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

[David Hildenbrand]

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 -- balloon implements isolation as one would expect it 
(disallow freeing while isolated).

> > 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.

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

--
Cheers,

David / dhildenb