Messages in this thread | | | From | Waiman Long <> | Subject | Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab | Date | Mon, 3 May 2021 10:20:17 -0400 |
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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.
Cheers, Longman
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