On 2025-09-26 at 01:27 +1000, Zi Yan <[email protected]> wrote... > On 24 Sep 2025, at 19:45, Alistair Popple wrote: > > > On 2025-09-25 at 03:49 +1000, Zi Yan <[email protected]> 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 <[email protected]> > >>>>>>>> Cc: David Hildenbrand <[email protected]> > >>>>>>>> Cc: Zi Yan <[email protected]> > >>>>>>>> Cc: Joshua Hahn <[email protected]> > >>>>>>>> Cc: Rakie Kim <[email protected]> > >>>>>>>> Cc: Byungchul Park <[email protected]> > >>>>>>>> Cc: Gregory Price <[email protected]> > >>>>>>>> Cc: Ying Huang <[email protected]> > >>>>>>>> Cc: Alistair Popple <[email protected]> > >>>>>>>> Cc: Oscar Salvador <[email protected]> > >>>>>>>> Cc: Lorenzo Stoakes <[email protected]> > >>>>>>>> Cc: Baolin Wang <[email protected]> > >>>>>>>> Cc: "Liam R. Howlett" <[email protected]> > >>>>>>>> Cc: Nico Pache <[email protected]> > >>>>>>>> Cc: Ryan Roberts <[email protected]> > >>>>>>>> Cc: Dev Jain <[email protected]> > >>>>>>>> Cc: Barry Song <[email protected]> > >>>>>>>> Cc: Lyude Paul <[email protected]> > >>>>>>>> Cc: Danilo Krummrich <[email protected]> > >>>>>>>> Cc: David Airlie <[email protected]> > >>>>>>>> Cc: Simona Vetter <[email protected]> > >>>>>>>> Cc: Ralph Campbell <[email protected]> > >>>>>>>> Cc: Mika Penttilä <[email protected]> > >>>>>>>> Cc: Matthew Brost <[email protected]> > >>>>>>>> Cc: Francois Dugast <[email protected]> > >>>>>>>> --- > >>>>>>>> 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
