On 5/3/21 12:24 PM, Shakeel Butt wrote:
On Mon, May 3, 2021 at 8:32 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
On 5/3/21 4:20 PM, Waiman Long wrote:
On 5/3/21 8:22 AM, Vlastimil Babka wrote:
On 5/2/21 8:07 PM, Waiman Long wrote:
The obj_cgroup array (memcg_data) embedded in the page structure is
allocated at the first instance an accounted memory allocation happens.
With the right size object, it is possible that the allocated obj_cgroup
array comes from the same slab that requires memory accounting. If this
happens, the slab will never become empty again as there is at least one
object left (the obj_cgroup array) in the slab.
With instructmentation code added to detect this situation, I got 76
hits on the kmalloc-192 slab when booting up a test kernel on a VM.
So this can really happen.
To avoid the creation of these unfreeable slabs, a check is added to
memcg_alloc_page_obj_cgroups() to detect that and double the size
of the array in case it happens to make sure that it comes from a
different kmemcache.
This change, however, does not completely eliminate the presence
of unfreeable slabs which can still happen if a circular obj_cgroup
array dependency is formed.
Hm this looks like only a half fix then.
I'm afraid the proper fix is for kmemcg to create own set of caches for the
arrays. It would also solve the recursive kfree() issue.
Right, this is a possible solution. However, the objcg pointers array should
need that much memory. Creating its own set of kmemcaches may seem like an
overkill.
Well if we go that way, there might be additional benefits:
depending of gfp flags, kmalloc() would allocate from:
kmalloc-* caches that never have kmemcg objects, thus can be used for the objcg
pointer arrays
kmalloc-cg-* caches that have only kmemcg unreclaimable objects
kmalloc-rcl-* and dma-kmalloc-* can stay with on-demand
memcg_alloc_page_obj_cgroups()
This way we fully solve the issues that this patchset solves. In addition we get
better separation between kmemcg and !kmemcg thus save memory - no allocation of
the array as soon as a single object appears in slab. For "kmalloc-8" we now
have 8 bytes for the useful data and 8 bytes for the obj_cgroup pointer.
Yes this seems like a better approach.
OK, I will try to go this route then if there is no objection from others.
From slabinfo, the objs/slab numbers range from 4-512. That means we
need kmalloc-cg-{32,64,128,256,512,1k,2k,4k}. A init function to set up
the new kmemcaches and an allocation function that use the proper
kmemcaches to allocate from.
Cheers,
Longman
