On 2022/12/6 上午12:28, Peter Xu wrote:
Chuang,
No worry on the delay; you're faster than when I read yours. :)
On Mon, Dec 05, 2022 at 02:56:15PM +0800, Chuang Xu wrote:
As a start, maybe you can try with poison address_space_to_flatview() (by
e.g. checking the start_pack_mr_change flag and assert it is not set)
during this process to see whether any call stack can even try to
dereference a flatview.
It's just that I didn't figure a good way to "prove" its validity, even if
I think this is an interesting idea worth thinking to shrink the downtime.
Thanks for your sugguestions!
I used a thread local variable to identify whether the current thread is a
migration thread(main thread of target qemu) and I modified the code of
qemu_coroutine_switch to make sure the thread local variable true only in
process_incoming_migration_co call stack. If the target qemu detects that
start_pack_mr_change is set and address_space_to_flatview() is called in
non-migrating threads or non-migrating coroutine, it will crash.
Are you using the thread var just to avoid the assert triggering in the
migration thread when commiting memory changes?
I think _maybe_ another cleaner way to sanity check this is directly upon
the depth:
static inline FlatView *address_space_to_flatview(AddressSpace *as)
{
/*
* Before using any flatview, sanity check we're not during a memory
* region transaction or the map can be invalid. Note that this can
* also be called during commit phase of memory transaction, but that
* should also only happen when the depth decreases to 0 first.
*/
assert(memory_region_transaction_depth == 0);
return qatomic_rcu_read(&as->current_map);
}
That should also cover the safe cases of memory transaction commits during
migration.
Peter, I tried this way and found that the target qemu will crash.
Here is the gdb backtrace:
#0 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1 0x00007ff2929d851a in __GI_abort () at abort.c:118
#2 0x00007ff2929cfe67 in __assert_fail_base (fmt=<optimized out>,
assertion=assertion@entry=0x55a32578cdc0 "memory_region_transaction_depth == 0",
file=file@entry=0x55a32575d9b0 "/data00/migration/qemu-5.2.0/include/exec/memory.h",
line=line@entry=766, function=function@entry=0x55a32578d6e0
<__PRETTY_FUNCTION__.20463> "address_space_to_flatview") at assert.c:92
#3 0x00007ff2929cff12 in __GI___assert_fail (assertion=assertion@entry=0x55a32578cdc0
"memory_region_transaction_depth == 0", file=file@entry=0x55a32575d9b0
"/data00/migration/qemu-5.2.0/include/exec/memory.h", line=line@entry=766,
function=function@entry=0x55a32578d6e0 <__PRETTY_FUNCTION__.20463>
"address_space_to_flatview") at assert.c:101
#4 0x000055a324b2ed5e in address_space_to_flatview (as=0x55a326132580
<address_space_memory>) at
/data00/migration/qemu-5.2.0/include/exec/memory.h:766
#5 0x000055a324e79559 in address_space_to_flatview (as=0x55a326132580
<address_space_memory>) at ../softmmu/memory.c:811
#6 address_space_get_flatview (as=0x55a326132580 <address_space_memory>) at
../softmmu/memory.c:805
#7 0x000055a324e96474 in address_space_cache_init (cache=cache@entry=0x55a32a4fb000,
as=<optimized out>, addr=addr@entry=68404985856, len=len@entry=4096,
is_write=false) at ../softmmu/physmem.c:3307
#8 0x000055a324ea9cba in virtio_init_region_cache (vdev=0x55a32985d9a0, n=0)
at ../hw/virtio/virtio.c:185
#9 0x000055a324eaa615 in virtio_load (vdev=0x55a32985d9a0, f=<optimized out>,
version_id=<optimized out>) at ../hw/virtio/virtio.c:3203
#10 0x000055a324c6ab96 in vmstate_load_state (f=f@entry=0x55a329dc0c00,
vmsd=0x55a325fc1a60 <vmstate_virtio_scsi>, opaque=0x55a32985d9a0, version_id=1)
at ../migration/vmstate.c:143
#11 0x000055a324cda138 in vmstate_load (f=0x55a329dc0c00, se=0x55a329941c90) at
../migration/savevm.c:913
#12 0x000055a324cdda34 in qemu_loadvm_section_start_full (mis=0x55a3284ef9e0,
f=0x55a329dc0c00) at ../migration/savevm.c:2741
#13 qemu_loadvm_state_main (f=f@entry=0x55a329dc0c00,
mis=mis@entry=0x55a3284ef9e0) at ../migration/savevm.c:2939
#14 0x000055a324cdf66a in qemu_loadvm_state (f=0x55a329dc0c00) at
../migration/savevm.c:3021
#15 0x000055a324d14b4e in process_incoming_migration_co (opaque=<optimized
out>) at ../migration/migration.c:574
#16 0x000055a32501ae3b in coroutine_trampoline (i0=<optimized out>, i1=<optimized
out>) at ../util/coroutine-ucontext.c:173
#17 0x00007ff2929e8000 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#18 0x00007ffed80dc2a0 in ?? ()
#19 0x0000000000000000 in ?? ()
address_space_cache_init() is the only caller of address_space_to_flatview
I can find in vmstate_load call stack so far. Although I think the mr used
by address_space_cache_init() won't be affected by the delay of
memory_region_transaction_commit(), we really need a mechanism to prevent
the modified mr from being used.
Maybe we can build a stale list:
If a subregion is added, add its parent to the stale list(considering that
new subregion's priority has uncertain effects on flatviews).
If a subregion is deleted, add itself to the stale list.
When memory_region_transaction_commit() regenerates flatviews, clear the
stale list.
when address_space_translate_internal() is called, check whether the mr
looked up matches one of mrs（or its child）in the stale list. If yes, a
crash will be triggered.
There may be many details to consider in this mechanism. Hope you can give
some suggestions on its feasibility.
I tested migration for lots of times, there was no crash. Does this prove
the validity to some extent?
Yes I think so, it's just that if we cannot 100% prove it's safe (e.g. you
cannot cover all the code paths in qemu that migration can trigger) then we
may need some sanity check like above along with the solution to make sure
even if something wrong it won't go wrong as weird.
And if we want to try this out, it'll better be at the start of a dev cycle
and we fix things or revert before the next rc0 releases.
I'm not sure whether that assert might be too strong, we can use an error
instead, but so far I don't see how that can happen and if that happens I
feel like it's bad enough, so maybe not so much. Then AFAICT we can
completely drop start_pack_mr_change with that stronger check.
If you agree with above, feel free to have two patches in the new version,
making the depth assert a separate patch. At the meantime, let's see
whether you can get some other comments from others.
Yes, start_pack_mr_change isn't needed any more. I'll drop it in the new
patches.
