Re: [PATCH 1/2] ext4/xfs: add page refcount helper

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Sep 25, 2020 at 1:45 PM Ralph Campbell <rcampbell@nvidia.com> wrote:
>
> There are several places where ZONE_DEVICE struct pages assume a reference
> count == 1 means the page is idle and free. Instead of open coding this,
> add a helper function to hide this detail.
>
> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
> ---
>  fs/dax.c            | 8 ++++----
>  fs/ext4/inode.c     | 2 +-
>  fs/xfs/xfs_file.c   | 2 +-
>  include/linux/dax.h | 5 +++++
>  4 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 994ab66a9907..8eddbcc0e149 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -358,7 +358,7 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
>         for_each_mapped_pfn(entry, pfn) {
>                 struct page *page = pfn_to_page(pfn);
>
> -               WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> +               WARN_ON_ONCE(trunc && !dax_layout_is_idle_page(page));
>                 WARN_ON_ONCE(page->mapping && page->mapping != mapping);
>                 page->mapping = NULL;
>                 page->index = 0;
> @@ -372,7 +372,7 @@ static struct page *dax_busy_page(void *entry)
>         for_each_mapped_pfn(entry, pfn) {
>                 struct page *page = pfn_to_page(pfn);
>
> -               if (page_ref_count(page) > 1)
> +               if (!dax_layout_is_idle_page(page))
>                         return page;
>         }
>         return NULL;
> @@ -560,11 +560,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
>
>  /**
>   * dax_layout_busy_page - find first pinned page in @mapping
> - * @mapping: address space to scan for a page with ref count > 1
> + * @mapping: address space to scan for a page with ref count > 0
>   *
>   * DAX requires ZONE_DEVICE mapped pages. These pages are never
>   * 'onlined' to the page allocator so they are considered idle when
> - * page->count == 1. A filesystem uses this interface to determine if
> + * page->count == 0. A filesystem uses this interface to determine if
>   * any page in the mapping is busy, i.e. for DMA, or other
>   * get_user_pages() usages.
>   *
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index bf596467c234..d9f8ad55523a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3927,7 +3927,7 @@ int ext4_break_layouts(struct inode *inode)
>                         return 0;
>
>                 error = ___wait_var_event(&page->_refcount,
> -                               atomic_read(&page->_refcount) == 1,
> +                               dax_layout_is_idle_page(page),
>                                 TASK_INTERRUPTIBLE, 0, 0,
>                                 ext4_wait_dax_page(ei));
>         } while (error == 0);
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index a29f78a663ca..29ab96541bc1 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -750,7 +750,7 @@ xfs_break_dax_layouts(
>
>         *retry = true;
>         return ___wait_var_event(&page->_refcount,
> -                       atomic_read(&page->_refcount) == 1, TASK_INTERRUPTIBLE,
> +                       dax_layout_is_idle_page(page), TASK_INTERRUPTIBLE,
>                         0, 0, xfs_wait_dax_page(inode));
>  }
>
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 43b39ab9de1a..3f78ed78d1d6 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -238,4 +238,9 @@ static inline bool dax_mapping(struct address_space *mapping)
>         return mapping->host && IS_DAX(mapping->host);
>  }
>
> +static inline bool dax_layout_is_idle_page(struct page *page)
> +{
> +       return page_ref_count(page) <= 1;

Why convert the check from "== 1" to "<= 1" and then back to the ==
operator in the next patch? A refcount < 1 in this path before your
other change is a bug.