On Thu, Jan 07, 2021 at 10:56:12AM +0000, Richard W.M. Jones wrote:
> On Thu, Jan 07, 2021 at 09:58:17AM +0000, Richard W.M. Jones wrote:
> > On Fri, Dec 04, 2020 at 01:27:13AM +0300, Vladimir Sementsov-Ogievskiy
> > wrote:
> > > Finally to be safe with calculations, to not calculate different
> > > maximums for different nodes (depending on cluster size and
> > > request_alignment), let's simply set QEMU_ALIGN_DOWN(INT64_MAX, 2^30)
> > > as absolute maximum bytes length for Qemu. Actually, it's not much less
> > > than INT64_MAX.
> >
> > > +/*
> > > + * We want allow aligning requests and disk length up to any 32bit
> > > alignment
> > > + * and don't afraid of overflow.
> > > + * To achieve it, and in the same time use some pretty number as maximum
> > > disk
> > > + * size, let's define maximum "length" (a limit for any offset/bytes
> > > request and
> > > + * for disk size) to be the greatest power of 2 less than INT64_MAX.
> > > + */
> > > +#define BDRV_MAX_ALIGNMENT (1L << 30)
> > > +#define BDRV_MAX_LENGTH (QEMU_ALIGN_DOWN(INT64_MAX, BDRV_MAX_ALIGNMENT))
> >
> > This change broke nbdkit tests.
>
> Actually that's not the only problem. It appears that we're unable to
> read or write the last sector of this disk:
>
> $ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -r -f raw "$uri" -c
> "r -v $(( 2**63 - 2**30 - 512 )) 512" '
> read failed: Input/output error
>
> $ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -f raw "$uri" -c "w
> -P 3 $(( 2**63 - 2**30 - 512 )) 512" '
> write failed: Input/output error
>
> You can play around with the constants. I found it's possible to read
> and write the non-aligned 512 bytes starting at 2^63-2^30-513. Could
> be a fencepost error somewhere in qemu?
Actually this is a pre-existing bug in qemu.
What happens is qemu-io calls qemu_strtosz("9223372035781033472")
which returns 0x7fffffffc0000000 and no error. That answer is plain
flat out wrong. The reason for that is qemu_strtosz uses floating
point for the calculation(!) so is limited to 53 bits of precision and
silently truncates.
It happened we were just lucky that our earlier test with
2^63 - 1024 worked.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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