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

[Sergey Senozhatsky]

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.