Messages in this thread | | | From | Shakeel Butt <> | Date | Wed, 20 Jun 2018 23:30:29 -0700 | Subject | Re: [PATCH] slub: track number of slabs irrespective of CONFIG_SLUB_DEBUG |
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On Wed, Jun 20, 2018 at 6:15 PM Christopher Lameter <cl@linux.com> wrote: > > On Wed, 20 Jun 2018, Shakeel Butt wrote: > > > For !CONFIG_SLUB_DEBUG, SLUB does not maintain the number of slabs > > allocated per node for a kmem_cache. Thus, slabs_node() in > > __kmem_cache_empty(), __kmem_cache_shrink() and __kmem_cache_destroy() > > will always return 0 for such config. This is wrong and can cause issues > > for all users of these functions. > > > CONFIG_SLUB_DEBUG is set by default on almost all builds. The only case > where CONFIG_SLUB_DEBUG is switched off is when we absolutely need to use > the minimum amount of memory (embedded or some such thing). > > > The right solution is to make slabs_node() work even for > > !CONFIG_SLUB_DEBUG. The commit 0f389ec63077 ("slub: No need for per node > > slab counters if !SLUB_DEBUG") had put the per node slab counter under > > CONFIG_SLUB_DEBUG because it was only read through sysfs API and the > > sysfs API was disabled on !CONFIG_SLUB_DEBUG. However the users of the > > per node slab counter assumed that it will work in the absence of > > CONFIG_SLUB_DEBUG. So, make the counter work for !CONFIG_SLUB_DEBUG. > > Please do not do this. Find a way to avoid these checks. The > objective of a !CONFIG_SLUB_DEBUG configuration is to not compile in > debuggin checks etc etc in order to reduce the code/data footprint to the > minimum necessary while sacrificing debuggability etc etc. > > Maybe make it impossible to disable CONFIG_SLUB_DEBUG if CGROUPs are in > use? >
Copying from the other thread:
On Wed, Jun 20, 2018 at 6:22 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote: > > On Thu, Jun 21, 2018 at 3:20 AM Christopher Lameter <cl@linux.com> wrote: > > > > NAK. Its easier to simply not allow !CONFIG_SLUB_DEBUG for cgroups based > > configs because in that case you certainly have enough memory to include > > the runtime debug code as well as the extended counters. > > > > FWIW, I ran into issues with a combination of KASAN+CONFIG_SLUB > without having CONFIG_SLUB_DEBUG, because KASAN was using functions > that were broken without CONFIG_SLUB_DEBUG, so while you're at it with > creating dependencies, you might want to also say KASAN+CONFIG_SLUB > ==> CONFIG_SLUB_DEBUG.
KASAN is the only user of __kmem_cache_empty(). So, enforcing KASAN+CONFIG_SLUB => CONFIG_SLUB_DEBUG makes sense but not sure about cgroups or memcg. Though is it ok let __kmem_cache_shrink() & __kmem_cache_shutdown() be broken for !CONFIG_SLUB_DEBUG?
For __kmem_cache_shutdown(), I can understand that shutting down a kmem_cache when there are still objects allocated from it, is broken and wrong. For __kmem_cache_shrink(), maybe wrong answer from it is tolerable.
Shakeel
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