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[2003:cb:c749:6600:b73a:466c:e610:686]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43656b119ccsm56313895e9.24.2024.12.19.08.31.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Dec 2024 08:31:15 -0800 (PST) Message-ID: <4104f64a-09c3-4f20-8e1a-5f4547fdcb25@redhat.com> Date: Thu, 19 Dec 2024 17:31:14 +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: Shakeel Butt , Zi Yan Cc: Bernd Schubert , Joanne Koong , miklos@szeredi.hu, linux-fsdevel@vger.kernel.org, jefflexu@linux.alibaba.com, josef@toxicpanda.com, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Oscar Salvador , Michal Hocko References: <0CF889CE-09ED-4398-88AC-920118D837A1@nvidia.com> <722A63E5-776E-4353-B3EE-DE202E4A4309@nvidia.com> <6FBDD501-25A0-4A21-8051-F8EE74AD177B@nvidia.com> <7qyun2waznrduxpf2i5eebqdvpigrd5ycu4rlpawu336kqkyvh@xmfmlsmr43gw> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: A61684001C X-Stat-Signature: 1474kbmqysic73wx3z9unzwrncmqeqpe X-Rspam-User: X-HE-Tag: 1734625846-383376 X-HE-Meta: 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 tG3VMKxl 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000006, 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 17:26, Shakeel Butt wrote: > On Thu, Dec 19, 2024 at 11:14:49AM -0500, Zi Yan wrote: >> On 19 Dec 2024, at 11:09, Bernd Schubert wrote: >> >>> On 12/19/24 17:02, Zi Yan wrote: >>>> On 19 Dec 2024, at 11:00, Zi Yan wrote: >>>>> On 19 Dec 2024, at 10:56, Bernd Schubert wrote: >>>>> >>>>>> On 12/19/24 16:55, Zi Yan wrote: >>>>>>> On 19 Dec 2024, at 10:53, Shakeel Butt wrote: >>>>>>> >>>>>>>> On Thu, Dec 19, 2024 at 04:47:18PM +0100, David Hildenbrand wrote: >>>>>>>>> On 19.12.24 16:43, Shakeel Butt wrote: >>>>>>>>>> On Thu, Dec 19, 2024 at 02:05:04PM +0100, 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. >>>>>>>>>> >>>>>>>>>> The page under writeback are already unmovable while they are under >>>>>>>>>> writeback. This patch is only making potentially unrelated tasks to >>>>>>>>>> synchronously wait on writeback completion for such pages which in worst >>>>>>>>>> case can be indefinite. This actually is solving an isolation issue on a >>>>>>>>>> multi-tenant machine. >>>>>>>>>> >>>>>>>>> Are you sure, because I read in the cover letter: >>>>>>>>> >>>>>>>>> "In the current FUSE writeback design (see commit 3be5a52b30aa ("fuse: >>>>>>>>> support writable mmap"))), a temp page is allocated for every dirty >>>>>>>>> page to be written back, the contents of the dirty page are copied over to >>>>>>>>> the temp page, and the temp page gets handed to the server to write back. >>>>>>>>> This is done so that writeback may be immediately cleared on the dirty >>>>>>>>> page," >>>>>>>>> >>>>>>>>> Which to me means that they are immediately movable again? >>>>>>>> >>>>>>>> Oh sorry, my mistake, yes this will become an isolation issue with the >>>>>>>> removal of the temp page in-between which this series is doing. I think >>>>>>>> the tradeoff is between extra memory plus slow write performance versus >>>>>>>> temporary unmovable memory. >>>>>>> >>>>>>> No, the tradeoff is slow FUSE performance vs whole system slowdown due to >>>>>>> memory fragmentation. AS_WRITEBACK_INDETERMINATE indicates it is not >>>>>>> temporary. >>>>>> >>>>>> Is there is a difference between FUSE TMP page being unmovable and >>>>>> AS_WRITEBACK_INDETERMINATE folios/pages being unmovable? >>>> >>>> (Fix my response location) >>>> >>>> Both are unmovable, but you can control where FUSE TMP page >>>> can come from to avoid spread across the entire memory space. For example, >>>> allocate a contiguous region as a TMP page pool. >>> >>> Wouldn't it make sense to have that for fuse writeback pages as well? >>> Fuse tries to limit dirty pages anyway. >> >> Can fuse constraint the location of writeback pages? Something like what >> I proposed[1], migrating pages to a location before their writeback? Will >> that be a performance concern? >> >> In terms of the number of dirty pages, you only need one page out of 512 >> pages to prevent 2MB THP from allocation. For CMA allocation, one unmovable >> page can kill one contiguous range. What is the limit of fuse dirty pages? >> >> [1] https://lore.kernel.org/linux-mm/90C41581-179F-40B6-9801-9C9DBBEB1AF4@nvidia.com/ > > I think this whole concern of fuse making system memory unmovable > forever is overblown. Fuse is already using a temp (unmovable) page Right, and we allocated in a way that we expect it to not be movable (e.g., not on ZONE_MOVABLE, usually in a UNMOVABLE pageblock etc). As another question, which effect does this change here have on folio_wait_writeback() users like arch/s390/kernel/uv.c or shrink_folio_list()? -- Cheers, David / dhildenb