On Tue, Dec 08, 2020 at 05:57:07PM +0900, Joonsoo Kim wrote: > On Mon, Dec 07, 2020 at 09:25:54AM -0800, Paul E. McKenney wrote: > > On Mon, Dec 07, 2020 at 06:02:53PM +0900, Joonsoo Kim wrote: > > > 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... > > > > Good to hear! ;-) > > > > > 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) > > > > Here both functions would say "true" for a pointer from kmalloc()? > > Or do I need to add a third function that is happy with a pointer from > > either source? > > I focused on separation and missed this case that the user sometimes > cannot know the object source (kmalloc/kmem_cache). At first step, > just checking whether it is a slab-object or not looks enough. > > int kmem_valid_obj() OK, I will update my current kmalloc_valid_obj() to kmem_valid_obj(), thank you! > > I do understand that people who don't want to distinguish could just do > > "kmem_cache_valid_ptr(p) || kmalloc_valid_ptr(p)". However, the two > > use cases in the series have no idea whether the pointer they have came > > from kmalloc(), kmem_cache_alloc(), or somewhere else entirely, even an > > on-stack variable. > > > > Are you asking me to choose between the _obj() and _ptr() suffixes? > > Yes, I prefer _obj(). Then _obj() it is. > > If not, please help me understand the distinction. > > > > Do we want "debug" in these names as well? > > I don't think so since it can be called without enabling the debug > option. OK, understood. > > > 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. > > > > I am quite happy to add the "debug", but my use cases have no idea > > how the pointer was allocated. In fact, the next version of the > > patch will also handle allocator return addresses from vmalloc(). > > > > And for kernels without sufficient debug enabled, I need to provide > > the name of the slab cache, and this also is to be in the next version. > > Okay. So, your code would be... > > if (kmem_valid_obj(ptr)) > kmalloc_debug_print_provenance(ptr) > else if (vmalloc_valid_obj(ptr)) > .... Suggestions on where to put the mem_dump_obj() or whatever name that executes this code? Left to myself, I will pick a likely on the theory that it can always be moved later. This structuring does cause double work, but this should be OK because all of the uses I know of are on error paths. > > > 3. If concrete error message is needed, please introduce more functions. > > > > > > void *kmalloc_debug_error(errno) > > > > Agreed, in fact, I was planning to have a function that printed out > > a suitable error-message continuation to the console for ease-of-use > > reasons. For example, why is the caller deciding how deep the stack > > frame is? ;-) > > > > So something like this? > > > > void kmalloc_debug_print_provenance(void *ptr); > > > > With the understanding that it will print something helpful regardless > > of where ptr came from, within the constraints of the kernel build and > > boot options? > > Looks good idea. I suggest a name, kmem_dump_obj(), for this function. > In this case, I don't think that "debug" keyword is needed since it shows > something useful (slab cache info) even if debug option isn't enabled. > > So, for summary, we need to introduce two functions to accomplish your > purpose. Please correct me if wrong. > > int kmem_valid_obj(ptr) > void kmem_dump_obj(ptr) Within slab, agreed. We course also need something like mem_dump_obj() to handle a pointer with unknown provenance, along with the vmalloc_valid_obj() and the vmalloc_dump_obj(). And similar functions should other allocation sources become important. Thanx, Paul
