Re: [PATCH] mm: clarify why we want kmalloc before falling backto vmallock

[Michal Hocko]

On Fri 19-05-17 17:46:58, John Hubbard wrote:
> On 05/17/2017 01:09 AM, Michal Hocko wrote:
> >From: Michal Hocko <mhocko@xxxxxxxx>
> >
> >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.
> >
> >Suggested-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> >---
> >  mm/util.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> >diff --git a/mm/util.c b/mm/util.c
> >index 464df3489903..87499f8119f2 100644
> >--- a/mm/util.c
> >+++ b/mm/util.c
> >@@ -357,7 +357,10 @@ 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
> >+	 * 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
> >  	 */
> 
> Thanks for adding this, it's great to have. Here's a slightly polished
> version of your words, if you like:
> 
> 	/*
> 	 * We want to attempt a large physically contiguous block first because
> 	 * it is less likely to fragment multiple larger blocks. This approach
> 	 * therefore contributes less to long term fragmentation than a vmalloc
> 	 * fallback would. However, make sure that larger requests are not too
> 	 * disruptive: no OOM killer and no allocation failure warnings, as we
> 	 * have a fallback.
> 	 */

Looks ok to me.

-- 
Michal Hocko
SUSE Labs