Re: [RFC PATCH v4 1/6] mm: split SWP_FILE into SWP_ACTIVATED and SWP_FS

[Nikolay Borisov <nborisov@suse.com>]

On 25.05.2018 00:41, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> The SWP_FILE flag serves two purposes: to make swap_{read,write}page()
> go through the filesystem, and to make swapoff() call
> ->swap_deactivate(). For Btrfs, we want the latter but not the former,
> so split this flag into two. This makes us always call
> ->swap_deactivate() if ->swap_activate() succeeded, not just if it
> didn't add any swap extents itself.
> 
> This also resolves the issue of the very misleading name of SWP_FILE,
> which is only used for swap files over NFS.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Generally looks good:

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

just one cleanup suggestion, see below.
> ---
>  include/linux/swap.h | 13 +++++++------
>  mm/page_io.c         |  6 +++---
>  mm/swapfile.c        | 13 ++++++++-----
>  3 files changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 2417d288e016..29dfd436435c 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -167,13 +167,14 @@ enum {
>  	SWP_SOLIDSTATE	= (1 << 4),	/* blkdev seeks are cheap */
>  	SWP_CONTINUED	= (1 << 5),	/* swap_map has count continuation */
>  	SWP_BLKDEV	= (1 << 6),	/* its a block device */
> -	SWP_FILE	= (1 << 7),	/* set after swap_activate success */
> -	SWP_AREA_DISCARD = (1 << 8),	/* single-time swap area discards */
> -	SWP_PAGE_DISCARD = (1 << 9),	/* freed swap page-cluster discards */
> -	SWP_STABLE_WRITES = (1 << 10),	/* no overwrite PG_writeback pages */
> -	SWP_SYNCHRONOUS_IO = (1 << 11),	/* synchronous IO is efficient */
> +	SWP_ACTIVATED	= (1 << 7),	/* set after swap_activate success */
> +	SWP_FS		= (1 << 8),	/* swap file goes through fs */
> +	SWP_AREA_DISCARD = (1 << 9),	/* single-time swap area discards */
> +	SWP_PAGE_DISCARD = (1 << 10),	/* freed swap page-cluster discards */
> +	SWP_STABLE_WRITES = (1 << 11),	/* no overwrite PG_writeback pages */
> +	SWP_SYNCHRONOUS_IO = (1 << 12),	/* synchronous IO is efficient */
>  					/* add others here before... */
> -	SWP_SCANNING	= (1 << 12),	/* refcount in scan_swap_map */
> +	SWP_SCANNING	= (1 << 13),	/* refcount in scan_swap_map */
>  };
>  
>  #define SWAP_CLUSTER_MAX 32UL
> diff --git a/mm/page_io.c b/mm/page_io.c
> index b41cf9644585..f2d06c1d0cc1 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -283,7 +283,7 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
>  	struct swap_info_struct *sis = page_swap_info(page);
>  
>  	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
> -	if (sis->flags & SWP_FILE) {
> +	if (sis->flags & SWP_FS) {

Not necessarily for this series but something to keep in mind:

nit: The way the fs case is tucked onto the __swap_writepage is a bit
ugly. How about factoring out the filesystem/blockdev casae into two
functions :

__swap_writepage_fs/__swap_writepage_bdev

then in swap_writepage we just have the if (sis->flags & SWP_FS) check
and dispatch to either write_page_fs or writepage_bdev.


>  		struct kiocb kiocb;
>  		struct file *swap_file = sis->swap_file;
>  		struct address_space *mapping = swap_file->f_mapping;
> @@ -364,7 +364,7 @@ int swap_readpage(struct page *page, bool synchronous)
>  		goto out;
>  	}
>  
> -	if (sis->flags & SWP_FILE) {
> +	if (sis->flags & SWP_FS) {
>  		struct file *swap_file = sis->swap_file;
>  		struct address_space *mapping = swap_file->f_mapping;
>  
> @@ -422,7 +422,7 @@ int swap_set_page_dirty(struct page *page)
>  {
>  	struct swap_info_struct *sis = page_swap_info(page);
>  
> -	if (sis->flags & SWP_FILE) {
> +	if (sis->flags & SWP_FS) {
>  		struct address_space *mapping = sis->swap_file->f_mapping;
>  
>  		VM_BUG_ON_PAGE(!PageSwapCache(page), page);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index cc2cf04d9018..886c9d89b144 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -973,7 +973,7 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
>  			goto nextsi;
>  		}
>  		if (cluster) {
> -			if (!(si->flags & SWP_FILE))
> +			if (!(si->flags & SWP_FS))
>  				n_ret = swap_alloc_cluster(si, swp_entries);
>  		} else
>  			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> @@ -2327,12 +2327,13 @@ static void destroy_swap_extents(struct swap_info_struct *sis)
>  		kfree(se);
>  	}
>  
> -	if (sis->flags & SWP_FILE) {
> +	if (sis->flags & SWP_ACTIVATED) {
>  		struct file *swap_file = sis->swap_file;
>  		struct address_space *mapping = swap_file->f_mapping;
>  
> -		sis->flags &= ~SWP_FILE;
> -		mapping->a_ops->swap_deactivate(swap_file);
> +		sis->flags &= ~SWP_ACTIVATED;
> +		if (mapping->a_ops->swap_deactivate)
> +			mapping->a_ops->swap_deactivate(swap_file);
>  	}
>  }
>  
> @@ -2428,8 +2429,10 @@ static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
>  
>  	if (mapping->a_ops->swap_activate) {
>  		ret = mapping->a_ops->swap_activate(sis, swap_file, span);
> +		if (ret >= 0)
> +			sis->flags |= SWP_ACTIVATED;
>  		if (!ret) {
> -			sis->flags |= SWP_FILE;
> +			sis->flags |= SWP_FS;
>  			ret = add_swap_extent(sis, 0, sis->max, 0);
>  			*span = sis->pages;
>  		}
>