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.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/20170517080932.21423-1-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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;