On Wed, May 05, 2021 at 08:02:06PM +0200, Vlastimil Babka wrote: > On 5/5/21 7:30 PM, Roman Gushchin wrote: > > On Wed, May 05, 2021 at 11:46:13AM -0400, Waiman Long wrote: > >> > >> 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. > > > > Unfortunately the positive effect of this change will be likely > > reversed by a lower utilization due to a larger number of caches. > > > > Btw, I wonder if we also need a change in the slab caches merging procedure? > > KMALLOC_NORMAL caches should not be merged with caches which can potentially > > include accounted objects. > > Good point. But looks like kmalloc* caches are extempt from all merging in > create_boot_cache() via > > s->refcount = -1; /* Exempt from merging for now */ Oh, interesting... I wonder if there is (still) a good reason for that? Maybe we can remove this limitation and save some memory? > > It wouldn't hurt though to create the kmalloc-cg-* caches with SLAB_ACCOUNT flag > to prevent accidental merging in case the above is ever removed. It would also > better reflect reality, and ensure that the array is allocated immediately with > the page, AFAICS. That wouldn't be enough, because a !SLAB_ACCOUNT cache can still have accounted allocations and be merged with kmalloc-* cache. What we might wanna do is to keep the no-merging rule for kmalloc-*, but relax it for kmalloc-cg-* caches. But we can do it later, as a separate change. Thanks!
