Re: [RFC PATCH v4 2/6] vfs: update swap_{,de}activate documentation

[Nikolay Borisov <nborisov@suse.com>]

On 25.05.2018 00:41, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> The documentation for these functions is wrong in several ways:
> 
> - swap_activate() is called with the inode locked
> - swap_activate() takes a swap_info_struct * and a sector_t *
> - swap_activate() can also return a positive number of extents it added
>   itself
> - swap_deactivate() does not return anything
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

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

> ---
>  Documentation/filesystems/Locking | 17 +++++++----------
>  Documentation/filesystems/vfs.txt | 12 ++++++++----
>  2 files changed, 15 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
> index 75d2d57e2c44..7f009e98fa3c 100644
> --- a/Documentation/filesystems/Locking
> +++ b/Documentation/filesystems/Locking
> @@ -211,8 +211,9 @@ prototypes:
>  	int (*launder_page)(struct page *);
>  	int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
>  	int (*error_remove_page)(struct address_space *, struct page *);
> -	int (*swap_activate)(struct file *);
> -	int (*swap_deactivate)(struct file *);
> +	int (*swap_activate)(struct swap_info_struct *, struct file *,
> +			     sector_t *);
> +	void (*swap_deactivate)(struct file *);
>  
>  locking rules:
>  	All except set_page_dirty and freepage may block
> @@ -236,8 +237,8 @@ putback_page:		yes
>  launder_page:		yes
>  is_partially_uptodate:	yes
>  error_remove_page:	yes
> -swap_activate:		no
> -swap_deactivate:	no
> +swap_activate:					yes
> +swap_deactivate:				no
>  
>  	->write_begin(), ->write_end() and ->readpage() may be called from
>  the request handler (/dev/loop).
> @@ -334,14 +335,10 @@ cleaned, or an error value if not. Note that in order to prevent the page
>  getting mapped back in and redirtied, it needs to be kept locked
>  across the entire operation.
>  
> -	->swap_activate will be called with a non-zero argument on
> -files backing (non block device backed) swapfiles. A return value
> -of zero indicates success, in which case this file can be used for
> -backing swapspace. The swapspace operations will be proxied to the
> -address space operations.
> +	->swap_activate is called from sys_swapon() with the inode locked.
>  
>  	->swap_deactivate() will be called in the sys_swapoff()
> -path after ->swap_activate() returned success.
> +path after ->swap_activate() returned success. The inode is not locked.
>  
>  ----------------------- file_lock_operations ------------------------------
>  prototypes:
> diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
> index 5fd325df59e2..0149109d94d1 100644
> --- a/Documentation/filesystems/vfs.txt
> +++ b/Documentation/filesystems/vfs.txt
> @@ -650,8 +650,9 @@ struct address_space_operations {
>  					unsigned long);
>  	void (*is_dirty_writeback) (struct page *, bool *, bool *);
>  	int (*error_remove_page) (struct mapping *mapping, struct page *page);
> -	int (*swap_activate)(struct file *);
> -	int (*swap_deactivate)(struct file *);
> +	int (*swap_activate)(struct swap_info_struct *, struct file *,
> +			     sector_t *);
> +	void (*swap_deactivate)(struct file *);
>  };
>  
>    writepage: called by the VM to write a dirty page to backing store.
> @@ -828,8 +829,11 @@ struct address_space_operations {
>  
>    swap_activate: Called when swapon is used on a file to allocate
>  	space if necessary and pin the block lookup information in
> -	memory. A return value of zero indicates success,
> -	in which case this file can be used to back swapspace.
> +	memory. If this returns zero, the swap system will call the address
> +	space operations ->readpage() and ->direct_IO(). Alternatively, this
> +	may call add_swap_extent() and return the number of extents added, in
> +	which case the swap system will use the provided blocks directly
> +	instead of going through the filesystem.
>  
>    swap_deactivate: Called during swapoff on files where swap_activate
>  	was successful.
>