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Thu, 19 Dec 2024 10:56:55 -0500 (EST) Message-ID: Date: Thu, 19 Dec 2024 16:56:54 +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 , Shakeel Butt Cc: David Hildenbrand , 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: <20241122232359.429647-1-joannelkoong@gmail.com> <20241122232359.429647-5-joannelkoong@gmail.com> From: Bernd Schubert Content-Language: en-US, de-DE, fr In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Stat-Signature: 1h4ndt4t6whxyy63bhdy1azcedirai31 X-Rspamd-Queue-Id: C8A8240012 X-Rspam-User: X-HE-Tag: 1734623769-269807 X-HE-Meta: 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 Ws/zrFk+ aQnmM1BHhRP9qzs7PAYytussZ/R64kMpk/Gu+6SbDpgWYlNxETnx+iIRfDH9uM+hY2mo7NKlGlQ1KVoEInW2sWXNgKkYBvLDjyHXDbitebcOO8bZJS/IqQhlngVGcX/0D1OF+WbAFl1kmc1Xno15j/X1mtLC/yc4uXhxKvoU37bbKtmBENcQBV4CbP2Oqkk9AcUAGZg0yH46OuQwYqJqYwi7gSxDyXIFT/xLH52XDMWg2uet/XCGqtKE0LxbxfkDFaXVM/xVxTezXXl/5pn6athwUsALduTdCTArSAZynodPHE0emVNT9LZKaQ/FLrYy+9OFg+4rK+43dg1OikYRaMU26nV+2o41YwhmUDr9tlfzrnJGAGYHk0o5AKB79OAao0eJ8PIatV8lQCr9gR4Pb7nACAAZF1TnqwbKhih2t5RUoA9plCvBo8d34fEaFife/tu1jXrJpe/RlYoLY2D2wWWnoVA== 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: List-Subscribe: List-Unsubscribe: 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? Thanks, Bernd AS_WRITEBACK_INDETERMINATE