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[2003:d8:2f23:5600:727b:b2f6:94a1:a51b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3f02f332c31sm1086606f8f.45.2025.09.19.00.55.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 19 Sep 2025 00:55:20 -0700 (PDT) Message-ID: <9d2c3e3e-439d-4695-b7c9-21fa52f48ced@redhat.com> Date: Fri, 19 Sep 2025 09:55:18 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit >> 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. For THP, I recall that only some pages might be tagged. So likely we want to check per page. > > 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. > 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 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); For KSM, similarly just bail out early. But still wondering if this is already checked somehow for KSM. -- Cheers David / dhildenb