On 12/22/24 12:23 AM, David Hildenbrand wrote: > On 21.12.24 03:28, Jingbo Xu wrote: >> >> >> On 12/21/24 2:01 AM, Shakeel Butt wrote: >>> On Fri, Dec 20, 2024 at 03:49:39PM +0100, David Hildenbrand wrote: >>>>>> I'm wondering if there would be a way to just "cancel" the >>>>>> writeback and >>>>>> mark the folio dirty again. That way it could be migrated, but not >>>>>> reclaimed. At least we could avoid the whole >>>>>> AS_WRITEBACK_INDETERMINATE >>>>>> thing. >>>>>> >>>>> >>>>> That is what I basically meant with short timeouts. Obviously it is >>>>> not >>>>> that simple to cancel the request and to retry - it would add in quite >>>>> some complexity, if all the issues that arise can be solved at all. >>>> >>>> At least it would keep that out of core-mm. >>>> >>>> AS_WRITEBACK_INDETERMINATE really has weird smell to it ... we >>>> should try to >>>> improve such scenarios, not acknowledge and integrate them, then >>>> work around >>>> using timeouts that must be manually configured, and ca likely no be >>>> default >>>> enabled because it could hurt reasonable use cases :( >>> >>> Just to be clear AS_WRITEBACK_INDETERMINATE is being used in two core-mm >>> parts. First is reclaim and second is compaction/migration. For reclaim, >>> it is a must have as explained by Jingbo in [1] i.e. due to potential >>> self deadlock by fuse server. If I understand you correctly, the main >>> concern you have is its usage in the second case. >>> >>> The reason for adding AS_WRITEBACK_INDETERMINATE in the second case was >>> to avoid untrusted fuse server causing pain to unrelated jobs on the >>> machine (fuse folks please correct me if I am wrong here). >> >> Right, IIUC direct MIGRATE_SYNC migration won't be triggered on the >> memory allocation path, i.e. the fuse server itself won't stumble into >> MIGRATE_SYNC migration. >> > > Maybe memory compaction (on higher-order allocations only) could trigger > it? > > gfp_compaction_allowed() checks __GFP_IO. GFP_KERNEL includes that. > But that (memory compaction on memory allocation, which can be triggered in the fuse server process context) only triggers MIGRATE_SYNC_LIGHT, which won't wait for writeback. AFAICS, MIGRATE_SYNC can be triggered during cma allocation, memory offline, or node compaction manually through sysctl. -- Thanks, Jingbo
