On Mon, 21 Nov 2022 at 12:29, Jonathan Cameron <jonathan.came...@huawei.com> wrote: > > On Mon, 21 Nov 2022 11:47:48 -0500 > Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > On Mon, 21 Nov 2022 at 11:22, Jonathan Cameron via > > <qemu-devel@nongnu.org> wrote: > > > > > > First CC list is a guess as I haven't managed to root cause where things > > > are > > > going wrong yet. > > > > > > Originally hit this whilst rebasing some CXL patches on v7.2.0-rc1. > > > CXL makes extensive use of memory-backends and most my tests happen > > > to use memory-backend-file > > > > > > Issue seen on arm64 and x86 though helpfully on x86 the crash appears in > > > an entirely > > > unrelated location (though the 'fix' works). > > > > > > Fairly minimal test command line. > > > > > > qemu-system-aarch64 \ > > > -M virt \ > > > -drive if=none,file=full.qcow2,format=qcow2,id=hd \ > > > -device virtio-blk,drive=hd \ > > > -object > > > memory-backend-file,id=cxl-mem1,mem-path=/tmp/cxltest.raw,size=256M,align=256M > > > \ > > > > > > Powerdown the machine or ctrl-c during boot gives a segfault. > > > On arm64 it was in a stable location that made at least some sense in that > > > bs in the below snippet is NULL. > > > > > > I added the follow work around and the segfault goes away... > > > > > > [PATCH] temp > > > > > > --- > > > block/io.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/block/io.c b/block/io.c > > > index b9424024f9..750e1366aa 100644 > > > --- a/block/io.c > > > +++ b/block/io.c > > > @@ -3324,6 +3324,9 @@ void bdrv_unregister_buf(BlockDriverState *bs, void > > > *host, size_t size) > > > { > > > BdrvChild *child; > > > > > > + if (!bs) { > > > + return; > > > + } > > > GLOBAL_STATE_CODE(); > > > if (bs->drv && bs->drv->bdrv_unregister_buf) { > > > bs->drv->bdrv_unregister_buf(bs, host, size); > > > > bdrv_*() APIs generally don't accept NULL bs arguments. > > > > I think blk_unregister_buf() needs to handle the blk_bs() NULL return > > value. Can you confirm that the parent function is > > blk_unregister_buf()? > > > > This bug may have been introduced by commit baf422684d73 ("virtio-blk: > > use BDRV_REQ_REGISTERED_BUF optimization hint"). > Got it in one. I just bisected to exactly that patch > > + using the below change indeed works just as well as the above. > Now I'd send this as a patch, but I don't yet sufficiently understand what > that change you > referenced did to break things Seems it registered a notifier that is getting > called for all ram blocks, not just the one virtio-blk ones? > > Perhaps better if you send a fix with an explanation :)
Sure, no problem. I have reproduced the bug and will send a patch. Thanks, Stefan