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[2003:cb:c749:6600:b73a:466c:e610:686]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43656b0145csm55403945e9.15.2024.12.19.07.50.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Dec 2024 07:50:11 -0800 (PST) Message-ID: <49dc2ccc-888f-4e40-bad9-ba014381554e@redhat.com> Date: Thu, 19 Dec 2024 16:50:10 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings To: Zi Yan Cc: Joanne Koong , miklos@szeredi.hu, linux-fsdevel@vger.kernel.org, shakeel.butt@linux.dev, jefflexu@linux.alibaba.com, josef@toxicpanda.com, bernd.schubert@fastmail.fm, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Oscar Salvador , Michal Hocko References: <20241122232359.429647-1-joannelkoong@gmail.com> <20241122232359.429647-5-joannelkoong@gmail.com> <485BC133-98F3-4E57-B459-A5C424428C0F@nvidia.com> <90C41581-179F-40B6-9801-9C9DBBEB1AF4@nvidia.com> <96EA65A5-EA67-4245-95DA-D0DAD7BE2E47@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: EC6F510000F X-Stat-Signature: groghqkdi94j959qa76ypo1u8y3wndf3 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1734623351-802345 X-HE-Meta: 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 ZaROvVEZ nEvSogF6E6oru0Pbmz8GHimIJ6xeoNIUn7F5rAkLGzA1DFl/+2vgfOEvAmEcI1QFb5mcPjNjYQJcRg7H7g6m1/e7OoU5sAGeWxGoUg/R0BBXLRJOBAJiKW8pbo71yTYfWjN19McbBt5skQVtaj+bZ8sd/qa7rt/MM05Mj5/jDBRLguY9pL9TXxQst9orGaROKaWFTS7XTra/TjkkEfTX32R/MPh4ecVs/O12hXUw56AvA55tAw5ce9wcFJ/JRW+8pgqjbCgW0le4r8fB2Fvi7pn04RnDPVq/+uObVXtCD52cD55xHpeWk8oOsjeBDmL2k7B7oQT44/mWxmuSaTHcVzSy1Ocg0YBGpZCfVl2uGfSmFwQfTaUy8lFBbsjjFE9Jq3RWyGqbWi3EcTusFtFHGidHaJqc0IVyse3vaSwnGJ+Ff4wjd7XHAretxa3O51HCBD6+LDiK+F0exJBREeF1Fd7pttA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.009558, 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 19.12.24 16:47, Zi Yan wrote: > On 19 Dec 2024, at 10:39, David Hildenbrand wrote: > >> On 19.12.24 16:08, Zi Yan wrote: >>> On 19 Dec 2024, at 9:19, Zi Yan wrote: >>> >>>> On 19 Dec 2024, at 8:05, David Hildenbrand wrote: >>>> >>>>> On 23.11.24 00:23, Joanne Koong wrote: >>>>>> For migrations called in MIGRATE_SYNC mode, skip migrating the folio if >>>>>> it is under writeback and has the AS_WRITEBACK_INDETERMINATE flag set on its >>>>>> mapping. If the AS_WRITEBACK_INDETERMINATE flag is set on the mapping, the >>>>>> writeback may take an indeterminate amount of time to complete, and >>>>>> waits may get stuck. >>>>>> >>>>>> Signed-off-by: Joanne Koong >>>>>> Reviewed-by: Shakeel Butt >>>>>> --- >>>>>> mm/migrate.c | 5 ++++- >>>>>> 1 file changed, 4 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/migrate.c b/mm/migrate.c >>>>>> index df91248755e4..fe73284e5246 100644 >>>>>> --- a/mm/migrate.c >>>>>> +++ b/mm/migrate.c >>>>>> @@ -1260,7 +1260,10 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, >>>>>> */ >>>>>> switch (mode) { >>>>>> case MIGRATE_SYNC: >>>>>> - break; >>>>>> + if (!src->mapping || >>>>>> + !mapping_writeback_indeterminate(src->mapping)) >>>>>> + break; >>>>>> + fallthrough; >>>>>> default: >>>>>> rc = -EBUSY; >>>>>> goto out; >>>>> >>>>> Ehm, doesn't this mean that any fuse user can essentially completely block CMA allocations, memory compaction, memory hotunplug, memory poisoning... ?! >>>>> >>>>> That sounds very bad. >>>> >>>> Yeah, these writeback folios become unmovable. It makes memory fragmentation >>>> unrecoverable. I do not know why AS_WRITEBACK_INDETERMINATE is allowed, since >>>> it is essentially a forever pin to writeback folios. Why not introduce a >>>> retry and timeout mechanism instead of waiting for the writeback forever? >>> >>> If there is no way around such indeterminate writebacks, to avoid fragment memory, >>> these to-be-written-back folios should be migrated to a physically contiguous region. Either you have a preallocated region or get free pages from MIGRATE_UNMOVABLE. >> >> But at what point? > > Before each writeback. And there should be a limit on the amount of unmovable > pages they can allocate. The question is if that is than still a performance win :) But yes, we can avoid another migration if we are already on allows-movable memory. > >> >> We surely don't want to make fuse consume only effectively-unmovable memory. > > Yes, that is undesirable, but the folio under writeback cannot be migrated, > since migration needs to wait until its finish. Right, and currently that works by immediately marking the folio clean again (IIUC after reading the cover letter). -- Cheers, David / dhildenb