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. Vlastimil > Cheers, > Longman >
