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Jul 2023 01:53:08 +0000 Message-ID: <65523232-58d4-a8c7-50ff-8f44f7ac23fc@intel.com> Date: Thu, 20 Jul 2023 09:52:58 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Firefox/102.0 Thunderbird/102.13.0 Subject: Re: [RFC PATCH v2 3/3] mm: mlock: update mlock_pte_range to handle large folio To: Hugh Dickins CC: Yosry Ahmed , Yu Zhao , , , , , , , References: <20230712060144.3006358-1-fengwei.yin@intel.com> <40cbc39e-5179-c2f4-3cea-0a98395aaff1@intel.com> <16844254-7248-f557-b1eb-b8b102c877a2@intel.com> <208aff10-8a32-6ab8-f03a-7f3c9d3ca0f7@intel.com> <438d6f6d-2571-69d9-844e-9af9e6b4f820@intel.com> <79f6822-f2f8-aba4-b517-b661d07e2d@google.com> Content-Language: en-US From: "Yin, Fengwei" In-Reply-To: <79f6822-f2f8-aba4-b517-b661d07e2d@google.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SI1PR02CA0012.apcprd02.prod.outlook.com (2603:1096:4:1f7::8) To CO1PR11MB4820.namprd11.prod.outlook.com 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3h50fYAvTaATBl2RPY8/z5zFJTzcqUI/M4D7LgvF14DtriCglZCo9grp5TO6VULnqtBkAR9aCnWpcSMNYzbhteHmB7tQsWxUqhacPnWbRXo7y0uhYI31xE+/Jl0DR/Dj4qpPNh3vdoVHq+elCDrMMPrqDw/wo7kFl6HqWyxeV7/xkpJ0MNRpxSXiKMdI1seuWjd1WrK+EPW/Ix+s5L0j/CoV5HEYRQayDCKEAyqoQZ/N3aSpd5QS28AlJO0vX2N9+MYnh4RVIzCP8gegh5XrZJjylQd+YHwb7OqS8+ZQg3Gnk6XZjv9eJd/ie97XSam7lJE3oq/nxue2ITeQ= 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: On 7/19/2023 10:26 PM, Hugh Dickins wrote: > On Wed, 19 Jul 2023, Yin Fengwei wrote: >>>>>>>>>>> Could this also happen against normal 4K page? I mean when user try to munlock >>>>>>>>>>> a normal 4K page and this 4K page is isolated. So it become unevictable page? >>>>>>>>>> Looks like it can be possible. If cpu 1 is in __munlock_folio() and >>>>>>>>>> cpu 2 is isolating the folio for any purpose: >>>>>>>>>> >>>>>>>>>> cpu1 cpu2 >>>>>>>>>> isolate folio >>>>>>>>>> folio_test_clear_lru() // 0 >>>>>>>>>> putback folio // add to unevictable list >>>>>>>>>> folio_test_clear_mlocked() >>>>>>> folio_set_lru() >> Let's wait the response from Huge and Yu. :). > > I haven't been able to give it enough thought, but I suspect you are right: > that the current __munlock_folio() is deficient when folio_test_clear_lru() > fails. > > (Though it has not been reported as a problem in practice: perhaps because > so few places try to isolate from the unevictable "list".) > > I forget what my order of development was, but it's likely that I first > wrote the version for our own internal kernel - which used our original > lruvec locking, which did not depend on getting PG_lru first (having got > lru_lock, it checked memcg, then tried again if that had changed). > > I was uneasy with the PG_lru aspect of upstream lru_lock implementation, > but it turned out to work okay - elsewhere; but it looks as if I missed > its implication when adapting __munlock_page() for upstream. > > If I were trying to fix this __munlock_folio() race myself (sorry, I'm > not), I would first look at that aspect: instead of folio_test_clear_lru() > behaving always like a trylock, could "folio_wait_clear_lru()" or whatever > spin waiting for PG_lru here? Considering following sequence: CPU1 (migration) CPU2 (mlock) isolation page (clear lru) mlock_pte_range try_to_migrate -> take_pte_lock try_to_migrate_one munlock_folio pvmw -> take pte lock __munlock_folio if batch full folio_wait_clear_lru deadlock may happen. Regards Yin, Fengwei > > Hugh