On 5/5/21 10:06 PM, Waiman Long wrote: > There are currently two problems in the way the objcg pointer array > (memcg_data) in the page structure is being allocated and freed. > > On its allocation, it is possible that the allocated objcg pointer > 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. > > When it is freed, the objcg pointer array object may be the last one > in its slab and hence causes kfree() to be called again. With the > right workload, the slab cache may be set up in a way that allows the > recursive kfree() calling loop to nest deep enough to cause a kernel > stack overflow and panic the system. > > One way to solve this problem is to split the kmalloc-<n> caches > (KMALLOC_NORMAL) into two separate sets - a new set of kmalloc-<n> > (KMALLOC_NORMAL) caches for unaccounted objects only and a new set of > kmalloc-cg-<n> (KMALLOC_CGROUP) caches for accounted objects only. All > the other caches can still allow a mix of accounted and unaccounted > objects. > > With this change, all the objcg pointer array objects will come from > KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So > both the recursive kfree() problem and non-freeable slab problem are > gone. > > Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer have > mixed accounted and unaccounted objects, this will slightly reduce the > number of objcg pointer arrays that need to be allocated and save a bit > of memory. On the other hand, creating a new set of kmalloc caches does > have the effect of reducing cache utilization. So it is properly a wash. > > The new KMALLOC_CGROUP is added between KMALLOC_NORMAL and > KMALLOC_RECLAIM so that the first for loop in create_kmalloc_caches() > will include the newly added caches without change. > > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> A last nitpick: the new caches -cg should perhaps not be created when cgroup_memory_nokmem == true because kmemcg was disabled by the respective boot param.
