Re: [PATCHv2 4/4] swap: change swap_list_head to plist, add swap_avail_head

[Mel Gorman <mgorman@suse.de>]

On Mon, May 12, 2014 at 12:38:20PM -0400, Dan Streetman wrote:
> Originally get_swap_page() started iterating through the singly-linked
> list of swap_info_structs using swap_list.next or highest_priority_index,
> which both were intended to point to the highest priority active swap
> target that was not full.  The first patch in this series changed the
> singly-linked list to a doubly-linked list, and removed the logic to start
> at the highest priority non-full entry; it starts scanning at the highest
> priority entry each time, even if the entry is full.
> 
> Replace the manually ordered swap_list_head with a plist, swap_active_head.
> Add a new plist, swap_avail_head.  The original swap_active_head plist
> contains all active swap_info_structs, as before, while the new
> swap_avail_head plist contains only swap_info_structs that are active and
> available, i.e. not full.  Add a new spinlock, swap_avail_lock, to protect
> the swap_avail_head list.
> 
> Mel Gorman suggested using plists since they internally handle ordering
> the list entries based on priority, which is exactly what swap was doing
> manually.  All the ordering code is now removed, and swap_info_struct
> entries and simply added to their corresponding plist and automatically
> ordered correctly.
> 
> Using a new plist for available swap_info_structs simplifies and
> optimizes get_swap_page(), which no longer has to iterate over full
> swap_info_structs.  Using a new spinlock for swap_avail_head plist
> allows each swap_info_struct to add or remove themselves from the
> plist when they become full or not-full; previously they could not
> do so because the swap_info_struct->lock is held when they change
> from full<->not-full, and the swap_lock protecting the main
> swap_active_head must be ordered before any swap_info_struct->lock.
> 
> Signed-off-by: Dan Streetman <ddstreet@ieee.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Shaohua Li <shli@fusionio.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> 

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs