Re: [PATCH v1 04/10] NFS: Keep the readdir pagecache cursor updated

[Trond Myklebust <trondmy@hammerspace.com>]

On Wed, 2021-01-20 at 11:59 -0500, Benjamin Coddington wrote:
> Whenever we successfully locate our dir_cookie within the pagecache,
> or
> finish emitting entries to userspace, update the pagecache cursor. 
> These
> updates provide marker points to validate pagecache pages in a future
> patch.

How isn't this going to end up subject to the exact same problem that
Dave Wysochanski's patchset had?

> 
> Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> ---
>  fs/nfs/dir.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> index 6626aff9f54d..7f6c84c8a412 100644
> --- a/fs/nfs/dir.c
> +++ b/fs/nfs/dir.c
> @@ -150,6 +150,10 @@ struct nfs_cache_array {
>         struct nfs_cache_array_entry array[];
>  };
>  
> +static const int cache_entries_per_page =
> +       (PAGE_SIZE - sizeof(struct nfs_cache_array)) /
> +       sizeof(struct nfs_cache_array_entry);
> +
>  struct nfs_readdir_descriptor {
>         struct file     *file;
>         struct page     *page;
> @@ -251,6 +255,21 @@ static bool nfs_readdir_array_is_full(struct
> nfs_cache_array *array)
>         return array->page_full;
>  }
>  
> +static void nfs_readdir_set_cursor(struct nfs_readdir_descriptor
> *desc, int index)
> +{
> +       desc->pgc.entry_index = index;
> +       desc->pgc.index_cookie = desc->dir_cookie;
> +}
> +
> +static void nfs_readdir_cursor_next(struct nfs_dir_page_cursor *pgc,
> u64 cookie)
> +{
> +       pgc->index_cookie = cookie;
> +       if (++pgc->entry_index == cache_entries_per_page) {
> +               pgc->entry_index = 0;
> +               pgc->page_index++;
> +       }
> +}
> +
>  /*
>   * the caller is responsible for freeing qstr.name
>   * when called by nfs_readdir_add_to_array, the strings will be
> freed in
> @@ -424,7 +443,7 @@ static int nfs_readdir_search_for_pos(struct
> nfs_cache_array *array,
>  
>         index = (unsigned int)diff;
>         desc->dir_cookie = array->array[index].cookie;
> -       desc->pgc.entry_index = index;
> +       nfs_readdir_set_cursor(desc, index);
>         return 0;
>  out_eof:
>         desc->eof = true;
> @@ -492,7 +511,7 @@ static int nfs_readdir_search_for_cookie(struct
> nfs_cache_array *array,
>                         else
>                                 desc->ctx->pos = new_pos;
>                         desc->prev_index = new_pos;
> -                       desc->pgc.entry_index = i;
> +                       nfs_readdir_set_cursor(desc, i);
>                         return 0;
>                 }
>         }
> @@ -519,9 +538,9 @@ static int nfs_readdir_search_array(struct
> nfs_readdir_descriptor *desc)
>                 status = nfs_readdir_search_for_cookie(array, desc);
>  
>         if (status == -EAGAIN) {
> -               desc->pgc.index_cookie = array->last_cookie;
> +               desc->pgc.entry_index = array->size - 1;
> +               nfs_readdir_cursor_next(&desc->pgc, array-
> >last_cookie);
>                 desc->current_index += array->size;
> -               desc->pgc.page_index++;
>         }
>         kunmap_atomic(array);
>         return status;
> @@ -1035,6 +1054,8 @@ static void nfs_do_filldir(struct
> nfs_readdir_descriptor *desc)
>                 desc->eof = true;
>  
>         kunmap(desc->page);
> +       desc->pgc.entry_index = i-1;
> +       nfs_readdir_cursor_next(&desc->pgc, desc->dir_cookie);
>         dfprintk(DIRCACHE, "NFS: nfs_do_filldir() filling ended @
> cookie %llu\n",
>                         (unsigned long long)desc->dir_cookie);
>  }

-- 
Trond Myklebust
CTO, Hammerspace Inc
4984 El Camino Real, Suite 208
Los Altos, CA 94022
​
www.hammer.space