Re: apparent memory leak from object-add+object-del of memory-backend-ram

[David Hildenbrand <david@redhat.com>]

On 19.08.24 18:24, Peter Maydell wrote:
> Hi; I'm looking at a memory leak apparently in the host memory backend
> code that you can see from the qmp-cmd-test. Repro instructions:

Hi Peter,

> 
> (1) build QEMU with '--cc=clang' '--cxx=clang++' '--enable-debug'
> '--target-list=x86_64-softmmu' '--enable-sanitizers'
> (2) run 'make check'. More specifically, to get just this
> failure ('make check' on current head-of-tree produces some
> other unrelated leak errors) you can run the relevant single test:
> 
> (cd build/asan && ASAN_OPTIONS="fast_unwind_on_malloc=0"
> QTEST_QEMU_BINARY=./qemu-system-x86_64 ./tests/qtest/qmp-cmd-test
> --tap -k -p /x86_64/qmp/object-add-failure-modes)
> 
> The test case is doing a variety of object-add then object-del
> of the "memory-backend-ram" object, and this add-del cycle seems
> to result in a fairly large leak:
> 
> Direct leak of 1572864 byte(s) in 6 object(s) allocated from:
>      #0 0x555c1336efd8 in __interceptor_calloc
> (/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/asan/qemu-system-x86_64+0x218efd8)
> (BuildId: fc7566a39db1253aed91d500b5b1784e0c438397)
>      #1 0x7f5bf3472c50 in g_malloc0 debian/build/deb/../../../glib/gmem.c:161:13
>      #2 0x555c155bb134 in bitmap_new
> /mnt/nvmedisk/linaro/qemu-from-laptop/qemu/include/qemu/bitmap.h:102:12
>      #3 0x555c155ba4ee in dirty_memory_extend system/physmem.c:1831:37
>      #4 0x555c15585a2b in ram_block_add system/physmem.c:1907:9
>      #5 0x555c15589e50 in qemu_ram_alloc_internal system/physmem.c:2109:5
>      #6 0x555c1558a096 in qemu_ram_alloc system/physmem.c:2129:12
>      #7 0x555c15518b69 in memory_region_init_ram_flags_nomigrate
> system/memory.c:1571:21
>      #8 0x555c1464fd27 in ram_backend_memory_alloc backends/hostmem-ram.c:34:12
>      #9 0x555c146510ac in host_memory_backend_memory_complete
> backends/hostmem.c:345:10
>      #10 0x555c1580bc90 in user_creatable_complete qom/object_interfaces.c:28:9
>      #11 0x555c1580c6f8 in user_creatable_add_type qom/object_interfaces.c:125:10
>      #12 0x555c1580ccc4 in user_creatable_add_qapi qom/object_interfaces.c:157:11
>      #13 0x555c15ff0e2c in qmp_object_add qom/qom-qmp-cmds.c:227:5
>      #14 0x555c161ce508 in qmp_marshal_object_add
> /mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/asan/qapi/qapi-commands-qom.c:337:5
>      #15 0x555c162a7139 in do_qmp_dispatch_bh qapi/qmp-dispatch.c:128:5
>      #16 0x555c16387921 in aio_bh_call util/async.c:171:5
>      #17 0x555c163887fc in aio_bh_poll util/async.c:218:13
>      #18 0x555c162e1288 in aio_dispatch util/aio-posix.c:423:5
>      #19 0x555c1638f7be in aio_ctx_dispatch util/async.c:360:5
>      #20 0x7f5bf3469d3a in g_main_dispatch
> debian/build/deb/../../../glib/gmain.c:3419:28
>      #21 0x7f5bf3469d3a in g_main_context_dispatch
> debian/build/deb/../../../glib/gmain.c:4137:7
>      #22 0x555c163935c9 in glib_pollfds_poll util/main-loop.c:287:9
>      #23 0x555c16391f03 in os_host_main_loop_wait util/main-loop.c:310:5
>      #24 0x555c16391acc in main_loop_wait util/main-loop.c:589:11
>      #25 0x555c14614917 in qemu_main_loop system/runstate.c:801:9
>      #26 0x555c16008b8c in qemu_default_main system/main.c:37:14
>      #27 0x555c16008bd7 in main system/main.c:48:12
>      #28 0x7f5bf12fbd8f in __libc_start_call_main
> csu/../sysdeps/nptl/libc_start_call_main.h:58:16
> 
> My initial suspicion here is that the problem is that
> TYPE_MEMORY_BACKEND has a UserCreatableClass::complete method which
> calls HostMemoryBackend::alloc, but there is no corresponding
> "now free this" in instance_finalize. So ram_backend_memory_alloc()
> calls memory_region_init_ram_flags_nomigrate(), which allocates
> RAM, dirty blocks, etc, but nothing ever destroys the MR and the
> memory is leaked when the TYPE_MEMORY_BACKEND object is finalized.
> 
> But there isn't a "free" method in HostMemoryBackendClass,
> only an "alloc", so this looks like an API with "leaks memory"
> baked into it. How is the freeing of the memory on object
> deletion intended to work?

I *think* during object_del(), we would be un-refing the contained 
memory-region, which in turn will make the refcount go to 0 and end up 
calling memory_region_finalize().

In memory_region_finalize, we do various things, including calling 
mr->destructor(mr).

For memory_region_init_ram_flags_nomigrate(), the deconstructor is set 
to memory_region_destructor_ram(). This is the place where we call 
qemu_ram_free(mr->ram_block);

There we clean up.

What we *don't* clean up is the allocation you are seeing: 
dirty_memory_extend() will extend the ram_list.dirty_memory bitmap as 
needed. It is not stored in the RAMBlock, it's a global list.

It's not really a leak I think: when we object_del + object_add *I 
think* that bitmap will simply get reused.

I think at some point I had a dirty_memory_shrink() implementation here, 
but I never upstreamed it, because it was not really worth the churn.

-- 
Cheers,

David / dhildenb