Re: [PATCH v1 12/29] mm/zsmalloc: stop using __ClearPageMovable()

Sergey Senozhatsky
2025-07-03T02:28:37Z

On (25/07/02 12:55), David Hildenbrand wrote:
> On 02.07.25 12:10, Sergey Senozhatsky wrote:
> > On (25/07/02 10:25), David Hildenbrand wrote:
> > > On 02.07.25 10:11, Sergey Senozhatsky wrote:
> > > > On (25/06/30 14:59), David Hildenbrand wrote:
> > > > [..]
> > > > > static int zs_page_migrate(struct page *newpage, struct page *page,
> > > > > @@ -1736,6 +1736,13 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
> > > > > unsigned long old_obj, new_obj;
> > > > > unsigned int obj_idx;
> > > > > + /*
> > > > > + * TODO: nothing prevents a zspage from getting destroyed while
> > > > > + * isolated: we should disallow that and defer it.
> > > > > + */
> > > >
> > > > Can you elaborate?
> > >
> > > We can only free a zspage in free_zspage() while the page is locked.
> > >
> > > After we isolated a zspage page for migration (under page lock!), we drop
> > ^^ a physical page? (IOW zspage chain page?)
> >
> > > the lock again, to retake the lock when trying to migrate it.
> > >
> > > That means, there is a window where a zspage can be freed although the page
> > > is isolated for migration.
> >
> > I see, thanks. Looks somewhat fragile. Is this a new thing?
>
> No, it's been like that forever. And I was surprised that only zsmalloc
> behaves that way

Oh, that makes two of us.

> > > While we currently keep that working (as far as I can see), in the future we
> > > want to remove that support from the core.
> >
> > Maybe comment can more explicitly distinguish zspage isolation and
> > physical page (zspage chain) isolation? zspages can get isolated
> > for compaction (defragmentation), for instance, which is a different
> > form of isolation.
>
> Well, it's confusing, as we have MM compaction (-> migration) and apparently
> zs_compact.

True.

> I'll try to clarify that we are talking about isolation for page migration
> purposes.

Thanks.