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

Sergey Senozhatsky
2025-07-03T03:22:36Z

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.