Re: [PATCH v2.1 1/8] fsdax: introduce page->share for fsdax in reflink mode

[Allison Henderson]

On Fri, 2022-12-02 at 09:23 +0000, Shiyang Ruan wrote:
> fsdax page is used not only when CoW, but also mapread. To make the
> it
> easily understood, use 'share' to indicate that the dax page is
> shared
> by more than one extent.  And add helper functions to use it.
> 
> Also, the flag needs to be renamed to PAGE_MAPPING_DAX_SHARED.
> 
The new changes look reasonable to me
Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx>

> Signed-off-by: Shiyang Ruan <ruansy.fnst@xxxxxxxxxxx>
> ---
>  fs/dax.c                   | 38 ++++++++++++++++++++++--------------
> --
>  include/linux/mm_types.h   |  5 ++++-
>  include/linux/page-flags.h |  2 +-
>  3 files changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index 1c6867810cbd..edbacb273ab5 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -334,35 +334,41 @@ static unsigned long dax_end_pfn(void *entry)
>         for (pfn = dax_to_pfn(entry); \
>                         pfn < dax_end_pfn(entry); pfn++)
>  
> -static inline bool dax_mapping_is_cow(struct address_space *mapping)
> +static inline bool dax_page_is_shared(struct page *page)
>  {
> -       return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> +       return (unsigned long)page->mapping ==
> PAGE_MAPPING_DAX_SHARED;
>  }
>  
>  /*
> - * Set the page->mapping with FS_DAX_MAPPING_COW flag, increase the
> refcount.
> + * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase
> the
> + * refcount.
>   */
> -static inline void dax_mapping_set_cow(struct page *page)
> +static inline void dax_page_bump_sharing(struct page *page)
>  {
> -       if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
> +       if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_SHARED) {
>                 /*
>                  * Reset the index if the page was already mapped
>                  * regularly before.
>                  */
>                 if (page->mapping)
> -                       page->index = 1;
> -               page->mapping = (void *)PAGE_MAPPING_DAX_COW;
> +                       page->share = 1;
> +               page->mapping = (void *)PAGE_MAPPING_DAX_SHARED;
>         }
> -       page->index++;
> +       page->share++;
> +}
> +
> +static inline unsigned long dax_page_drop_sharing(struct page *page)
> +{
> +       return --page->share;
>  }
>  
>  /*
> - * When it is called in dax_insert_entry(), the cow flag will
> indicate that
> + * When it is called in dax_insert_entry(), the shared flag will
> indicate that
>   * whether this entry is shared by multiple files.  If so, set the
> page->mapping
> - * FS_DAX_MAPPING_COW, and use page->index as refcount.
> + * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
>   */
>  static void dax_associate_entry(void *entry, struct address_space
> *mapping,
> -               struct vm_area_struct *vma, unsigned long address,
> bool cow)
> +               struct vm_area_struct *vma, unsigned long address,
> bool shared)
>  {
>         unsigned long size = dax_entry_size(entry), pfn, index;
>         int i = 0;
> @@ -374,8 +380,8 @@ static void dax_associate_entry(void *entry,
> struct address_space *mapping,
>         for_each_mapped_pfn(entry, pfn) {
>                 struct page *page = pfn_to_page(pfn);
>  
> -               if (cow) {
> -                       dax_mapping_set_cow(page);
> +               if (shared) {
> +                       dax_page_bump_sharing(page);
>                 } else {
>                         WARN_ON_ONCE(page->mapping);
>                         page->mapping = mapping;
> @@ -396,9 +402,9 @@ static void dax_disassociate_entry(void *entry,
> struct address_space *mapping,
>                 struct page *page = pfn_to_page(pfn);
>  
>                 WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> -               if (dax_mapping_is_cow(page->mapping)) {
> -                       /* keep the CoW flag if this page is still
> shared */
> -                       if (page->index-- > 0)
> +               if (dax_page_is_shared(page)) {
> +                       /* keep the shared flag if this page is still
> shared */
> +                       if (dax_page_drop_sharing(page) > 0)
>                                 continue;
>                 } else
>                         WARN_ON_ONCE(page->mapping && page->mapping
> != mapping);
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 500e536796ca..f46cac3657ad 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -103,7 +103,10 @@ struct page {
>                         };
>                         /* See page-flags.h for PAGE_MAPPING_FLAGS */
>                         struct address_space *mapping;
> -                       pgoff_t index;          /* Our offset within
> mapping. */
> +                       union {
> +                               pgoff_t index;          /* Our offset
> within mapping. */
> +                               unsigned long share;    /* share
> count for fsdax */
> +                       };
>                         /**
>                          * @private: Mapping-private opaque data.
>                          * Usually used for buffer_heads if
> PagePrivate.
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 0b0ae5084e60..c8a3aa02278d 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -641,7 +641,7 @@ PAGEFLAG_FALSE(VmemmapSelfHosted,
> vmemmap_self_hosted)
>   * Different with flags above, this flag is used only for fsdax
> mode.  It
>   * indicates that this page->mapping is now under reflink case.
>   */
> -#define PAGE_MAPPING_DAX_COW   0x1
> +#define PAGE_MAPPING_DAX_SHARED        0x1
>  
>  static __always_inline bool folio_mapping_flags(struct folio *folio)
>  {