Re: [Qemu-devel] [PATCH v2 1/3] block: Respect underlying file's EOF

[Max Reitz]

On 2014-10-22 at 15:40, Kevin Wolf wrote:
> Am 22.10.2014 um 15:24 hat Max Reitz geschrieben:
> > When falling through to the underlying file in
> > bdrv_co_get_block_status(), if it returns that the query offset is
> > beyond the file end (by setting *pnum to 0), return the range to be
> > zero and do not let the number of sectors for which information could be
> > obtained be overwritten.
> >
> > Signed-off-by: Max Reitz <address@hidden>
> > ---
> >   block.c | 16 ++++++++++++++--
> >   1 file changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/block.c b/block.c
> > index 773e87e..68dc11d 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -3954,13 +3954,25 @@ static int64_t coroutine_fn 
> > bdrv_co_get_block_status(BlockDriverState *bs,
> >       if (bs->file &&
> >           (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) &&
> >           (ret & BDRV_BLOCK_OFFSET_VALID)) {
> > +        int backing_pnum;
> Sounds as if it were about bs->backing_hd, whereas it really is about
> bs->file. Perhaps we can find a better name.

Good question why I called it that way. I don't know and just haven't 
thought about it afterwards.

> > +
> >           ret2 = bdrv_co_get_block_status(bs->file, ret >> BDRV_SECTOR_BITS,
> > -                                        *pnum, pnum);
> > +                                        *pnum, &backing_pnum);
> >           if (ret2 >= 0) {
> >               /* Ignore errors.  This is just providing extra information, it
> >                * is useful but not necessary.
> >                */
> > -            ret |= (ret2 & BDRV_BLOCK_ZERO);
> > +            if (!backing_pnum) {
> > +                /* !backing_pnum indicates an offset at or beyond the EOF; it 
> > is
> > +                 * perfectly valid for the format block driver to point to such
> > +                 * offsets, so catch it and mark everything as unallocated and
> > +                 * empty */
> > +                ret = BDRV_BLOCK_ZERO;
> I don't think this is correct. Even though the area is unallocated in
> bs->file, it's certainly allocated in bs (BDRV_BLOCK_ALLOCATED) and is
> read from bs->file (BDRV_BLOCK_DATA). It is arguable whether an offset
> beyond EOF is valid (BDRV_BLOCK_OFFSET_VALID), but in terms of the qemu
> block layer I'm inclined to say yes.
>
> So I'd suggest ret |= BDRV_BLOCK_ZERO instead.

You're right, this fits in better with the intention of this block of 
code (the whole recursion to the protocol layer), too.

Thanks,

Max

> > +            } else {
> > +                /* Limit request to the range reported by the protocol driver 
> > */
> > +                *pnum = backing_pnum;
> > +                ret |= (ret2 & BDRV_BLOCK_ZERO);
> > +            }
> >           }
> >       }
> Kevin