On Sun, May 10, 2020 at 05:27:41PM -0400, Qian Cai wrote: > On May 9, 2020, at 5:44 AM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Thu, May 07, 2020 at 01:29:04PM -0400, Qian Cai wrote: > >> On May 7, 2020, at 1:16 PM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > >>> I don't mind adding additional tracking info if it helps with debugging. > >>> But if it's for improving false positives, I'd prefer to look deeper > >>> into figure out why the pointer reference graph tracking failed. > >> > >> No, the task struct leaks are real leaks. It is just painful to figure > >> out the missing or misplaced put_task_struct() from the kmemleak > >> reports at the moment. > > > > We could log the callers to get_task_struct() and put_task_struct(), > > something like __builtin_return_address(0) (how does this work if the > > function is inlined?). If it's not the full backtrace, it shouldn't slow > > down kmemleak considerably. I don't think it's worth logging only the > > first/last calls to get/put. You'd hope that put is called in reverse > > order to get. > > > > I think it may be better if this is added as a new allocation pointed to > > from kmemleak_object rather than increasing this structure since it will > > be added on a case by case basis. When dumping the leak information, it > > would also dump the get/put calls, in the order they were called. We > > could add some simple refcount tracking (++ for get, -- for put) to > > easily notice any imbalance. > > > > I'm pretty busy next week but happy to review if you have a patch ;). > > I am still thinking about a more generic way for all those > refcount-based leaks without needing of manual annotation of all those > places. Today, I had another one, > > unreferenced object 0xe6ff008924f28500 (size 128): > comm "qemu-kvm", pid 4835, jiffies 4295141828 (age 6944.120s) > hex dump (first 32 bytes): > 01 00 00 00 6b 6b 6b 6b 00 00 00 00 ad 4e ad de ....kkkk.....N.. > ff ff ff ff 6b 6b 6b 6b ff ff ff ff ff ff ff ff ....kkkk........ > backtrace: > [<000000005ed1a868>] slab_post_alloc_hook+0x74/0x9c > [<00000000c65ee7dc>] kmem_cache_alloc_trace+0x2b4/0x3d4 > [<000000009efa9e6e>] do_eventfd+0x54/0x1ac > [<000000001146e724>] __arm64_sys_eventfd2+0x34/0x44 > [<0000000096fc3a61>] do_el0_svc+0x128/0x1dc > [<000000005ae8f980>] el0_sync_handler+0xd0/0x268 > [<0000000043f2c790>] el0_sync+0x164/0x180 > > That is eventfd_ctx_fileget() / eventfd_ctx_put() pairs. In this case it uses kref_get() to increment the refcount. We could add a kmemleak_add_trace() which allocates a new array and stores the stack trace, linked to the original object. Similarly for kref_put(). If we do this for each inc/dec call, I'd leave it off as default and only enable it explicitly by cmdline argument or /sys/kerne/debug/kmemleak when needed. In most cases you'd hope there is no leak, so no point in tracking additional metadata. But if you do hit a problem, just enable the additional tracking to help with the debugging. -- Catalin
