[PATCH RFC 6/6] mm, slub: sheaf prefilling for guaranteed allocations

[Vlastimil Babka]

On 11/18/24 14:13, Hyeonggon Yoo wrote:
> On Wed, Nov 13, 2024 at 1:39 AM Vlastimil Babka <vbabka at suse.cz> wrote:
>> +
>> +/*
>> + * Allocate from a sheaf obtained by kmem_cache_prefill_sheaf()
>> + *
>> + * Guaranteed not to fail as many allocations as was the requested count.
>> + * After the sheaf is emptied, it fails - no fallback to the slab cache itself.
>> + *
>> + * The gfp parameter is meant only to specify __GFP_ZERO or __GFP_ACCOUNT
>> + * memcg charging is forced over limit if necessary, to avoid failure.
>> + */
>> +void *
>> +kmem_cache_alloc_from_sheaf_noprof(struct kmem_cache *s, gfp_t gfp,
>> +                                  struct slab_sheaf *sheaf)
>> +{
>> +       void *ret = NULL;
>> +       bool init;
>> +
>> +       if (sheaf->size == 0)
>> +               goto out;
>> +
>> +       ret = sheaf->objects[--sheaf->size];
>> +
>> +       init = slab_want_init_on_alloc(gfp, s);
>> +
>> +       /* add __GFP_NOFAIL to force successful memcg charging */
>> +       slab_post_alloc_hook(s, NULL, gfp | __GFP_NOFAIL, 1, &ret, init, s->object_size);
> 
> Maybe I'm missing something, but how can this be used for non-sleepable contexts
> if __GFP_NOFAIL is used? I think we have to charge them when the sheaf

AFAIK it forces memcg to simply charge even if allocated memory goes over
the memcg limit. So there's no issue with a non-sleepable context, there
shouldn't be memcg reclaim happening in that case.

> is returned
> via kmem_cache_prefill_sheaf(), just like users of bulk alloc/free?

That would be very costly to charge/uncharge if most of the objects are not
actually used - it's what we want to avoid here.
Going over the memcgs limit a bit in a very rare case isn't considered such
an issue, for example Linus advocated such approach too in another context.

> Best,
> Hyeonggon
> 
>> +out:
>> +       trace_kmem_cache_alloc(_RET_IP_, ret, s, gfp, NUMA_NO_NODE);
>> +
>> +       return ret;
>> +}
>> +
>>  /*
>>   * To avoid unnecessary overhead, we pass through large allocation requests
>>   * directly to the page allocator. We use __GFP_COMP, because we will need to
>>
>> --
>> 2.47.0
>>