Re: [PATCH sl-b 1/6] mm: Add kmem_last_alloc() to return last allocation for memory block

[Joonsoo Kim]

Hello, Paul.

On Fri, Dec 04, 2020 at 04:40:52PM -0800, paulmck@xxxxxxxxxx wrote:
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> 
> There are kernel facilities such as per-CPU reference counts that give
> error messages in generic handlers or callbacks, whose messages are
> unenlightening.  In the case of per-CPU reference-count underflow, this
> is not a problem when creating a new use of this facility because in that
> case the bug is almost certainly in the code implementing that new use.
> However, trouble arises when deploying across many systems, which might
> exercise corner cases that were not seen during development and testing.
> Here, it would be really nice to get some kind of hint as to which of
> several uses the underflow was caused by.
> 
> This commit therefore exposes a new kmem_last_alloc() function that
> takes a pointer to dynamically allocated memory and returns the return
> address of the call that allocated it.  This pointer can reference the
> middle of the block as well as the beginning of the block, as needed
> by things like RCU callback functions and timer handlers that might not
> know where the beginning of the memory block is.  These functions and
> handlers can use the return value from kmem_last_alloc() to give the
> kernel hacker a better hint as to where the problem might lie.

I agree with exposing allocation caller information to the other
subsystem to help the debugging. Some suggestions...

1. It's better to separate a slab object check (validity check) and
retrieving the allocation caller. Someone else would want to check
only a validity. And, it doesn't depend on the debug configuration so
it's not good to bind it to the debug function.

kmem_cache_valid_(obj|ptr)
kmalloc_valid_(obj|ptr)

2. rename kmem_last_alloc to ...

int kmem_cache_debug_alloc_caller(cache, obj, &ret_addr)
int kmalloc_debug_alloc_caller(obj, &ret_addr)

or debug_kmem_cache_alloc_caller()

I think that function name need to include the keyword 'debug' to show
itself as a debugging facility (enabled at the debugging). And, return
errno and get caller address by pointer argument.

3. If concrete error message is needed, please introduce more functions.

void *kmalloc_debug_error(errno)

Thanks.