Re: [v6 01/15] mm/zone_device: support large zone device private folios

[Alistair Popple <apopple@nvidia.com>]

On 2025-09-26 at 01:27 +1000, Zi Yan <ziy@nvidia.com> wrote...
> On 24 Sep 2025, at 19:45, Alistair Popple wrote:
> 
> > On 2025-09-25 at 03:49 +1000, Zi Yan <ziy@nvidia.com> wrote...
> >> On 24 Sep 2025, at 7:04, David Hildenbrand wrote:
> >>
> >>> On 23.09.25 05:47, Balbir Singh wrote:
> >>>> On 9/19/25 23:26, Zi Yan wrote:
> >>>>> On 19 Sep 2025, at 1:01, Balbir Singh wrote:
> >>>>>
> >>>>>> On 9/18/25 12:49, Zi Yan wrote:
> >>>>>>> On 16 Sep 2025, at 8:21, Balbir Singh wrote:
> >>>>>>>
> >>>>>>>> Add routines to support allocation of large order zone device folios
> >>>>>>>> and helper functions for zone device folios, to check if a folio is
> >>>>>>>> device private and helpers for setting zone device data.
> >>>>>>>>
> >>>>>>>> When large folios are used, the existing page_free() callback in
> >>>>>>>> pgmap is called when the folio is freed, this is true for both
> >>>>>>>> PAGE_SIZE and higher order pages.
> >>>>>>>>
> >>>>>>>> Zone device private large folios do not support deferred split and
> >>>>>>>> scan like normal THP folios.
> >>>>>>>>
> >>>>>>>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> >>>>>>>> Cc: David Hildenbrand <david@redhat.com>
> >>>>>>>> Cc: Zi Yan <ziy@nvidia.com>
> >>>>>>>> Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
> >>>>>>>> Cc: Rakie Kim <rakie.kim@sk.com>
> >>>>>>>> Cc: Byungchul Park <byungchul@sk.com>
> >>>>>>>> Cc: Gregory Price <gourry@gourry.net>
> >>>>>>>> Cc: Ying Huang <ying.huang@linux.alibaba.com>
> >>>>>>>> Cc: Alistair Popple <apopple@nvidia.com>
> >>>>>>>> Cc: Oscar Salvador <osalvador@suse.de>
> >>>>>>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >>>>>>>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> >>>>>>>> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> >>>>>>>> Cc: Nico Pache <npache@redhat.com>
> >>>>>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
> >>>>>>>> Cc: Dev Jain <dev.jain@arm.com>
> >>>>>>>> Cc: Barry Song <baohua@kernel.org>
> >>>>>>>> Cc: Lyude Paul <lyude@redhat.com>
> >>>>>>>> Cc: Danilo Krummrich <dakr@kernel.org>
> >>>>>>>> Cc: David Airlie <airlied@gmail.com>
> >>>>>>>> Cc: Simona Vetter <simona@ffwll.ch>
> >>>>>>>> Cc: Ralph Campbell <rcampbell@nvidia.com>
> >>>>>>>> Cc: Mika Penttilä <mpenttil@redhat.com>
> >>>>>>>> Cc: Matthew Brost <matthew.brost@intel.com>
> >>>>>>>> Cc: Francois Dugast <francois.dugast@intel.com>
> >>>>>>>> ---
> >>>>>>>>   include/linux/memremap.h | 10 +++++++++-
> >>>>>>>>   mm/memremap.c            | 34 +++++++++++++++++++++-------------
> >>>>>>>>   mm/rmap.c                |  6 +++++-
> >>>>>>>>   3 files changed, 35 insertions(+), 15 deletions(-)
> >>>>>>>>
> >>>>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> >>>>>>>> index e5951ba12a28..9c20327c2be5 100644
> >>>>>>>> --- a/include/linux/memremap.h
> >>>>>>>> +++ b/include/linux/memremap.h
> >>>>>>>> @@ -206,7 +206,7 @@ static inline bool is_fsdax_page(const struct page *page)
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>>   #ifdef CONFIG_ZONE_DEVICE
> >>>>>>>> -void zone_device_page_init(struct page *page);
> >>>>>>>> +void zone_device_folio_init(struct folio *folio, unsigned int order);
> >>>>>>>>   void *memremap_pages(struct dev_pagemap *pgmap, int nid);
> >>>>>>>>   void memunmap_pages(struct dev_pagemap *pgmap);
> >>>>>>>>   void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
> >>>>>>>> @@ -215,6 +215,14 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn);
> >>>>>>>>   bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
> >>>>>>>>
> >>>>>>>>   unsigned long memremap_compat_align(void);
> >>>>>>>> +
> >>>>>>>> +static inline void zone_device_page_init(struct page *page)
> >>>>>>>> +{
> >>>>>>>> +	struct folio *folio = page_folio(page);
> >>>>>>>> +
> >>>>>>>> +	zone_device_folio_init(folio, 0);
> >>>>>>>
> >>>>>>> I assume it is for legacy code, where only non-compound page exists?
