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[2003:cb:c724:4300:430f:1c83:1abc:1d66]) by smtp.gmail.com with ESMTPSA id r19-20020a05600c321300b004167b0819aasm7036068wmp.0.2024.04.11.07.42.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Apr 2024 07:42:36 -0700 (PDT) Message-ID: <30c6934c-d7a1-4ed2-91a9-34b282a0b839@redhat.com> Date: Thu, 11 Apr 2024 16:42:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free To: Ryan Roberts , Lance Yang Cc: akpm@linux-foundation.org, 21cnbao@gmail.com, mhocko@suse.com, fengwei.yin@intel.com, zokeefe@google.com, shy828301@gmail.com, xiehuan09@gmail.com, wangkefeng.wang@huawei.com, songmuchun@bytedance.com, peterx@redhat.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240408042437.10951-1-ioworker0@gmail.com> <20240408042437.10951-2-ioworker0@gmail.com> <38c4add8-53a2-49ca-9f1b-f62c2ee3e764@arm.com> <3cda8e87-7095-4aad-beb1-6a420912df34@arm.com> <8d674b15-ef74-4d96-bc27-8794f744517c@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-Queue-Id: 02AD7C0015 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 53jwax3rytezaumpmcughf8qxboemapz X-HE-Tag: 1712846562-749281 X-HE-Meta: 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 JC0nvfH8 4BcEMjjglsd0heoYGgAqe1osR/699f0332awM7yNX79BdCjY/j5phE0nFnfUDS9qRJVoAY1+ZRoOglP+bqwIRV1XyZ2woMjXP53VH5Bk7KDsaALuHAPd8j1YZx3f6/kNwDRwyg/v6/H75w8IhD7MLS7dD4ns5QCNtduWAAx8Z6K3F1H5iIxESjrieW3/6lvg9ICk6iZKAmWtXDboqzyqaew0Wrg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000229, 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 11.04.24 16:39, Ryan Roberts wrote: > On 11/04/2024 15:07, Lance Yang wrote: >> On Thu, Apr 11, 2024 at 9:48 PM Ryan Roberts wrote: >>> >>> [...] >>> >>>>>> + >>>>>> + if (!folio_trylock(folio)) >>>>>> + continue; >>>>> >>>>> This is still wrong. This should all be protected by the "if >>>>> (folio_test_swapcache(folio) || folio_test_dirty(folio))" as it was previously >>>>> so that you only call folio_trylock() if that condition is true. You are >>>>> unconditionally locking here, then unlocking, then relocking below if the >>>>> condition is met. Just put everything inside the condition and lock once. >>>> >>>> I'm not sure if it's safe to call folio_mapcount() without holding the >>>> folio lock. >>>> >>>> As mentioned earlier by David in the v2[1] >>>>> What could work for large folios is making sure that #ptes that map the >>>>> folio here correspond to the folio_mapcount(). And folio_mapcount() >>>>> should be called under folio lock, to avoid racing with swapout/migration. >>>> >>>> [1] https://lore.kernel.org/all/5cc05529-eb80-410e-bc26-233b0ba0b21f@redhat.com/ >>> >>> But I'm not suggesting that you should call folio_mapcount() without the lock. >>> I'm proposing this: >>> >>> if (folio_test_swapcache(folio) || folio_test_dirty(folio)) { >>> if (!folio_trylock(folio)) >>> continue; >>> /* >>> - * If folio is shared with others, we mustn't clear >>> - * the folio's dirty flag. >>> + * If we have a large folio at this point, we know it is >>> + * fully mapped so if its mapcount is the same as its >>> + * number of pages, it must be exclusive. >>> */ >>> - if (folio_mapcount(folio) != 1) { >>> + if (folio_mapcount(folio) != folio_nr_pages(folio)) { >>> folio_unlock(folio); >>> continue; >>> } >> >> IIUC, if the folio is clean and not in the swapcache, we still need to >> compare the number of batched PTEs against folio_mapcount(). > > Why? That's not how the old code worked. In fact the comment says that the > reason for the exclusive check is to avoid marking a dirty *folio* as clean if > shared; that would be bad because we could throw away data that others relied > upon. It's perfectly safe to clear the dirty flag from the *pte* even if it is > shared; the ptes are private to the process so that won't affect sharers. > > You should just follow the pattern already estabilished by the original code. > The only difference is that because the folio is now (potentially) large, you > have to change the way to detect exclusivity. +1 -- Cheers, David / dhildenb