On Wed, Aug 28, 2024 at 5:20 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > On Wed, Aug 28, 2024 at 04:25:30PM GMT, Yosry Ahmed wrote: > > On Tue, Aug 27, 2024 at 4:52 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > > > [...] > > > + > > > + /* Ignore KMALLOC_NORMAL cache to avoid circular dependency. */ > > > + if ((s->flags & KMALLOC_TYPE) == SLAB_KMALLOC) > > > + return true; > > > > Taking a step back here, why do we need this? Which circular > > dependency are we avoiding here? > > commit 494c1dfe855ec1f70f89552fce5eadf4a1717552 > Author: Waiman Long <longman@xxxxxxxxxx> > Date: Mon Jun 28 19:37:38 2021 -0700 > > mm: memcg/slab: create a new set of kmalloc-cg-<n> caches > > 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. > ... Thanks for the reference, this makes sense. Wouldn't it be easier to special case the specific slab cache used for the objcg vector or use a dedicated cache for it instead of using kmalloc caches to begin with? Anyway, I am fine with any approach you and/or the slab maintainers prefer, as long as we make things clear. If you keep the following approach as-is, please expand the comment or refer to the commit you just referenced. Personally, I prefer either explicitly special casing the slab cache used for the objcgs vector, explicitly tagging KMALLOC_NORMAL allocations, or having a dedicated documented helper that finds the slab cache kmalloc type (if any) or checks if it is a KMALLOC_NORMAL cache.
