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From: Lance Yang Subject: Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation To: David Hildenbrand 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> Content-Language: en-US In-Reply-To: <1a55f9f3-f5b1-4761-97ba-423756c707fe@redhat.com> 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-20250626_065334_614500_97659AF2 X-CRM114-Status: GOOD ( 18.35 ) 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 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?