Re: [RFC PATCH] mm/slub: enable debugging memory wasting of kmalloc

[Feng Tang]

Hi Andrew,

Thanks for the review!

On Wed, Jun 29, 2022 at 07:30:06PM -0700, Andrew Morton wrote:
> On Thu, 30 Jun 2022 09:47:15 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:
> 
> > kmalloc's API family is critical for mm, with one shortcoming that
> > its object size is fixed to be power of 2. When user requests memory
> > for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so
> > in worst case, there is around 50% memory space waste.
> > 
> > We've met a kernel boot OOM panic, and from the dumped slab info:
> > 
> >     [   26.062145] kmalloc-2k            814056KB     814056KB
> > 
> > >From debug we found there are huge number of 'struct iova_magazine',
> > whose size is 1032 bytes (1024 + 8), so each allocation will waste
> > 1016 bytes. Though the issue is solved by giving the right(bigger)
> > size of RAM, it is still better to optimize the size (either use
> > a kmalloc friendly size or create a dedicated slab for it).
> 
> Well that's nice, and additional visibility is presumably a good thing.
> 
> But what the heck is going on with iova_magazine?  Is anyone looking at
> moderating its impact?

Yes, I have a very simple patch at hand

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index db77aa675145..5422e67bb4b5 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -614,7 +614,7 @@ EXPORT_SYMBOL_GPL(reserve_iova);
  * dynamic size tuning described in the paper.
  */
 
-#define IOVA_MAG_SIZE 128
+#define IOVA_MAG_SIZE 127
 #define MAX_GLOBAL_MAGS 32	/* magazines per bin */
 
 struct iova_magazine {

I guess changing it from 128 to 127 will not hurt much, and plan to
send it out soon.

Thanks,
Feng