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

[David Hildenbrand]

On 03.07.25 05:22, Sergey Senozhatsky wrote:
> On (25/07/03 11:28), Sergey Senozhatsky 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.
>
> I sort of wonder if zs_page_migrate() VM_BUG_ON_PAGE() removal and
> zspage check addition need to be landed outside of this series, as
> a zsmalloc fixup.

Not sure if there is real value for that; given the review status, I 
assume this series won't take too long to be ready for upstream. Of 
course, if that is not the case we could try pulling them out.

--
Cheers,

David / dhildenb