Re: [PATCH v1] mm/slub: enable debugging memory wasting of kmalloc

[Hyeonggon Yoo]

On Fri, Jul 01, 2022 at 11:04:51PM +0800, Feng Tang wrote:
> Hi Christoph,
> 
> On Fri, Jul 01, 2022 at 04:37:00PM +0200, Christoph Lameter wrote:
> > On Fri, 1 Jul 2022, Feng Tang wrote:
> > 
> > >  static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
> > > -			  unsigned long addr, struct kmem_cache_cpu *c)
> > > +			  unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size)
> > >  {
> > 
> > It would be good to avoid expanding the basic slab handling functions for
> > kmalloc. Can we restrict the mods to the kmalloc related functions?
> 
> Yes, this is the part that concerned me. I tried but haven't figured
> a way.
> 
> I started implemting it several month ago, and stuck with several
> kmalloc APIs in a hacky way like dump_stack() when there is a waste
> over 1/4 of the object_size of the kmalloc_caches[][].
> 
> Then I found one central API which has all the needed info (object_size &
> orig_size) that we can yell about the waste :
> 
> static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_lru *lru,
>                 gfp_t gfpflags, int node, unsigned long addr, size_t orig_size)
> 
> which I thought could be still hacky, as the existing 'alloc_traces'
> can't be resued which already has the count/call-stack info. Current
> solution leverage it at the cost of adding 'orig_size' parameters, but
> I don't know how to pass the 'waste' info through as track/location is
> in the lowest level.

If adding cost of orig_size parameter for non-debugging case is concern,
what about doing this in userspace script that makes use of kmalloc
tracepoints?

	kmalloc: call_site=tty_buffer_alloc+0x43/0x90 ptr=00000000b78761e1
	bytes_req=1056 bytes_alloc=2048 gfp_flags=GFP_ATOMIC|__GFP_NOWARN
	accounted=false

calculating sum of (bytes_alloc - bytes_req) for each call_site
may be an alternative solution.

Thanks,
Hyeonggon

> Thanks,
> Feng
> 
> 
>