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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id k7-20020a5d6287000000b0034f3f756293sm3812174wru.6.2024.05.07.01.59.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 07 May 2024 01:59:02 -0700 (PDT) Message-ID: <099a2c9e-f85e-4fe7-99dd-81b61115935c@redhat.com> Date: Tue, 7 May 2024 10:59:01 +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> <5b770715-7516-42a8-9ea0-3f61572d92af@redhat.com> <7dc2084e-d8b1-42f7-b854-38981839f82e@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: efmw79ynbwps8ykcby8ezi1nz3c5m4y4 X-Rspamd-Queue-Id: 80B01140002 X-Rspamd-Server: rspam10 X-Rspam-User: X-HE-Tag: 1715072346-885478 X-HE-Meta: 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 /byVbWCZ yoDwPFnZcPkOG6PZF/R4ePJS5aYscjM9M/FkadLM7uC+v3nfFQAl+ke8xcRjSgYbSEQ7+XEFyb2lDMYjCc6vxnwcMyvF8MxPGUM2G8tvAuZxe9fQLWpQdhMH/Jgafeozr+iYSZNbZ13GRIS4Eyn4oRNC6/aI9mlO90QQBVsnqjDo7H623v1nU8Zz+twNqJAUr32PGnZebWZQ0bodvdzxP1ZVh6jw4wUNfuJVEnyv1z2XRFtMJXFnlyQ+jgOjNVvIa8fcerOEYhzau6hw/r7Dc77GUvru3qlIh+Ga7Ayp0+HqenUDi0ZO/mRPHrKWrOtj9KuHHrEe128FOXOSh6hPt3Fci72cl6W+mEjdpCr3BHuKQ6C1eeqIFDBLQlz3r8RA93MmQ3Pob2XDmvIo7cHQn/2ZgmtiGIkmxWUKV78og6W2HlJnaPALnUH0sjvJsCIIYB4ds X-Bogosity: Ham, tests=bogofilter, spamicity=0.000007, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> Let's assume a single subpage of a large folio is no longer mapped. >> Then, we'd have: >> >> nr_pages == folio_nr_pages(folio) - 1. >> >> You could simply map+reuse most of the folio without COWing. > > yes. This is good but the pte which is no longer mapped could be > anyone within the nr_pages PTEs. so it could be quite tricky for > set_ptes. The swap batching logic should take care of that, otherwise it would be buggy. > >> >> Once we support COW reuse of PTE-mapped THP we'd do the same. Here, it's >> just easy to detect that the folio is exclusive (folio_ref_count(folio) >> == 1 before mapping anything). >> >> If you really want to mimic what do_wp_page() currently does, you should >> have: >> >> exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio)) > > I actually dislike the part that do_wp_page() handles the reuse of a large > folio which is entirely mapped. For example, A forks B, B exit, we write > A's large folio, we get nr_pages CoW of small folios. Ideally, we can > reuse the whole folios for writing. Yes, see the link I shared to what I am working on. There isn't really a question if what we do right now needs to be improved and all these scenarios are pretty obvious clear. > >> >> Personally, I think we should keep it simple here and use: >> >> exclusive || folio_ref_count(folio) == 1 > > I feel this is still better than > exclusive || (folio_ref_count(folio) == 1 && !folio_test_large(folio)) > as we reuse the whole large folio. the do_wp_page() behaviour > doesn't have this. Yes, but there is the comment "Same logic as in do_wp_page();". We already ran into issues having different COW reuse logic all over the place. For this case here, I don't care if we leave it as "exclusive || folio_ref_count(folio) == 1" But let's not try inventing new stuff here. -- Cheers, David / dhildenb