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