> >>>>>>>
> >>>>>>> It seems that you assume @page is always order-0, but there is no check
> >>>>>>> for it. Adding VM_WARN_ON_ONCE_FOLIO(folio_order(folio) != 0, folio)
> >>>>>>> above it would be useful to detect misuse.
> >>>>>>>
> >>>>>>>> +}
> >>>>>>>> +
> >>>>>>>>   #else
> >>>>>>>>   static inline void *devm_memremap_pages(struct device *dev,
> >>>>>>>>   		struct dev_pagemap *pgmap)
> >>>>>>>> diff --git a/mm/memremap.c b/mm/memremap.c
> >>>>>>>> index 46cb1b0b6f72..a8481ebf94cc 100644
> >>>>>>>> --- a/mm/memremap.c
> >>>>>>>> +++ b/mm/memremap.c
> >>>>>>>> @@ -416,20 +416,19 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
> >>>>>>>>   void free_zone_device_folio(struct folio *folio)
> >>>>>>>>   {
> >>>>>>>>   	struct dev_pagemap *pgmap = folio->pgmap;
> >>>>>>>> +	unsigned long nr = folio_nr_pages(folio);
> >>>>>>>> +	int i;
> >>>>>>>>
> >>>>>>>>   	if (WARN_ON_ONCE(!pgmap))
> >>>>>>>>   		return;
> >>>>>>>>
> >>>>>>>>   	mem_cgroup_uncharge(folio);
> >>>>>>>>
> >>>>>>>> -	/*
> >>>>>>>> -	 * Note: we don't expect anonymous compound pages yet. Once supported
> >>>>>>>> -	 * and we could PTE-map them similar to THP, we'd have to clear
> >>>>>>>> -	 * PG_anon_exclusive on all tail pages.
> >>>>>>>> -	 */
> >>>>>>>>   	if (folio_test_anon(folio)) {
> >>>>>>>> -		VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> >>>>>>>> -		__ClearPageAnonExclusive(folio_page(folio, 0));
> >>>>>>>> +		for (i = 0; i < nr; i++)
> >>>>>>>> +			__ClearPageAnonExclusive(folio_page(folio, i));
> >>>>>>>> +	} else {
> >>>>>>>> +		VM_WARN_ON_ONCE(folio_test_large(folio));
> >>>>>>>>   	}
> >>>>>>>>
> >>>>>>>>   	/*
> >>>>>>>> @@ -456,8 +455,8 @@ void free_zone_device_folio(struct folio *folio)
> >>>>>>>>   	case MEMORY_DEVICE_COHERENT:
> >>>>>>>>   		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->page_free))
> >>>>>>>>   			break;
> >>>>>>>> -		pgmap->ops->page_free(folio_page(folio, 0));
> >>>>>>>> -		put_dev_pagemap(pgmap);
> >>>>>>>> +		pgmap->ops->page_free(&folio->page);
> >>>>>>>> +		percpu_ref_put_many(&folio->pgmap->ref, nr);
> >>>>>>>>   		break;
> >>>>>>>>
> >>>>>>>>   	case MEMORY_DEVICE_GENERIC:
> >>>>>>>> @@ -480,14 +479,23 @@ void free_zone_device_folio(struct folio *folio)
> >>>>>>>>   	}
> >>>>>>>>   }
> >>>>>>>>
> >>>>>>>> -void zone_device_page_init(struct page *page)
> >>>>>>>> +void zone_device_folio_init(struct folio *folio, unsigned int order)
> >>>>>>>>   {
> >>>>>>>> +	struct page *page = folio_page(folio, 0);
> >>>>>>>
> >>>>>>> It is strange to see a folio is converted back to page in
> >>>>>>> a function called zone_device_folio_init().
