linux-kernel - Re: [PATCH v1 03/29] mm/zsmalloc: drop PageIsolated() related VM_BUG

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Message-ID: <8b64f02a-8afb-40e6-bcfc-3eb90cd58e0a@lucifer.local>
Date: Tue, 1 Jul 2025 09:57:27 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
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Subject: Re: [PATCH v1 03/29] mm/zsmalloc: drop PageIsolated() related
 VM_BUG_ONs

On Tue, Jul 01, 2025 at 10:03:57AM +0200, David Hildenbrand wrote:
> > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > > index 999b513c7fdff..7f1431f2be98f 100644
> > > --- a/mm/zsmalloc.c
> > > +++ b/mm/zsmalloc.c
> > > @@ -1719,8 +1719,6 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
> > >   	 * Page is locked so zspage couldn't be destroyed. For detail, look at
> > >   	 * lock_zspage in free_zspage.
> > >   	 */
> > > -	VM_BUG_ON_PAGE(PageIsolated(page), page);
> > > -
> > >   	return true;
> > >   }
> > >
> > > @@ -1739,8 +1737,6 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
> > >   	unsigned long old_obj, new_obj;
> > >   	unsigned int obj_idx;
> > >
> > > -	VM_BUG_ON_PAGE(!zpdesc_is_isolated(zpdesc), zpdesc_page(zpdesc));
> > > -
> > >   	/* The page is locked, so this pointer must remain valid */
> > >   	zspage = get_zspage(zpdesc);
> > >   	pool = zspage->pool;
> > > @@ -1811,7 +1807,6 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
> > >
> > >   static void zs_page_putback(struct page *page)
> > >   {
> > > -	VM_BUG_ON_PAGE(!PageIsolated(page), page);
> > >   }
> >
> > Can we just drop zs_page_putback from movable_operations() now this is empty?
>
> Common code expects there to be a callback, and I don't want to change that.
> Long-term I assume it will rather indicate a BUG if there is no putback
> handler, not something we want to encourage.
>
> Likely, once we rework that isolated pages cannot get freed here, we'd have
> to handle stuff on the putback path (realize that the page can be freed and
> free it) -- TODO for that is added in #12.

Ack thanks!