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[2003:cb:c74b:bf00:182c:d606:87cf:6fea]) by smtp.gmail.com with ESMTPSA id j7-20020a5d5647000000b0034e52844757sm9248623wrw.95.2024.05.06.06.16.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 May 2024 06:16:11 -0700 (PDT) Message-ID: <5b770715-7516-42a8-9ea0-3f61572d92af@redhat.com> Date: Mon, 6 May 2024 15:16:09 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 6/6] mm: swap: entirely map large folios found in swapcache To: Barry Song <21cnbao@gmail.com> Cc: Ryan Roberts , akpm@linux-foundation.org, linux-mm@kvack.org, baolin.wang@linux.alibaba.com, chrisl@kernel.org, hanchuanhua@oppo.com, hannes@cmpxchg.org, hughd@google.com, kasong@tencent.com, linux-kernel@vger.kernel.org, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yosryahmed@google.com, yuzhao@google.com, ziy@nvidia.com References: <20240503005023.174597-1-21cnbao@gmail.com> <20240503005023.174597-7-21cnbao@gmail.com> <0b4d4d4b-91d8-4fd5-af4e-aebe9ee08b89@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 47CA5100014 X-Stat-Signature: zna8k1c7uxfy5ewrk89mmb7jatukoy6b X-Rspam-User: X-HE-Tag: 1715001375-530601 X-HE-Meta: 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 GzreobaX 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 06.05.24 14:58, Barry Song wrote: > On Tue, May 7, 2024 at 12:38 AM Barry Song <21cnbao@gmail.com> wrote: >> >> On Tue, May 7, 2024 at 12:07 AM David Hildenbrand wrote: >>> >>> On 04.05.24 01:23, Barry Song wrote: >>>> On Fri, May 3, 2024 at 6:50 PM Ryan Roberts wrote: >>>>> >>>>> On 03/05/2024 01:50, Barry Song wrote: >>>>>> From: Chuanhua Han >>>>>> >>>>>> When a large folio is found in the swapcache, the current implementation >>>>>> requires calling do_swap_page() nr_pages times, resulting in nr_pages >>>>>> page faults. This patch opts to map the entire large folio at once to >>>>>> minimize page faults. Additionally, redundant checks and early exits >>>>>> for ARM64 MTE restoring are removed. >>>>>> >>>>>> Signed-off-by: Chuanhua Han >>>>>> Co-developed-by: Barry Song >>>>>> Signed-off-by: Barry Song >>>>> >>>>> With the suggested changes below: >>>>> >>>>> Reviewed-by: Ryan Roberts >>>>> >>>>>> --- >>>>>> mm/memory.c | 60 ++++++++++++++++++++++++++++++++++++++++++----------- >>>>>> 1 file changed, 48 insertions(+), 12 deletions(-) >>>>>> >>>>>> diff --git a/mm/memory.c b/mm/memory.c >>>>>> index 22e7c33cc747..940fdbe69fa1 100644 >>>>>> --- a/mm/memory.c >>>>>> +++ b/mm/memory.c >>>>>> @@ -3968,6 +3968,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>>>> pte_t pte; >>>>>> vm_fault_t ret = 0; >>>>>> void *shadow = NULL; >>>>>> + int nr_pages = 1; >>>>>> + unsigned long page_idx = 0; >>>>>> + unsigned long address = vmf->address; >>>>>> + pte_t *ptep; >>>>> >>>>> nit: Personally I'd prefer all these to get initialised just before the "if >>>>> (folio_test_large()..." block below. That way it is clear they are fresh (incase >>>>> any logic between here and there makes an adjustment) and its clear that they >>>>> are only to be used after that block (the compiler will warn if using an >>>>> uninitialized value). >>>> >>>> right. I agree this will make the code more readable. >>>> >>>>> >>>>>> >>>>>> if (!pte_unmap_same(vmf)) >>>>>> goto out; >>>>>> @@ -4166,6 +4170,36 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>>>> goto out_nomap; >>>>>> } >>>>>> >>>>>> + ptep = vmf->pte; >>>>>> + if (folio_test_large(folio) && folio_test_swapcache(folio)) { >>>>>> + int nr = folio_nr_pages(folio); >>>>>> + unsigned long idx = folio_page_idx(folio, page); >>>>>> + unsigned long folio_start = vmf->address - idx * PAGE_SIZE; >>>>>> + unsigned long folio_end = folio_start + nr * PAGE_SIZE; >>>>>> + pte_t *folio_ptep; >>>>>> + pte_t folio_pte; >>>>>> + >>>>>> + if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start))) >>>>>> + goto check_folio; >>>>>> + if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end))) >>>>>> + goto check_folio; >>>>>> + >>>>>> + folio_ptep = vmf->pte - idx; >>>>>> + folio_pte = ptep_get(folio_ptep); >>>>>> + if (!pte_same(folio_pte, pte_move_swp_offset(vmf->orig_pte, -idx)) || >>>>>> + swap_pte_batch(folio_ptep, nr, folio_pte) != nr) >>>>>> + goto