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Commit 8079b1c8 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
Browse files

mm: clarify the radix_tree exceptional cases 



Make the radix_tree exceptional cases, mostly in filemap.c, clearer.

It's hard to devise a suitable snappy name that illuminates the use by
shmem/tmpfs for swap, while keeping filemap/pagecache/radix_tree
generality.  And akpm points out that /* radix_tree_deref_retry(page) */
comments look like calls that have been commented out for unknown
reason.

Skirt the naming difficulty by rearranging these blocks to handle the
transient radix_tree_deref_retry(page) case first; then just explain the
remaining shmem/tmpfs swap case in a comment.

Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e504f3fd

mm/filemap.c

+44 −22
Original line number Diff line number Diff line
@@ -700,10 +700,14 @@ repeat: 
		if (unlikely(!page))

			goto out;

		if (radix_tree_exception(page)) {

			if (radix_tree_exceptional_entry(page))

				goto out;

			/* radix_tree_deref_retry(page) */

			if (radix_tree_deref_retry(page))

				goto repeat;

			/*

			 * Otherwise, shmem/tmpfs must be storing a swap entry

			 * here as an exceptional entry: so return it without

			 * attempting to raise page count.

			 */

			goto out;

		}

		if (!page_cache_get_speculative(page))

			goto repeat;
@@ -838,16 +842,22 @@ repeat: 
			continue;



		if (radix_tree_exception(page)) {

			if (radix_tree_exceptional_entry(page))

				continue;

			if (radix_tree_deref_retry(page)) {

				/*

			 * radix_tree_deref_retry(page):

			 * can only trigger when entry at index 0 moves out of

			 * or back to root: none yet gotten, safe to restart.

				 * Transient condition which can only trigger

				 * when entry at index 0 moves out of or back

				 * to root: none yet gotten, safe to restart.

				 */

				WARN_ON(start | i);

				goto restart;

			}

			/*

			 * Otherwise, shmem/tmpfs must be storing a swap entry

			 * here as an exceptional entry: so skip over it -

			 * we only reach this from invalidate_mapping_pages().

			 */

			continue;

		}



		if (!page_cache_get_speculative(page))

			goto repeat;
@@ -904,15 +914,21 @@ repeat: 
			continue;



		if (radix_tree_exception(page)) {

			if (radix_tree_exceptional_entry(page))

				break;

			if (radix_tree_deref_retry(page)) {

				/*

			 * radix_tree_deref_retry(page):

			 * can only trigger when entry at index 0 moves out of

			 * or back to root: none yet gotten, safe to restart.

				 * Transient condition which can only trigger

				 * when entry at index 0 moves out of or back

				 * to root: none yet gotten, safe to restart.

				 */

				goto restart;

			}

			/*

			 * Otherwise, shmem/tmpfs must be storing a swap entry

			 * here as an exceptional entry: so stop looking for

			 * contiguous pages.

			 */

			break;

		}



		if (!page_cache_get_speculative(page))

			goto repeat;
@@ -973,14 +989,20 @@ repeat: 
			continue;



		if (radix_tree_exception(page)) {

			BUG_ON(radix_tree_exceptional_entry(page));

			if (radix_tree_deref_retry(page)) {

				/*

			 * radix_tree_deref_retry(page):

			 * can only trigger when entry at index 0 moves out of

			 * or back to root: none yet gotten, safe to restart.

				 * Transient condition which can only trigger

				 * when entry at index 0 moves out of or back

				 * to root: none yet gotten, safe to restart.

				 */

				goto restart;

			}

			/*

			 * This function is never used on a shmem/tmpfs

			 * mapping, so a swap entry won't be found here.

			 */

			BUG();

		}



		if (!page_cache_get_speculative(page))

			goto repeat;

mm/mincore.c

+1 −0
Original line number Diff line number Diff line
@@ -72,6 +72,7 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff) 
	 */

	page = find_get_page(mapping, pgoff);

#ifdef CONFIG_SWAP

	/* shmem/tmpfs may return swap: account for swapcache page too. */

	if (radix_tree_exceptional_entry(page)) {

		swp_entry_t swap = radix_to_swp_entry(page);

		page = find_get_page(&swapper_space, swap.val);

mm/shmem.c

+8 −4
Original line number Diff line number Diff line
@@ -332,10 +332,14 @@ repeat: 
		if (unlikely(!page))

			continue;

		if (radix_tree_exception(page)) {

			if (radix_tree_exceptional_entry(page))

				goto export;

			/* radix_tree_deref_retry(page) */

			if (radix_tree_deref_retry(page))

				goto restart;

			/*

			 * Otherwise, we must be storing a swap entry

			 * here as an exceptional entry: so return it

			 * without attempting to raise page count.

			 */

			goto export;

		}

		if (!page_cache_get_speculative(page))

			goto repeat;