Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH v4 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches | Date | Wed, 5 May 2021 19:19:23 -0400 |
| |
On 5/5/21 5:41 PM, Vlastimil Babka wrote: > 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@suse.cz> >> Signed-off-by: Waiman Long <longman@redhat.com> >> Reviewed-by: Shakeel Butt <shakeelb@google.com> > 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. > It is a nice to have feature. However, the nokmem kernel parameter isn't used that often. The cgroup_memory_nokmem variable is private to memcontrol.c and is not directly accessible. I will take a look on that, but it will be a follow-on patch. I am not planning to change the current patchset unless there are other issues coming up.
Cheers, Longman
| |