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[2003:d8:2f3c:2f00:8afc:23fe:3813:776c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538a3b2fd7sm22569255e9.17.2025.06.26.07.39.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 26 Jun 2025 07:39:06 -0700 (PDT) Message-ID: <6fbcf806-eb3c-4bcd-8daf-8d87fd759d2b@redhat.com> Date: Thu, 26 Jun 2025 16:39:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation To: Lance Yang Cc: 21cnbao@gmail.com, akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, chrisl@kernel.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, lorenzo.stoakes@oracle.com, ryan.roberts@arm.com, v-songbaohua@oppo.com, x86@kernel.org, ying.huang@intel.com, zhengtangquan@oppo.com, Lance Yang References: <20250626092905.31305-1-ioworker0@gmail.com> <20250626124445.77865-1-ioworker0@gmail.com> <1a55f9f3-f5b1-4761-97ba-423756c707fe@redhat.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-20250626_153915_903483_22294FB7 X-CRM114-Status: GOOD ( 21.71 ) X-BeenThere: linux-arm-kernel@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-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 26.06.25 15:52, Lance Yang wrote: > > > On 2025/6/26 21:16, David Hildenbrand wrote: >> On 26.06.25 14:44, Lance Yang wrote: >>> >>> On 2025/6/26 17:29, Lance Yang wrote: >>>> Before I send out the real patch, I'd like to get some quick feedback to >>>> ensure I've understood the discussion correctly ;) >>>> >>>> Does this look like the right direction? >>>> >>>> diff --git a/mm/rmap.c b/mm/rmap.c >>>> index fb63d9256f09..5ebffe2137e4 100644 >>>> --- a/mm/rmap.c >>>> +++ b/mm/rmap.c >>>> @@ -1845,23 +1845,37 @@ void folio_remove_rmap_pud(struct folio >>>> *folio, struct page *page, >>>>    #endif >>>>    } >>>> -/* We support batch unmapping of PTEs for lazyfree large folios */ >>>> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr, >>>> -            struct folio *folio, pte_t *ptep) >>>> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >>>> +            struct page_vma_mapped_walk *pvmw, >>>> +            enum ttu_flags flags, pte_t pte) >>>>    { >>>>        const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>>> -    int max_nr = folio_nr_pages(folio); >>>> -    pte_t pte = ptep_get(ptep); >>>> +    unsigned long end_addr, addr = pvmw->address; >>>> +    struct vm_area_struct *vma = pvmw->vma; >>>> +    unsigned int max_nr; >>>> + >>>> +    if (flags & TTU_HWPOISON) >>>> +        return 1; >>>> +    if (!folio_test_large(folio)) >>>> +        return 1; >>>> +    /* We may only batch within a single VMA and a single page >>>> table. */ >>>> +    end_addr = pmd_addr_end(addr, vma->vm_end); >>>> +    max_nr = (end_addr - addr) >> PAGE_SHIFT; >>>> + >>>> +    /* We only support lazyfree batching for now ... */ >>>>        if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >>>> -        return false; >>>> +        return 1; >>>>        if (pte_unused(pte)) >>>> -        return false; >>>> -    if (pte_pfn(pte) != folio_pfn(folio)) >>>> -        return false; >>>> +        return 1; >>>> + >>>> +    /* ... where we must be able to batch the whole folio. */ >>>> +    if (pte_pfn(pte) != folio_pfn(folio) || max_nr != >>>> folio_nr_pages(folio)) >>>> +        return 1; >>>> +    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, >>>> fpb_flags, >>>> +                 NULL, NULL, NULL); >>>> -    return folio_pte_batch(folio, addr, ptep, pte, max_nr, >>>> fpb_flags, NULL, >>>> -                   NULL, NULL) == max_nr; >>>> +    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr; >>>>    } >>>>    /* >>>> @@ -2024,9 +2038,7 @@ static bool try_to_unmap_one(struct folio >>>> *folio, struct vm_area_struct *vma, >>>>                if (pte_dirty(pteval)) >>>>                    folio_mark_dirty(folio); >>>>            } else if (likely(pte_present(pteval))) { >>>> -            if (folio_test_large(folio) && !(flags & TTU_HWPOISON) && >>>> -                can_batch_unmap_folio_ptes(address, folio, pvmw.pte)) >>>> -                nr_pages = folio_nr_pages(folio); >>>> +            nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags, >>>> pteval); >>>>                end_addr = address + nr_pages * PAGE_SIZE; >>>>                flush_cache_range(vma, address, end_addr); >>>> @@ -2206,13 +2218,16 @@ static bool try_to_unmap_one(struct folio >>>> *folio, struct vm_area_struct *vma, >>>>                hugetlb_remove_rmap(folio); >>>>            } else { >>>>                folio_remove_rmap_ptes(folio, subpage, nr_pages, vma); >>>> -            folio_ref_sub(folio, nr_pages - 1); >>>>            } >>>>            if (vma->vm_flags & VM_LOCKED) >>>>                mlock_drain_local(); >>>> -        folio_put(folio); >>>> -        /* We have already batched the entire folio */ >>>> -        if (nr_pages > 1) >>>> +        folio_put_refs(folio, nr_pages); >>>> + >>>> +        /* >>>> +         * If we are sure that we batched the entire folio and cleared >>>> +         * all PTEs, we can just optimize and stop right here. >>>> +         */ >>>> +        if (nr_pages == folio_nr_pages(folio)) >>>>                goto walk_done; >>>>            continue; >>>>    walk_abort: >>>> -- >>> >>> Oops ... Through testing on my machine, I found that the logic doesn't >>> behave as expected because I messed up the meaning of max_nr (the >>> available >>> scan room in the page table) with folio_nr_pages(folio) :( >>> >>> With the following change: >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 5ebffe2137e4..b1407348e14e 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1850,9 +1850,9 @@ static inline unsigned int >>> folio_unmap_pte_batch(struct folio *folio, >>>               enum ttu_flags flags, pte_t pte) >>>   { >>>       const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >>> +    unsigned int max_nr, nr_pages = folio_nr_pages(folio); >>>       unsigned long end_addr, addr = pvmw->address; >>>       struct vm_area_struct *vma = pvmw->vma; >>> -    unsigned int max_nr; >>>       if (flags & TTU_HWPOISON) >>>           return 1; >>> @@ -1870,12 +1870,13 @@ static inline unsigned int >>> folio_unmap_pte_batch(struct folio *folio, >>>           return 1; >>>       /* ... where we must be able to batch the whole folio. */ >> >> Why is that still required? :) > > Sorry ... I was still stuck in the "all-or-nothing" mindset ... > > So, IIUC, you mean we should completely remove the "max_nr < nr_pages" > check and just let folio_pte_batch handle whatever partial batch it > safely can. > >> >>> -    if (pte_pfn(pte) != folio_pfn(folio) || max_nr != >>> folio_nr_pages(folio)) >>> +    if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages) >>>           return 1; >>> -    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, >>> fpb_flags, >>> -                 NULL, NULL, NULL); >>> -    return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr; >>> +    max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages, >>> +                 fpb_flags, NULL, NULL, NULL); >>> + >>> +    return (max_nr != nr_pages) ? 1 : max_nr; >> >> Why is that still required? :) > > Then simply return the number of PTEs that consecutively map to the > large folio. Right? Yes. Any part of the large folio. Just return folio_pte_batch() ... -- Cheers, David / dhildenb