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jefflexu@linux.alibaba.com, josef@toxicpanda.com, linux-mm@kvack.org, kernel-team@meta.com, Matthew Wilcox , Oscar Salvador , Michal Hocko Subject: Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings Date: Thu, 19 Dec 2024 11:00:32 -0500 X-Mailer: MailMate (1.14r6065) Message-ID: <0CF889CE-09ED-4398-88AC-920118D837A1@nvidia.com> In-Reply-To: References: <20241122232359.429647-1-joannelkoong@gmail.com> <20241122232359.429647-5-joannelkoong@gmail.com> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BLAPR03CA0079.namprd03.prod.outlook.com (2603:10b6:208:329::24) To BL4PR12MB9478.namprd12.prod.outlook.com (2603:10b6:208:58e::9) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BL4PR12MB9478:EE_|BL1PR12MB5996:EE_ X-MS-Office365-Filtering-Correlation-Id: 93e3c1a7-2436-45b8-ac9a-08dd204644e0 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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For migrations called in MIGRATE_SYNC mode, skip migrating the fo= lio 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 map= ping, the >>>>>>> writeback may take an indeterminate amount of time to complete, a= nd >>>>>>> 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 =3D -EBUSY; >>>>>>> goto out; >>>>>> >>>>>> Ehm, doesn't this mean that any fuse user can essentially complete= ly block >>>>>> CMA allocations, memory compaction, memory hotunplug, memory poiso= ning... ?! >>>>>> >>>>>> 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 ("fus= e: >>>> support writable mmap"))), a temp page is allocated for every dirty >>>> page to be written back, the contents of the dirty page are copied o= ver 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 dir= ty >>>> page," >>>> >>>> Which to me means that they are immediately movable again? >>> >>> Oh sorry, my mistake, yes this will become an isolation issue with th= e >>> removal of the temp page in-between which this series is doing. I thi= nk >>> the tradeoff is between extra memory plus slow write performance vers= us >>> 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? 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.