Re: [RFC PATCH v2 06/19] fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range

[Vivek Goyal]

On Fri, Aug 09, 2019 at 03:58:20PM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Callers of dax_layout_busy_page() are only rarely operating on the
> entire file of concern.
> 
> Teach dax_layout_busy_page() to operate on a sub-range of the
> address_space provided.  Specifying 0 - ULONG_MAX however, will continue
> to operate on the "entire file" and XFS is split out to a separate patch
> by this method.
> 
> This could potentially speed up dax_layout_busy_page() as well.

I need this functionality as well for virtio_fs and posted a patch for
this.

https://lkml.org/lkml/2019/8/21/825

Given this is an optimization which existing users can benefit from already,
this patch could probably be pushed upstream independently.

> 
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> ---
> Changes from RFC v1
> 	Fix 0-day build errors
> 
>  fs/dax.c            | 15 +++++++++++----
>  fs/ext4/ext4.h      |  2 +-
>  fs/ext4/extents.c   |  6 +++---
>  fs/ext4/inode.c     | 19 ++++++++++++-------
>  fs/xfs/xfs_file.c   |  3 ++-
>  include/linux/dax.h |  6 ++++--
>  6 files changed, 33 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index a14ec32255d8..3ad19c384454 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -573,8 +573,11 @@ bool dax_mapping_is_dax(struct address_space *mapping)
>  EXPORT_SYMBOL_GPL(dax_mapping_is_dax);
>  
>  /**
> - * dax_layout_busy_page - find first pinned page in @mapping
> + * dax_layout_busy_page - find first pinned page in @mapping within
> + *                        the range @off - @off + @len
>   * @mapping: address space to scan for a page with ref count > 1
> + * @off: offset to start at
> + * @len: length to scan through
>   *
>   * DAX requires ZONE_DEVICE mapped pages. These pages are never
>   * 'onlined' to the page allocator so they are considered idle when
> @@ -587,9 +590,13 @@ EXPORT_SYMBOL_GPL(dax_mapping_is_dax);
>   * to be able to run unmap_mapping_range() and subsequently not race
>   * mapping_mapped() becoming true.
>   */
> -struct page *dax_layout_busy_page(struct address_space *mapping)
> +struct page *dax_layout_busy_page(struct address_space *mapping,
> +				  loff_t off, loff_t len)
>  {
> -	XA_STATE(xas, &mapping->i_pages, 0);
> +	unsigned long start_idx = off >> PAGE_SHIFT;
> +	unsigned long end_idx = (len == ULONG_MAX) ? ULONG_MAX
> +				: start_idx + (len >> PAGE_SHIFT);
> +	XA_STATE(xas, &mapping->i_pages, start_idx);
>  	void *entry;
>  	unsigned int scanned = 0;
>  	struct page *page = NULL;
> @@ -612,7 +619,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
>  	unmap_mapping_range(mapping, 0, 0, 1);

Should we unmap only those pages which fall in the range specified by caller.
Unmapping whole file seems to be less efficient.

Thanks
Vivek