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