Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

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On 5/5/21 2:11 PM, Waiman Long wrote:
On 5/5/21 1:30 PM, Roman Gushchin wrote:


Btw, I wonder if we also need a change in the slab caches merging procedure? KMALLOC_NORMAL caches should not be merged with caches which can potentially
include accounted objects.

Thank for catching this omission.

I will take a look and modify the merging procedure in a new patch. Accounting is usually specified at kmem_cache_create() time. Though, I did find one instance of setting ACCOUNT flag in kmem_cache_alloc(), I will ignore this case and merge accounted, but unreclaimable caches to KMALLOC_CGROUP.

In mm/slab_common.c:

#define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
                         SLAB_CACHE_DMA32 | SLAB_ACCOUNT)

struct kmem_cache *find_mergeable(unsigned int size, unsigned int align,
  :
                if ((flags & SLAB_MERGE_SAME) != (s->flags & SLAB_MERGE_SAME))
                        continue;

By making sure kmalloc-cg-* has SLAB_ACCOUNT bit set, a kmemcache created with with SLAB_ACCOUNT may merge with kmalloc-cg-* whereas one without SLAB_ACCOUNT may merge with kmalloc-* for now. So the current code should work fine for most cases. Though, if the ACCOUNT flag is set at kmem_cache_alloc() and the cache happens to be merged into kmalloc-*, we will have the rare case that an objcg pointer array may have to be added to a kmalloc-* cache. However, this is not a common practice, and the three cases (not one, sorry) that I found so far is in

arch/x86/kvm/x86.c:     ctxt = kmem_cache_zalloc(x86_emulator_cache, GFP_KERNEL_ACCOUNT); fs/hostfs/hostfs_kern.c:        hi = kmem_cache_alloc(hostfs_inode_cache, GFP_KERNEL_ACCOUNT); virt/kvm/kvm_main.c:    vcpu = kmem_cache_zalloc(kvm_vcpu_cache, GFP_KERNEL_ACCOUNT);

We will have to advise against doing that.

Cheers,
Longman





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