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[2003:d8:2f07:dd00:ca9c:199e:d2b6:9099]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3ee0fbf1d35sm3664976f8f.55.2025.09.18.04.57.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 18 Sep 2025 04:57:22 -0700 (PDT) Message-ID: Date: Thu, 18 Sep 2025 13:57:20 +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: Usama Arif , =?UTF-8?B?UXVuLXdlaSBMaW4gKOael+e+pOW0tCk=?= , "catalin.marinas@arm.com" , "linux-mm@kvack.org" , "yuzhao@google.com" , "akpm@linux-foundation.org" Cc: "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> <52175d87-50b5-49f8-bb68-6071e6b03557@gmail.com> 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-20250918_045727_386572_5FB3C11B X-CRM114-Status: GOOD ( 13.94 ) 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 18.09.25 13:42, Usama Arif wrote: > > > On 18/09/2025 09:56, David Hildenbrand wrote: >> On 18.09.25 10:53, Qun-wei Lin (林群崴) wrote: >>> On Fri, 2024-08-30 at 11:03 +0100, Usama Arif wrote: >>>> From: Yu Zhao >>>> >>>> Here being unused means containing only zeros and inaccessible to >>>> userspace. When splitting an isolated thp under reclaim or migration, >>>> the unused subpages can be mapped to the shared zeropage, hence >>>> saving >>>> memory. This is particularly helpful when the internal >>>> fragmentation of a thp is high, i.e. it has many untouched subpages. >>>> >>>> This is also a prerequisite for THP low utilization shrinker which >>>> will >>>> be introduced in later patches, where underutilized THPs are split, >>>> and >>>> the zero-filled pages are freed saving memory. >>>> >>>> Signed-off-by: Yu Zhao >>>> Tested-by: Shuang Zhai >>>> Signed-off-by: Usama Arif >>>> --- >>>>   include/linux/rmap.h |  7 ++++- >>>>   mm/huge_memory.c     |  8 ++--- >>>>   mm/migrate.c         | 72 ++++++++++++++++++++++++++++++++++++++---- >>>> -- >>>>   mm/migrate_device.c  |  4 +-- >>>>   4 files changed, 75 insertions(+), 16 deletions(-) >>>> >>>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h >>>> index 91b5935e8485..d5e93e44322e 100644 >>>> --- a/include/linux/rmap.h >>>> +++ b/include/linux/rmap.h >>>> @@ -745,7 +745,12 @@ int folio_mkclean(struct folio *); >>>>   int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, >>>> pgoff_t pgoff, >>>>                 struct vm_area_struct *vma); >>>>   -void remove_migration_ptes(struct folio *src, struct folio *dst, >>>> bool locked); >>>> +enum rmp_flags { >>>> +    RMP_LOCKED        = 1 << 0, >>>> +    RMP_USE_SHARED_ZEROPAGE    = 1 << 1, >>>> +}; >>>> + >>>> +void remove_migration_ptes(struct folio *src, struct folio *dst, int >>>> flags); >>>>     /* >>>>    * rmap_walk_control: To control rmap traversing for specific needs >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 0c48806ccb9a..af60684e7c70 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -3020,7 +3020,7 @@ bool unmap_huge_pmd_locked(struct >>>> vm_area_struct *vma, unsigned long addr, >>>>       return false; >>>>   } >>>>   -static void remap_page(struct folio *folio, unsigned long nr) >>>> +static void remap_page(struct folio *folio, unsigned long nr, int >>>> flags) >>>>   { >>>>       int i = 0; >>>>   @@ -3028,7 +3028,7 @@ static void remap_page(struct folio *folio, >>>> unsigned long nr) >>>>       if (!folio_test_anon(folio)) >>>>           return; >>>>       for (;;) { >>>> -        remove_migration_ptes(folio, folio, true); >>>> +        remove_migration_ptes(folio, folio, RMP_LOCKED | >>>> flags); >>>>           i += folio_nr_pages(folio); >>>>           if (i >= nr) >>>>               break; >>>> @@ -3240,7 +3240,7 @@ static void __split_huge_page(struct page >>>> *page, struct list_head *list, >>>>         if (nr_dropped) >>>>           shmem_uncharge(folio->mapping->host, nr_dropped); >>>> -    remap_page(folio, nr); >>>> +    remap_page(folio, nr, PageAnon(head) ? >>>> RMP_USE_SHARED_ZEROPAGE : 0); >>>>         /* >>>>        * set page to its compound_head when split to non order-0 >>>> pages, so >>>> @@ -3542,7 +3542,7 @@ int split_huge_page_to_list_to_order(struct >>>> page *page, struct list_head *list, >>>>           if (mapping) >>>>               xas_unlock(&xas); >>>>           local_irq_enable(); >>>> -        remap_page(folio, folio_nr_pages(folio)); >>>> +        remap_page(folio, folio_nr_pages(folio), 0); >>>>           ret = -EAGAIN; >>>>       } >>>>   diff --git a/mm/migrate.c b/mm/migrate.c >>>> index 6f9c62c746be..d039863e014b 100644 >>>> --- a/mm/migrate.c >>>> +++ b/mm/migrate.c >>>> @@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio, >>>> struct list_head *list) >>>>       return true; >>>>   } >>>>   +static bool try_to_map_unused_to_zeropage(struct >>>> page_vma_mapped_walk *pvmw, >>>> +                      struct folio *folio, >>>> +                      unsigned long idx) >>>> +{ >>>> +    struct page *page = folio_page(folio, idx); >>>> +    bool contains_data; >>>> +    pte_t newpte; >>>> +    void *addr; >>>> + >>>> +    VM_BUG_ON_PAGE(PageCompound(page), page); >>>> +    VM_BUG_ON_PAGE(!PageAnon(page), page); >>>> +    VM_BUG_ON_PAGE(!PageLocked(page), page); >>>> +    VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page); >>>> + >>>> +    if (folio_test_mlocked(folio) || (pvmw->vma->vm_flags & >>>> VM_LOCKED) || >>>> +        mm_forbids_zeropage(pvmw->vma->vm_mm)) >>>> +        return false; >>>> + >>>> +    /* >>>> +     * The pmd entry mapping the old thp was flushed and the pte >>>> mapping >>>> +     * this subpage has been non present. If the subpage is only >>>> zero-filled >>>> +     * then map it to the shared zeropage. >>>> +     */ >>>> +    addr = kmap_local_page(page); >>>> +    contains_data = memchr_inv(addr, 0, PAGE_SIZE); >>>> +    kunmap_local(addr); >>>> + >>>> +    if (contains_data) >>>> +        return false; >>>> + >>>> +    newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address), >>>> +                    pvmw->vma->vm_page_prot)); >>>> +    set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, >>>> newpte); >>>> + >>>> +    dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio)); >>>> +    return true; >>>> +} >>>> + >>>> +struct rmap_walk_arg { >>>> +    struct folio *folio; >>>> +    bool map_unused_to_zeropage; >>>> +}; >>>> + >>>>   /* >>>>    * Restore a potential migration pte to a working pte entry >>>>    */ >>>>   static bool remove_migration_pte(struct folio *folio, >>>> -        struct vm_area_struct *vma, unsigned long addr, void >>>> *old) >>>> +        struct vm_area_struct *vma, unsigned long addr, void >>>> *arg) >>>>   { >>>> -    DEFINE_FOLIO_VMA_WALK(pvmw, old, vma, addr, PVMW_SYNC | >>>> PVMW_MIGRATION); >>>> +    struct rmap_walk_arg *rmap_walk_arg = arg; >>>> +    DEFINE_FOLIO_VMA_WALK(pvmw, rmap_walk_arg->folio, vma, addr, >>>> PVMW_SYNC | PVMW_MIGRATION); >>>>         while (page_vma_mapped_walk(&pvmw)) { >>>>           rmap_t rmap_flags = RMAP_NONE; >>>> @@ -234,6 +278,9 @@ static bool remove_migration_pte(struct folio >>>> *folio, >>>>               continue; >>>>           } >>>>   #endif >>>> +        if (rmap_walk_arg->map_unused_to_zeropage && >>>> +            try_to_map_unused_to_zeropage(&pvmw, folio, >>>> idx)) >>>> +            continue; >>>>             folio_get(folio); >>>>           pte = mk_pte(new, READ_ONCE(vma->vm_page_prot)); >>>> @@ -312,14 +359,21 @@ static bool remove_migration_pte(struct folio >>>> *folio, >>>>    * Get rid of all migration entries and replace them by >>>>    * references to the indicated page. >>>>    */ >>>> -void remove_migration_ptes(struct folio *src, struct folio *dst, >>>> bool locked) >>>> +void remove_migration_ptes(struct folio *src, struct folio *dst, int >>>> flags) >>>>   { >>>> +    struct