check_folio; >>>>>> + >>>>>> + page_idx = idx; >>>>>> + address = folio_start; >>>>>> + ptep = folio_ptep; >>>>>> + nr_pages = nr; >>>>>> + entry = folio->swap; >>>>>> + page = &folio->page; >>>>>> + } >>>>>> + >>>>>> +check_folio: >>>>> >>>>> Is this still the correct label name, given the checks are now above the new >>>>> block? Perhaps "one_page" or something like that? >>>> >>>> not quite sure about this, as the code after one_page can be multiple_pages. >>>> On the other hand, it seems we are really checking folio after "check_folio" >>>> :-) >>>> >>>> >>>> BUG_ON(!folio_test_anon(folio) && folio_test_mappedtodisk(folio)); >>>> BUG_ON(folio_test_anon(folio) && PageAnonExclusive(page)); >>>> >>>> /* >>>> * Check under PT lock (to protect against concurrent fork() sharing >>>> * the swap entry concurrently) for certainly exclusive pages. >>>> */ >>>> if (!folio_test_ksm(folio)) { >>>> >>>> >>>>> >>>>>> + >>>>>> /* >>>>>> * PG_anon_exclusive reuses PG_mappedtodisk for anon pages. A swap pte >>>>>> * must never point at an anonymous page in the swapcache that is >>>>>> @@ -4225,12 +4259,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>>>> * We're already holding a reference on the page but haven't mapped it >>>>>> * yet. >>>>>> */ >>>>>> - swap_free_nr(entry, 1); >>>>>> + swap_free_nr(entry, nr_pages); >>>>>> if (should_try_to_free_swap(folio, vma, vmf->flags)) >>>>>> folio_free_swap(folio); >>>>>> >>>>>> - inc_mm_counter(vma->vm_mm, MM_ANONPAGES); >>>>>> - dec_mm_counter(vma->vm_mm, MM_SWAPENTS); >>>>>> + folio_ref_add(folio, nr_pages - 1); >>>>>> + add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages); >>>>>> + add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages); >>>>>> pte = mk_pte(page, vma->vm_page_prot); >>>>>> >>>>>> /* >>>>>> @@ -4240,34 +4275,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >>>>>> * exclusivity. >>>>>> */ >>>>>> if (!folio_test_ksm(folio) && >>>>>> - (exclusive || folio_ref_count(folio) == 1)) { >>>>>> + (exclusive || (folio_ref_count(folio) == nr_pages && >>>>>> + folio_nr_pages(folio) == nr_pages))) { >>>>> >>>>> I think in practice there is no change here? If nr_pages > 1 then the folio is >>>>> in the swapcache, so there is an extra ref on it? I agree with the change for >>>>> robustness sake. Just checking my understanding. >>>> >>>> This is the code showing we are reusing/(mkwrite) a folio either >>>> 1. we meet a small folio and we are the only one hitting the small folio >>>> 2. we meet a large folio and we are the only one hitting the large folio >>>> >>>> any corner cases besides the above two seems difficult. for example, >>>> >>>> while we hit a large folio in swapcache but if we can't entirely map it >>>> (nr_pages==1) due to partial unmap, we will have folio_ref_count(folio) >>>> == nr_pages == 1 >>> >>> No, there would be other references from the swapcache and >>> folio_ref_count(folio) > 1. See my other reply. >> >> right. can be clearer by: > > Wait, do we still need folio_nr_pages(folio) == nr_pages even if we use > folio_ref_count(folio) == 1 and moving folio_ref_add(folio, nr_pages - 1)? I don't think that we will "need" it. > > one case is that we have a large folio with 16 PTEs, and we unmap > 15 swap PTE entries, thus we have only one swap entry left. Then > we hit the large folio in swapcache. but we have only one PTE thus we will > map only one PTE. lacking folio_nr_pages(folio) == nr_pages, we reuse the > large folio for one PTE. with it, do_wp_page() will migrate the large > folio to a small one? We will set PAE bit and do_wp_page() will unconditionally reuse that page. Note that this is the same as if we had pte_swp_exclusive() set and would have run into "exclusive=true" here. If we'd want a similar "optimization" as we have in wp_can_reuse_anon_folio(), you'd want something like exclusive || (folio_ref_count(folio) == 1 && (!folio_test_large(folio) || nr_pages > 1) ... but I am not sure if that is really worth the complexity here. > > 1AM, tired and sleepy. not quite sure I am correct. > I look forward to seeing your reply tomorrow morning :-) Heh, no need to dream about this ;) -- Cheers, David / dhildenb