> >>>>>>>
> >>>>>>>> +
> >>>>>>>> +	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >>>>>>>> +
> >>>>>>>>   	/*
> >>>>>>>>   	 * Drivers shouldn't be allocating pages after calling
> >>>>>>>>   	 * memunmap_pages().
> >>>>>>>>   	 */
> >>>>>>>> -	WARN_ON_ONCE(!percpu_ref_tryget_live(&page_pgmap(page)->ref));
> >>>>>>>> -	set_page_count(page, 1);
> >>>>>>>> +	WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order));
> >>>>>>>> +	folio_set_count(folio, 1);
> >>>>>>>>   	lock_page(page);
> >>>>>>>> +
> >>>>>>>> +	if (order > 1) {
> >
> > Why is this only called for order > 1 rather than order > 0 ?
> >
> >>>>>>>> +		prep_compound_page(page, order);
> >>>>>>>> +		folio_set_large_rmappable(folio);
> >>>>>>>> +	}
> >>>>>>>
> >>>>>>> OK, so basically, @folio is not a compound page yet when zone_device_folio_init()
> >>>>>>> is called.
> >>>>>>>
> >>>>>>> I feel that your zone_device_page_init() and zone_device_folio_init()
> >>>>>>> implementations are inverse. They should follow the same pattern
> >>>>>>> as __alloc_pages_noprof() and __folio_alloc_noprof(), where
> >>>>>>> zone_device_page_init() does the actual initialization and
> >>>>>>> zone_device_folio_init() just convert a page to folio.
> >>>>>>>
> >>>>>>> Something like:
> >>>>>>>
> >>>>>>> void zone_device_page_init(struct page *page, unsigned int order)
> >>>>>>> {
> >>>>>>> 	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >>>>>>>
> >>>>>>> 	/*
> >>>>>>> 	 * Drivers shouldn't be allocating pages after calling
> >>>>>>> 	 * memunmap_pages().
> >>>>>>> 	 */
> >>>>>>>
> >>>>>>>      WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order));
> >>>>>>> 	
> >>>>>>> 	/*
> >>>>>>> 	 * anonymous folio does not support order-1, high order file-backed folio
> >>>>>>> 	 * is not supported at all.
> >>>>>>> 	 */
> >
> > I guess that answers my question :-)
> >
> >>>>>>> 	VM_WARN_ON_ONCE(order == 1);
> >>>>>>>
> >>>>>>> 	if (order > 1)
> >>>>>>> 		prep_compound_page(page, order);
> >>>>>>>
> >>>>>>> 	/* page has to be compound head here */
> >>>>>>> 	set_page_count(page, 1);
> >>>>>>> 	lock_page(page);
> >>>>>>> }
> >>>>>>>
> >>>>>>> void zone_device_folio_init(struct folio *folio, unsigned int order)
> >>>>>>> {
> >>>>>>> 	struct page *page = folio_page(folio, 0);
> >>>>>>>
> >>>>>>> 	zone_device_page_init(page, order);
> >>>>>>> 	page_rmappable_folio(page);
> >>>>>>> }
> >>>>>>>
> >>>>>>> Or
> >>>>>>>
> >>>>>>> struct folio *zone_device_folio_init(struct page *page, unsigned int order)
> >>>>>>> {
> >>>>>>> 	zone_device_page_init(page, order);
> >>>>>>> 	return page_rmappable_folio(page);
> >>>>>>> }
> >>>>>>>
> >>>>>>>
> >>>>>>> Then, it comes to free_zone_device_folio() above,
> >>>>>>> I feel that pgmap->ops->page_free() should take an additional order
> >>>>>>> parameter to free a compound page like free_frozen_pages().
> >>>>>>>
> >>>>>>>
> >>>>>>> This is my impression after reading the patch and zone device page code.
> >>>>>>>
> >>>>>>> Alistair and David can correct me if this is wrong, since I am new to
> >>>>>>> zone device page code.
> >>>>>>> 	
> >>>>>>
> >>>>>> Thanks, I did not want to change zone_device_page_init() for several
> >>>>>> drivers (outside my test scope) that already assume it has an order size of 0.
> >
> > It's a trivial change, so I don't think avoiding changes to other drivers should
> > be a concern.