rmap_walk_arg rmap_walk_arg = { >>>> +        .folio = src, >>>> +        .map_unused_to_zeropage = flags & >>>> RMP_USE_SHARED_ZEROPAGE, >>>> +    }; >>>> + >>>>       struct rmap_walk_control rwc = { >>>>           .rmap_one = remove_migration_pte, >>>> -        .arg = src, >>>> +        .arg = &rmap_walk_arg, >>>>       }; >>>>   -    if (locked) >>>> +    VM_BUG_ON_FOLIO((flags & RMP_USE_SHARED_ZEROPAGE) && (src != >>>> dst), src); >>>> + >>>> +    if (flags & RMP_LOCKED) >>>>           rmap_walk_locked(dst, &rwc); >>>>       else >>>>           rmap_walk(dst, &rwc); >>>> @@ -934,7 +988,7 @@ static int writeout(struct address_space >>>> *mapping, struct folio *folio) >>>>        * At this point we know that the migration attempt cannot >>>>        * be successful. >>>>        */ >>>> -    remove_migration_ptes(folio, folio, false); >>>> +    remove_migration_ptes(folio, folio, 0); >>>>         rc = mapping->a_ops->writepage(&folio->page, &wbc); >>>>   @@ -1098,7 +1152,7 @@ static void migrate_folio_undo_src(struct folio >>>> *src, >>>>                      struct list_head *ret) >>>>   { >>>>       if (page_was_mapped) >>>> -        remove_migration_ptes(src, src, false); >>>> +        remove_migration_ptes(src, src, 0); >>>>       /* Drop an anon_vma reference if we took one */ >>>>       if (anon_vma) >>>>           put_anon_vma(anon_vma); >>>> @@ -1336,7 +1390,7 @@ static int migrate_folio_move(free_folio_t >>>> put_new_folio, unsigned long private, >>>>           lru_add_drain(); >>>>         if (old_page_state & PAGE_WAS_MAPPED) >>>> -        remove_migration_ptes(src, dst, false); >>>> +        remove_migration_ptes(src, dst, 0); >>>>     out_unlock_both: >>>>       folio_unlock(dst); >>>> @@ -1474,7 +1528,7 @@ static int unmap_and_move_huge_page(new_folio_t >>>> get_new_folio, >>>>         if (page_was_mapped) >>>>           remove_migration_ptes(src, >>>> -            rc == MIGRATEPAGE_SUCCESS ? dst : src, >>>> false); >>>> +            rc == MIGRATEPAGE_SUCCESS ? dst : src, 0); >>>>     unlock_put_anon: >>>>       folio_unlock(dst); >>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>> index 8d687de88a03..9cf26592ac93 100644 >>>> --- a/mm/migrate_device.c >>>> +++ b/mm/migrate_device.c >>>> @@ -424,7 +424,7 @@ static unsigned long >>>> migrate_device_unmap(unsigned long *src_pfns, >>>>               continue; >>>>             folio = page_folio(page); >>>> -        remove_migration_ptes(folio, folio, false); >>>> +        remove_migration_ptes(folio, folio, 0); >>>>             src_pfns[i] = 0; >>>>           folio_unlock(folio); >>>> @@ -840,7 +840,7 @@ void migrate_device_finalize(unsigned long >>>> *src_pfns, >>>>               dst = src; >>>>           } >>>>   -        remove_migration_ptes(src, dst, false); >>>> +        remove_migration_ptes(src, dst, 0); >>>>           folio_unlock(src); >>>>             if (folio_is_zone_device(src)) >>> >>> Hi, >>> >>> This patch has been in the mainline for some time, but we recently >>> discovered an issue when both mTHP and MTE (Memory Tagging Extension) >>> are enabled. >>> >>> It seems that remapping to the same zeropage might causes MTE tag >>> mismatches, since MTE tags are associated with physical addresses. >> >> Does this only trigger when the VMA has mte enabled? Maybe we'll have to bail out if we detect that mte is enabled. >> > > I believe MTE is all or nothing? i.e. all the memory is tagged when enabled, but will let the arm folks confirm. > I think for a process it's only active when PROT_MTE was called. In khugepaged, I recall that we use copy_mc_user_highpage()->copy_user_highpage() that translates on arm64 to its custom copy_highpage() where tags are copied as well. So that's why khugepaged works. KSM? Hmmm. Not sure how that is fenced off. We'd likely need some hidden page_mte_tagged() check somewhere. -- Cheers David / dhildenb