[PATCH] Smarter retry of costly-order allocations

[Nishanth Aravamudan <nacc@us.ibm.com>]

Because of page order checks in __alloc_pages(), hugepage (and similarly
large order) allocations will not retry unless explicitly marked
__GFP_REPEAT. However, the current retry logic is nearly an infinite
loop (or until reclaim does no progress whatsoever). For these costly
allocations, that seems like overkill and could potentially never
terminate.

Modify try_to_free_pages() to indicate how many pages were reclaimed.
Use that information in __alloc_pages() to eventually fail a large
__GFP_REPEAT allocation when we've reclaimed an order of pages equal to
or greater than the allocation's order. This relies on lumpy reclaim
functioning as advertised. Due to fragmentation, lumpy reclaim may not
be able to free up the order needed in one invocation, so multiple
iterations may be requred. In other words, the more fragmented memory
is, the more retry attempts __GFP_REPEAT will make (particularly for
higher order allocations).

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1db36da..1a0cc4d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1541,7 +1541,8 @@ __alloc_pages_internal(gfp_t gfp_mask, unsigned int order,
 	struct task_struct *p = current;
 	int do_retry;
 	int alloc_flags;
-	int did_some_progress;
+	unsigned long did_some_progress;
+	unsigned long pages_reclaimed = 0;
 
 	might_sleep_if(wait);
 
@@ -1691,15 +1692,26 @@ nofail_alloc:
 	 * Don't let big-order allocations loop unless the caller explicitly
 	 * requests that.  Wait for some write requests to complete then retry.
 	 *
-	 * In this implementation, either order <= PAGE_ALLOC_COSTLY_ORDER or
-	 * __GFP_REPEAT mean __GFP_NOFAIL, but that may not be true in other
+	 * In this implementation, order <= PAGE_ALLOC_COSTLY_ORDER
+	 * means __GFP_NOFAIL, but that may not be true in other
 	 * implementations.
+	 *
+	 * For order > PAGE_ALLOC_COSTLY_ORDER, if __GFP_REPEAT is
+	 * specified, then we retry until we no longer reclaim any pages
+	 * (above), or we've reclaimed an order of pages at least as
+	 * large as the allocation's order. In both cases, if the
+	 * allocation still fails, we stop retrying.
 	 */
+	pages_reclaimed += did_some_progress;
 	do_retry = 0;
 	if (!(gfp_mask & __GFP_NORETRY)) {
-		if ((order <= PAGE_ALLOC_COSTLY_ORDER) ||
-						(gfp_mask & __GFP_REPEAT))
+		if (order <= PAGE_ALLOC_COSTLY_ORDER) {
 			do_retry = 1;
+		} else {
+			if (gfp_mask & __GFP_REPEAT &&
+				pages_reclaimed < (1 << order))
+					do_retry = 1;
+		}
 		if (gfp_mask & __GFP_NOFAIL)
 			do_retry = 1;
 	}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 83f42c9..d106b2c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1319,6 +1319,9 @@ static unsigned long shrink_zones(int priority, struct zonelist *zonelist,
  * hope that some of these pages can be written.  But if the allocating task
  * holds filesystem locks which prevent writeout this might not work, and the
  * allocation attempt will fail.
+ *
+ * returns:	0, if no pages reclaimed
+ * 		else, the number of pages reclaimed
  */
 static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 					struct scan_control *sc)
@@ -1368,7 +1371,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 		}
 		total_scanned += sc->nr_scanned;
 		if (nr_reclaimed >= sc->swap_cluster_max) {
-			ret = 1;
+			ret = nr_reclaimed;
 			goto out;
 		}
 
@@ -1391,7 +1394,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 	}
 	/* top priority shrink_caches still had more to do? don't OOM, then */
 	if (!sc->all_unreclaimable && scan_global_lru(sc))
-		ret = 1;
+		ret = nr_reclaimed;
 out:
 	/*
 	 * Now that we've scanned all the zones at this priority level, note

-- 
Nishanth Aravamudan <nacc@us.ibm.com>
IBM Linux Technology Center

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