The smp_mb() is becaus sync_page() doesn't have PG_locked while it
accesses page_mapping(page). The comments in the patch (the entire
patch is the addition of this comment) try to explain further how
and why smp_mb() is used.
mm/filemap.c: 93595c327bbdc43fcea91b513fd750d1a73edfec
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -139,7 +139,25 @@ static int sync_page(void *word)
page = container_of((page_flags_t *)word, struct page, flags);
/*
- * FIXME, fercrissake. What is this barrier here for?
+ * page_mapping() is being called without PG_locked held.
+ * Some knowledge of the state and use of the page is used to
+ * reduce the requirements down to a memory barrier.
+ * The danger here is of a stale page_mapping() return value
+ * indicating a struct address_space different from the one it's
+ * associated with when it is associated with one.
+ * After smp_mb(), it's either the correct page_mapping() for
+ * the page, or an old page_mapping() and the page's own
+ * page_mapping() has gone NULL.
+ * The ->sync_page() address_space operation must tolerate
+ * page_mapping() going NULL. By an amazing coincidence,
+ * this comes about because none of the users of the page
+ * in the ->sync_page() methods make essential use of the
+ * page_mapping(), merely passing the page down to the backing
+ * device's unplug functions when it's non-NULL, which in turn
+ * ignore it for all cases but swap, where only page->private is
+ * of interest. When page_mapping() does go NULL, the entire
+ * call stack gracefully ignores the page and returns.
+ * -- wli
*/
smp_mb();
mapping = page_mapping(page);
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
