Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings

[David Hildenbrand <david@redhat.com>]

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.

-- 
Cheers,

David / dhildenb