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

[David Hildenbrand <david@redhat.com>]

On 02.01.25 19:54, Joanne Koong wrote:
> On Mon, Dec 30, 2024 at 12:11 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>>
>> On Mon, Dec 30, 2024 at 08:52:04PM +0100, David Hildenbrand wrote:
>>>
>> [...]
>>>> I'm looking back at some of the discussions in v2 [1] and I'm still
>>>> not clear on how memory fragmentation for non-movable pages differs
>>>> from memory fragmentation from movable pages and whether one is worse
>>>> than the other. Currently fuse uses movable temp pages (allocated with
>>>> gfp flags GFP_NOFS | __GFP_HIGHMEM), and these can run into the same
>>>
>>> Why are they movable? Do you also specify __GFP_MOVABLE?
>>>
>>> If not, they are unmovable and are never allocated from
>>> ZONE_MOVABLE/MIGRATE_CMA -- and usually only from MIGRATE_UNMOVBALE, to
>>> group these unmovable pages.
>>>
>>
>> Yes, these temp pages are non-movable. (Must be a typo in Joanne's
>> email).
> 
> Sorry for the confusion, that should have been "non-movable temp pages".
> 
>>
>> [...]
>>>
>>> I assume not regarding fragmentation.
>>>
>>>
>>> In general, I see two main issues:
>>>
>>> A) We are no longer waiting on writeback, even though we expect in sane
>>> environments that writeback will happen and we it might be worthwhile to
>>> just wait for writeback so we can migrate these folios.
>>>
>>> B) We allow turning movable pages to be unmovable, possibly forever/long
>>> time, and there is no way to make them movable again (e.g., cancel
>>> writeback).
>>>
>>>
>>> I'm wondering if A) is actually a new issue introduced by this change. Can
>>> folios with busy temp pages (writeback cleared on folio, but temp pages are
>>> still around) be migrated? I will look into some details once I'm back from
>>> vacation.
>>>
> 
> Folios with busy temp pages can be migrated since fuse will clear
> writeback on the folio immediately once it's copied to the temp page.

I was rather wondering if there is something else that prevents 
migrating these folios: for example, if there is a raised refcount on 
the folio while the temp pages exist. If that is not the case, then it 
should indeed just work.

> 
> To me, these two issues seem like one and the same. No longer waiting
> on writeback renders it unmovable, which prevents
> compaction/migration.
> 
>>
>> My suggestion is to just drop the patch related to A as it is not
>> required for deadlock avoidance. For B, I think we need a long term
>> solution which is usable by other filesystems as well.
> 
> Sounds good. With that, we need to take this patchset out of
> mm-unstable or this could lead to migration infinitely waiting on
> folio writeback without the migrate patch there.

I want to try triggering it with NFS next week when I am back from PTO, 
to see if it is indeed a problem there as well on connection loss.

In any case, having movable pages be turned unmovable due to persistent 
writaback is something that must be fixed, not worked around. Likely a 
good topic for LSF/MM.

-- 
Cheers,

David / dhildenb