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

[Feng Tang]

On Thu, Jul 14, 2022 at 10:11:32PM +0200, Vlastimil Babka wrote:
> On 7/13/22 09:36, Feng Tang wrote:
> > Hi Vlastimil,
> > 
> > On Mon, Jul 11, 2022 at 10:15:21AM +0200, Vlastimil Babka wrote:
> >> On 7/1/22 15:59, Feng Tang 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 (v5.10), 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 was solved by giving the right (bigger)
> >> > size of RAM, it is still nice to optimize the size (either use a
> >> > kmalloc friendly size or create a dedicated slab for it).
> > [...]
> >> 
> >> Hi and thanks.
> >> I would suggest some improvements to consider:
> >> 
> >> - don't use the struct track to store orig_size, although it's an obvious
> >> first choice. It's unused waste for the free_track, and also for any
> >> non-kmalloc caches. I'd carve out an extra int next to the struct tracks.
> >> Only for kmalloc caches (probably a new kmem cache flag set on creation will
> >> be needed to easily distinguish them).
> >> Besides the saved space, you can then set the field from ___slab_alloc()
> >> directly and not need to pass the orig_size also to alloc_debug_processing()
> >> etc.
> >  
> > Here is a draft patch fowlling your suggestion, please check if I missed
> > anything? (Quick test showed it achived similar effect as v1 patch). Thanks!
> 
> Thanks, overal it looks at first glance!

Thanks!

> > ---
> > diff --git a/include/linux/slab.h b/include/linux/slab.h
> > index 0fefdf528e0d..d3dacb0f013f 100644
> > --- a/include/linux/slab.h
> > +++ b/include/linux/slab.h
> > @@ -29,6 +29,8 @@
> >  #define SLAB_RED_ZONE		((slab_flags_t __force)0x00000400U)
> >  /* DEBUG: Poison objects */
> >  #define SLAB_POISON		((slab_flags_t __force)0x00000800U)
> > +/* Indicate a slab of kmalloc */
> 
> "Indicate a kmalloc cache" would be more precise.
 
Will use this in next version.

> > +#define SLAB_KMALLOC		((slab_flags_t __force)0x00001000U)
> >  /* Align objs on cache lines */
> >  #define SLAB_HWCACHE_ALIGN	((slab_flags_t __force)0x00002000U)
> >  /* Use GFP_DMA memory */
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 26b00951aad1..3b0f80927817 100644
> 
> <snip>
> 
> > 
> >> - the knowledge of actual size could be used to improve poisoning checks as
> >> well, detect cases when there's buffer overrun over the orig_size but not
> >> cache's size. e.g. if you kmalloc(48) and overrun up to 64 we won't detect
> >> it now, but with orig_size stored we could?
> > 
> > The above patch doesn't touch this. As I have a question, for the
> > [orib_size, object_size) area, shall we fill it with POISON_XXX no matter
> > REDZONE flag is set or not?
> 
> Ah, looks like we use redzoning, not poisoning, for padding from
> s->object_size to word boundary. So it would be more consistent to use the
> redzone pattern (RED_ACTIVE) and check with the dynamic orig_size. Probably
> no change for RED_INACTIVE handling is needed though.

Thanks for clarifying, will go this way and do more test. Also I'd 
make it a separate patch, as it is logically different from the space
wastage.

Thanks,
Feng