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: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/filemap.c | 66 ++++++++++++++++++++++++++++++++-----------------
mm/mincore.c | 1
mm/shmem.c | 12 +++++---
3 files changed, 53 insertions(+), 26 deletions(-)
--- mmotm.orig/mm/filemap.c 2011-07-08 18:57:15.142704312 -0700
+++ mmotm/mm/filemap.c 2011-07-19 11:13:31.945882037 -0700
@@ -703,10 +703,14 @@ repeat:
if (unlikely(!page))
goto out;
if (radix_tree_exception(page)) {
- if (radix_tree_exceptional_entry(page))
- goto out;
- /* radix_tree_deref_retry(page) */
- goto repeat;
+ 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;
@@ -841,15 +845,21 @@ repeat:
continue;
if (radix_tree_exception(page)) {
- if (radix_tree_exceptional_entry(page))
- continue;
+ if (radix_tree_deref_retry(page)) {
+ /*
+ * 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;
+ }
/*
- * 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.
+ * 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().
*/
- WARN_ON(start | i);
- goto restart;
+ continue;
}
if (!page_cache_get_speculative(page))
@@ -907,14 +917,20 @@ repeat:
continue;
if (radix_tree_exception(page)) {
- if (radix_tree_exceptional_entry(page))
- break;
+ if (radix_tree_deref_retry(page)) {
+ /*
+ * 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;
+ }
/*
- * 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.
+ * Otherwise, shmem/tmpfs must be storing a swap entry
+ * here as an exceptional entry: so stop looking for
+ * contiguous pages.
*/
- goto restart;
+ break;
}
if (!page_cache_get_speculative(page))
@@ -976,13 +992,19 @@ repeat:
continue;
if (radix_tree_exception(page)) {
- BUG_ON(radix_tree_exceptional_entry(page));
+ if (radix_tree_deref_retry(page)) {
+ /*
+ * 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;
+ }
/*
- * 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.
+ * This function is never used on a shmem/tmpfs
+ * mapping, so a swap entry won't be found here.
*/
- goto restart;
+ BUG();
}
if (!page_cache_get_speculative(page))
--- mmotm.orig/mm/mincore.c 2011-07-08 18:57:15.174704475 -0700
+++ mmotm/mm/mincore.c 2011-07-19 11:13:31.949882063 -0700
@@ -72,6 +72,7 @@ static unsigned char mincore_page(struct
*/
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);
--- mmotm.orig/mm/shmem.c 2011-07-19 11:11:33.709295729 -0700
+++ mmotm/mm/shmem.c 2011-07-19 11:13:31.953882076 -0700
@@ -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) */
- goto restart;
+ 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;
--
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