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

[Joanne Koong <joannelkoong@gmail.com>]

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.

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.


Thanks,
Joanne