linux-kernel - Re: [PATCH v6 1/5] mm/zone_device: Reinitialize large zone device private folios

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20260116172052.GC961572@ziepe.ca>
Date: Fri, 16 Jan 2026 13:20:52 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Francois Dugast <francois.dugast@...el.com>,
	intel-xe@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	Matthew Brost <matthew.brost@...el.com>, Zi Yan <ziy@...dia.com>,
	Alistair Popple <apopple@...dia.com>,
	adhavan Srinivasan <maddy@...ux.ibm.com>,
	Nicholas Piggin <npiggin@...il.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
	Felix Kuehling <Felix.Kuehling@....com>,
	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>,
	David Hildenbrand <david@...nel.org>,
	Oscar Salvador <osalvador@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Leon Romanovsky <leon@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, Balbir Singh <balbirs@...dia.com>,
	linuxppc-dev@...ts.ozlabs.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
	nouveau@...ts.freedesktop.org, linux-mm@...ck.org,
	linux-cxl@...r.kernel.org
Subject: Re: [PATCH v6 1/5] mm/zone_device: Reinitialize large zone device
 private folios

On Fri, Jan 16, 2026 at 05:07:09PM +0100, Vlastimil Babka wrote:
> On 1/16/26 12:10, Francois Dugast wrote:
> > From: Matthew Brost <matthew.brost@...el.com>
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 63c6ab4fdf08..ac7be07e3361 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -477,10 +477,43 @@ void free_zone_device_folio(struct folio *folio)
> >  	}
> >  }
> >  
> > -void zone_device_page_init(struct page *page, unsigned int order)
> > +void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
> > +			   unsigned int order)
> >  {
> > +	struct page *new_page = page;
> > +	unsigned int i;
> > +
> >  	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >  
> > +	for (i = 0; i < (1UL << order); ++i, ++new_page) {
> > +		struct folio *new_folio = (struct folio *)new_page;
> > +
> > +		/*
> > +		 * new_page could have been part of previous higher order folio
> > +		 * which encodes the order, in page + 1, in the flags bits. We
> > +		 * blindly clear bits which could have set my order field here,
> > +		 * including page head.
> > +		 */
> > +		new_page->flags.f &= ~0xffUL;	/* Clear possible order, page head */
> > +
> > +#ifdef NR_PAGES_IN_LARGE_FOLIO
> > +		/*
> > +		 * This pointer math looks odd, but new_page could have been
> > +		 * part of a previous higher order folio, which sets _nr_pages
> > +		 * in page + 1 (new_page). Therefore, we use pointer casting to
> > +		 * correctly locate the _nr_pages bits within new_page which
> > +		 * could have modified by previous higher order folio.
> > +		 */
> > +		((struct folio *)(new_page - 1))->_nr_pages = 0;
> > +#endif
> > +
> > +		new_folio->mapping = NULL;
> > +		new_folio->pgmap = pgmap;	/* Also clear compound head */
> > +		new_folio->share = 0;   /* fsdax only, unused for device private */
> > +		VM_WARN_ON_FOLIO(folio_ref_count(new_folio), new_folio);
> > +		VM_WARN_ON_FOLIO(!folio_is_zone_device(new_folio), new_folio);
> > +	}
> > +
> >  	/*
> >  	 * Drivers shouldn't be allocating pages after calling
> >  	 * memunmap_pages().
> 
> Can't say I'm a fan of this. It probably works now (so I'm not nacking) but
> seems rather fragile. It seems likely to me somebody will try to change some
> implementation detail in the page allocator and not notice it breaks this,
> for example. I hope we can eventually get to something more robust.

These pages shouldn't be in the buddy allocator at all? The driver
using the ZONE_DEVICE pages is responsible to provide its own
allocator.

Did you mean something else?

Jason