() || ret) > return ret; > > /* > * If the page content is identical but at least one of the pages is > * tagged, return non-zero to avoid KSM merging. If only one of the > * pages is tagged, __set_ptes() may zero or change the tags of the > * other page via mte_sync_tags(). > */ > if (page_mte_tagged(page1) || page_mte_tagged(page2)) > return addr1 != addr2; > > return ret; > } > > IIUC, if either page is MTE-tagged, memcmp_pages() intentionally returns > a non-zero value, which in turn causes pages_identical() to return false. Cool, so we should likely just use that then in the shrinker code. Can you send a fix? -- Cheers David / dhildenb