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

[Andrew Morton]

On Thu, 30 Jun 2022 10:38:44 +0800 Feng Tang <feng.tang@xxxxxxxxx> wrote:

> 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
> 
> --- 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

Well OK.  Would benefit from a comment explaining the reasoning.

But we still have eleventy squillion of these things in flight.  Why?

>  #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.