Re: [PATCH v4 14/25] rmap: Add support for PUD sized mappings to rmap

[David Hildenbrand <david@redhat.com>]

On 17.12.24 06:12, Alistair Popple wrote:
> The rmap doesn't currently support adding a PUD mapping of a
> folio. This patch adds support for entire PUD mappings of folios,
> primarily to allow for more standard refcounting of device DAX
> folios. Currently DAX is the only user of this and it doesn't require
> support for partially mapped PUD-sized folios so we don't support for
> that for now.
> 
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> 
> ---
> 
> David - Thanks for your previous comments, I'm less familiar with the
> rmap code so I would appreciate you taking another look. In particular
> I haven't added a stat for PUD mapped folios as it seemed like
> overkill for just the device DAX case but let me know if you think
> otherwise.
> 
> Changes for v4:
> 
>   - New for v4, split out rmap changes as suggested by David.
> ---
>   include/linux/rmap.h | 15 ++++++++++++-
>   mm/rmap.c            | 56 +++++++++++++++++++++++++++++++++++++++++++++-
>   2 files changed, 71 insertions(+)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index 683a040..7043914 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -192,6 +192,7 @@ typedef int __bitwise rmap_t;
>   enum rmap_level {
>   	RMAP_LEVEL_PTE = 0,
>   	RMAP_LEVEL_PMD,
> +	RMAP_LEVEL_PUD,
>   };
>   
>   static inline void __folio_rmap_sanity_checks(const struct folio *folio,
> @@ -228,6 +229,14 @@ static inline void __folio_rmap_sanity_checks(const struct folio *folio,
>   		VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PMD_NR, folio);
>   		VM_WARN_ON_FOLIO(nr_pages != HPAGE_PMD_NR, folio);
>   		break;
> +	case RMAP_LEVEL_PUD:
> +		/*
> +		 * Assume that we are creating * a single "entire" mapping of the
> +		 * folio.
> +		 */
> +		VM_WARN_ON_FOLIO(folio_nr_pages(folio) != HPAGE_PUD_NR, folio);
> +		VM_WARN_ON_FOLIO(nr_pages != HPAGE_PUD_NR, folio);
> +		break;
>   	default:
>   		VM_WARN_ON_ONCE(true);
>   	}
> @@ -251,12 +260,16 @@ void folio_add_file_rmap_ptes(struct folio *, struct page *, int nr_pages,
>   	folio_add_file_rmap_ptes(folio, page, 1, vma)
>   void folio_add_file_rmap_pmd(struct folio *, struct page *,
>   		struct vm_area_struct *);
> +void folio_add_file_rmap_pud(struct folio *, struct page *,
> +		struct vm_area_struct *);
>   void folio_remove_rmap_ptes(struct folio *, struct page *, int nr_pages,
>   		struct vm_area_struct *);
>   #define folio_remove_rmap_pte(folio, page, vma) \
>   	folio_remove_rmap_ptes(folio, page, 1, vma)
>   void folio_remove_rmap_pmd(struct folio *, struct page *,
>   		struct vm_area_struct *);
> +void folio_remove_rmap_pud(struct folio *, struct page *,
> +		struct vm_area_struct *);
>   
>   void hugetlb_add_anon_rmap(struct folio *, struct vm_area_struct *,
>   		unsigned long address, rmap_t flags);
> @@ -341,6 +354,7 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
>   		atomic_add(orig_nr_pages, &folio->_large_mapcount);
>   		break;
>   	case RMAP_LEVEL_PMD:
> +	case RMAP_LEVEL_PUD:
>   		atomic_inc(&folio->_entire_mapcount);
>   		atomic_inc(&folio->_large_mapcount);
>   		break;
> @@ -437,6 +451,7 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
>   		atomic_add(orig_nr_pages, &folio->_large_mapcount);
>   		break;
>   	case RMAP_LEVEL_PMD:
> +	case RMAP_LEVEL_PUD:
>   		if (PageAnonExclusive(page)) {
>   			if (unlikely(maybe_pinned))
>   				return -EBUSY;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index c6c4d4e..39d0439 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1203,6 +1203,11 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
>   		}
>   		atomic_inc(&folio->_large_mapcount);
>   		break;
> +	case RMAP_LEVEL_PUD:
> +		/* We only support entire mappings of PUD sized folios in rmap */
> +		atomic_inc(&folio->_entire_mapcount);
> +		atomic_inc(&folio->_large_mapcount);
> +		break;


This way you don't account the pages at all as mapped, whereby PTE-mapping it
would? And IIRC, these PUD-sized pages can be either mapped using PTEs or
using a single PUD.

I suspect what you want is to

diff --git a/mm/rmap.c b/mm/rmap.c
index c6c4d4ea29a7e..1477028d3a176 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1187,12 +1187,19 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
                 atomic_add(orig_nr_pages, &folio->_large_mapcount);
                 break;
         case RMAP_LEVEL_PMD:
+       case RMAP_LEVEL_PUD:
                 first = atomic_inc_and_test(&folio->_entire_mapcount);
                 if (first) {
                         nr = atomic_add_return_relaxed(ENTIRELY_MAPPED, mapped);
                         if (likely(nr < ENTIRELY_MAPPED + ENTIRELY_MAPPED)) {
-                               *nr_pmdmapped = folio_nr_pages(folio);
-                               nr = *nr_pmdmapped - (nr & FOLIO_PAGES_MAPPED);
+                               nr_pages = folio_nr_pages(folio);
+                               /*
+                                * We only track PMD mappings of PMD-sized
+                                * folios separately.
+                                */
+                               if (level == RMAP_LEVEL_PMD)
+                                       *nr_pmdmapped = nr_pages;
+                               nr = nr_pages - (nr & FOLIO_PAGES_MAPPED);
                                 /* Raced ahead of a remove and another add? */
                                 if (unlikely(nr < 0))
                                         nr = 0;

Similar on the removal path.


-- 
Cheers,

David / dhildenb