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

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Message-ID: <4e100125-ed70-4ec9-a0fa-5214bccd7cf3@suse.cz>
Date: Fri, 16 Jan 2026 18:27:37 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Jason Gunthorpe <jgg@...pe.ca>
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 1/16/26 18:20, Jason Gunthorpe wrote:
> 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?

Yeah sorry that was imprecise. I meant the struct page/folio layout
implementation details (which may or may not be related to the page allocator).

> Jason
>