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
