mm: clarify why we want kmalloc before falling backto vmallock

While converting drm_[cm]alloc* helpers to kvmalloc* variants Chris Wilson has wondered why we want to try kmalloc before vmalloc fallback even for larger allocations requests. Let's clarify that one larger physically contiguous block is less likely to fragment memory than many scattered pages which can prevent more large blocks from being created. [[email protected]: coding-style fixes] Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Michal Hocko <[email protected]> Suggested-by: Chris Wilson <[email protected]> Reviewed-by: Chris Wilson <[email protected]> Acked-by: Vlastimil Babka <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
josf · Jun 2, 2017 · 4f4f2ba · 4f4f2ba
1 parent 60b0a8c
commit 4f4f2ba
Showing 1 changed file with 5 additions and 2 deletions.
diff --git a/mm/util.c b/mm/util.c
@@ -357,8 +357,11 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
 	WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
 
 	/*
-	 * Make sure that larger requests are not too disruptive - no OOM
-	 * killer and no allocation failure warnings as we have a fallback
+	 * We want to attempt a large physically contiguous block first because
+	 * it is less likely to fragment multiple larger blocks and therefore
+	 * contribute to a long term fragmentation less than vmalloc fallback.
+	 * However make sure that larger requests are not too disruptive - no
+	 * OOM killer and no allocation failure warnings as we have a fallback.
 	 */
 	if (size > PAGE_SIZE) {
 		kmalloc_flags |= __GFP_NOWARN;