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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0862BCAC59A for ; Fri, 19 Sep 2025 12:19:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:References:Cc:To:From:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=EiVKkL22YbljXIkdVcER9mqBNCLAbO3Otj7ELKVqmVE=; b=a7qpfg8nWWn0OgAwCGwRcB6Hqr IJJyrqpmAchDn+7hOjjL9V1ecGFIu7VFBUHionxxWkf4tRI9+1XuxE3gdadn6NlvKcq70/Fz84MvA 4ZP2H9xysnE3NSz4j9FbmuVuLMcEtbpVnf5b8zQ/BDPR/6dli/OuE3CLhFEs/Kn2ZJDz+dTGo4Ixz PfKGPAdZiSNQdl7SwM/5Fo1nkE/Siv2fXUo6bXPcGje/z+qPnpA3Ttd9sZ9sBinaYYzZhoZFxmAO5 l8543MyhnrD78sUTsUG1SiLiuvwIcgoyh3oMn8pXNUPLW40dufBP3KNHg9zYA+ECOSdJPOcRJsOsh yhcFiuYw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uza5U-00000002nrl-3hUS; Fri, 19 Sep 2025 12:19:40 +0000 Received: from out-188.mta0.migadu.com ([2001:41d0:1004:224b::bc]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1uza5S-00000002nqx-0vNq for linux-mediatek@lists.infradead.org; Fri, 19 Sep 2025 12:19:39 +0000 Message-ID: <9395a9ca-d865-42d7-9ea1-8e693e4e38e0@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758284374; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=EiVKkL22YbljXIkdVcER9mqBNCLAbO3Otj7ELKVqmVE=; b=M5wUJJhGOKGalv8K65cSpDEtiAkwo7VxeOJE3XDf7FsY3sXohsexAOis/J4GO681ypJuJ7 ZhDFcFD2GZhk7ws4vP0mQDNRFfEFOunPk2f12QPiFu5iraacdY/1DdY5A7jTeqrCyb5fxR 82ubdEbzh3Ym39f3zPHEqae0+2V302w= Date: Fri, 19 Sep 2025 20:19:25 +0800 MIME-Version: 1.0 Subject: Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when splitting isolated thp Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang To: David Hildenbrand 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> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250919_051938_487571_5CF14677 X-CRM114-Status: GOOD ( 21.47 ) 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 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. Cheers, Lance