> >
> >>>>>
> >>>>> But my proposed zone_device_page_init() should still work for order-0
> >>>>> pages. You just need to change call site to add 0 as a new parameter.
> >>>>>
> >>>>
> >>>> I did not want to change existing callers (increases testing impact)
> >>>> without a strong reason.
> >>>>
> >>>>>
> >>>>> One strange thing I found in the original zone_device_page_init() is
> >>>>> the use of page_pgmap() in
> >>>>> WARN_ON_ONCE(!percpu_ref_tryget_many(&page_pgmap(page)->ref, 1 << order)).
> >>>>> page_pgmap() calls page_folio() on the given page to access pgmap field.
> >>>>> And pgmap field is only available in struct folio. The code initializes
> >>>>> struct page, but in middle it suddenly finds the page is actually a folio,
> >>>>> then treat it as a page afterwards. I wonder if it can be done better.
> >>>>>
> >>>>> This might be a question to Alistair, since he made the change.
> >
> > Hello! I might be him :)
> >
> > I think this situation is just historical - when I originally wrote
> > zone_device_page_init() the pgmap was stored on the page rather than the folio.
> > That only changed fairly recently with commit 82ba975e4c43 ("mm: allow compound
> > zone device pages").
> >
> > The reason pgmap is now only available on the folio is described in the
> > commit log. The TLDR is switching FS DAX to use compound pages required
> > page->compound_head to be available for use, and that was being shared
> > with page->pgmap. So the solution was to move pgmap to the folio freeing up
> > page->compound_head for use on tail pages.
> >
> > The whole percpu pgmap->ref could actually now go away - I've debated removing
> > it but haven't found the motivation as it provides a small advantage on driver
> > tear down. Basically it just tracks how many pages are allocated in the pgmap
> > so drivers could use that to determine if they need to trigger migrations before
> > tearing down the pgmap.
> >
> > The alternative is just to loop over every page in the pgmap to ensure the
> > folio/page refcounts are 0 before tear down.
> >
> >>>>>
> >>>>
> >>>> I'll let him answer it :)
> >>>
> >>> Not him, but I think this goes back to my question raised in my other reply: When would we allocate "struct folio" in the future.
> >>>
> >>> If it's "always" then actually most of the zone-device code would only ever operate on folios and never on pages in the future.
> >>>
> >>> I recall during a discussion at LSF/MM I raised that, and the answer was (IIRC) that we will allocate "struct folio" as we will initialize the memmap for dax.
> >
> > Sounds about right.
> >
> >>> So essentially, we'd always have folios and would never really have to operate on pages.
> >
> > Yeah, I think I mentioned to Matthew at LSF/MM that I thought ZONE_DEVICE (and
> > in particular ZONE_DEVICE_PRIVATE) might be a good candidate to experiment with
> > removing struct pages entirely and switching to memdesc's or whatever. Because
> > we should, in theory at least, only need to operate on folio's. But I'm still a
> > little vague on the details how that would actually work. It's been on my TODO
> > list for a while, so myabe I will try and look at it for LPC as a healthy bit of
> > conference driven development.
> >
> >> Hmm, then what is the point of having “struct folio”, which originally is
> >> added to save compound_head() calls, where everything is a folio in device
> >> private world? We might need DAX people to explain the rationale of
> >> “always struct folio”.
> >
> > Longer term isn't there an aim to remove struct page? So I assumed moving to
> 
> Right. But my current impression based on my code reading and this patchset
> is that every device private page is a folio. To form a high order folio,
> each device private folio is converted to page, prep_compound*()’d, then
> converted back to folio. Based on what you said above, this weird conversion
> might be temporary until the code is switched to memdesc.
> 
> I am looking forward to more details on how device private will be switched
> to memdesc from you. :)

Thanks, so am I :-P

For device private I think the first step is to move away from using
pfn_to_page()/page_to_pfn() and instead create a "device pfn" that doesn't exist
in the physical direct map. That in itself would solve some problems (such as
supporting device private pages on ARM) and I hope to have something posted in
the next couple of weeks.

> > folio's was part of that effort. As you say though many of the clean-ups thus
> > far related to switching ZONE_DEVICE to folios have indeed just been about
> > removing compound_head() calls.
> 
> 
> 
> Best Regards,
> Yan, Zi