mm/swap: fix comment about swap extent

Since commit 4efaceb1 ("mm, swap: use rbtree for swap_extent"), rbtree
is used for swap extent.  Also curr_swap_extent is removed at that time. 
Update the corresponding comment.

Link: https://lkml.kernel.org/r/20220509131416.17553-16-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swap.h

@@ -168,8 +168,8 @@ struct zone;
 
 /*
  * A swap extent maps a range of a swapfile's PAGE_SIZE pages onto a range of
- * disk blocks.  A list of swap extents maps the entire swapfile.  (Where the
- * term `swapfile' refers to either a blockdevice or an IS_REG file.  Apart
+ * disk blocks.  A rbtree of swap extents maps the entire swapfile (Where the
+ * term `swapfile' refers to either a blockdevice or an IS_REG file). Apart
  * from setup, they're handled identically.
  *
  * We always assume that blocks are of size PAGE_SIZE.

mm/swapfile.c

@@ -2208,8 +2208,8 @@ EXPORT_SYMBOL_GPL(add_swap_extent);
 
 /*
  * A `swap extent' is a simple thing which maps a contiguous range of pages
- * onto a contiguous range of disk blocks.  An ordered list of swap extents
- * is built at swapon time and is then used at swap_writepage/swap_readpage
+ * onto a contiguous range of disk blocks.  A rbtree of swap extents is
+ * built at swapon time and is then used at swap_writepage/swap_readpage
  * time for locating where on disk a page belongs.
  *
  * If the swapfile is an S_ISBLK block device, a single extent is installed.
@@ -2217,12 +2217,12 @@ EXPORT_SYMBOL_GPL(add_swap_extent);
  * swap files identically.
  *
  * Whether the swapdev is an S_ISREG file or an S_ISBLK blockdev, the swap
- * extent list operates in PAGE_SIZE disk blocks.  Both S_ISREG and S_ISBLK
+ * extent rbtree operates in PAGE_SIZE disk blocks.  Both S_ISREG and S_ISBLK
  * swapfiles are handled *identically* after swapon time.
  *
  * For S_ISREG swapfiles, setup_swap_extents() will walk all the file's blocks
- * and will parse them into an ordered extent list, in PAGE_SIZE chunks.  If
- * some stray blocks are found which do not fall within the PAGE_SIZE alignment
+ * and will parse them into a rbtree, in PAGE_SIZE chunks.  If some stray
+ * blocks are found which do not fall within the PAGE_SIZE alignment
  * requirements, they are simply tossed out - we will never use those blocks
  * for swapping.
  *
@@ -2231,10 +2231,7 @@ EXPORT_SYMBOL_GPL(add_swap_extent);
  *
  * The amount of disk space which a single swap extent represents varies.
  * Typically it is in the 1-4 megabyte range.  So we can have hundreds of
- * extents in the list.  To avoid much list walking, we cache the previous
- * search location in `curr_swap_extent', and start new searches from there.
- * This is extremely effective.  The average number of iterations in
- * map_swap_page() has been measured at about 0.3 per page.  - akpm.
+ * extents in the rbtree. - akpm.
  */
 static int setup_swap_extents(struct swap_info_struct *sis, sector_t *span)
 {