Re: [PATCH v4 12/21] parallels: Let image extensions work in RW mode

[Alexander Ivanov]

On 1/18/24 14:31, Denis V. Lunev wrote:
> On 1/16/24 15:45, Denis V. Lunev wrote:
> > On 12/28/23 11:12, Alexander Ivanov wrote:
> > > Now we support extensions saving and can let to work with them in
> > > read-write mode.
> > >
> > > Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
> > > ---
> > >   block/parallels-ext.c |  4 ----
> > >   block/parallels.c     | 17 ++++-------------
> > >   2 files changed, 4 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/block/parallels-ext.c b/block/parallels-ext.c
> > > index c83d1ea393..195b01b109 100644
> > > --- a/block/parallels-ext.c
> > > +++ b/block/parallels-ext.c
> > > @@ -175,10 +175,6 @@ parallels_load_bitmap(BlockDriverState *bs, 
> > > uint8_t *data, size_t data_size,
> > >           return NULL;
> > >       }
> > >   -    /* We support format extension only for RO parallels images. */
> > > -    assert(!(bs->open_flags & BDRV_O_RDWR));
> > > -    bdrv_dirty_bitmap_set_readonly(bitmap, true);
> > > -
> > >       return bitmap;
> > >   }
> > >   diff --git a/block/parallels.c b/block/parallels.c
> > > index a49922c6a7..d5d87984cf 100644
> > > --- a/block/parallels.c
> > > +++ b/block/parallels.c
> > > @@ -1374,19 +1374,10 @@ static int parallels_open(BlockDriverState 
> > > *bs, QDict *options, int flags,
> > >       }
> > >         if (ph.ext_off) {
> > > -        if (flags & BDRV_O_RDWR) {
> > > -            /*
> > > -             * It's unsafe to open image RW if there is an 
> > > extension (as we
> > > -             * don't support it). But parallels driver in QEMU 
> > > historically
> > > -             * ignores the extension, so print warning and don't care.
> > > -             */
> > > -            warn_report("Format Extension ignored in RW mode");
> > > -        } else {
> > > -            ret = parallels_read_format_extension(
> > > -                    bs, le64_to_cpu(ph.ext_off) << 
> > > BDRV_SECTOR_BITS, errp);
> > > -            if (ret < 0) {
> > > -                goto fail;
> > > -            }
> > > +        ret = parallels_read_format_extension(
> > > +                bs, le64_to_cpu(ph.ext_off) << BDRV_SECTOR_BITS, 
> > > errp);
> > > +        if (ret < 0) {
> > > +            goto fail;
> > >           }
> > >       }
> > Reviewed-by: Denis V. Lunev <den@openvz.org>
> This patch also deserves a note, what will happen with
> format extensions clusters. According to the current
> policy, we have only transient extensions, i.e.
> CBT. Cluster allocation mechanism will reuse these
> clusters as they are not marked as used.
> Thus we should either set format extension offset
> in the header to 0 or perform any other correct
> measures to properly handle this.
Yes, all the clusters used by extensions are marked as unused
on loading. In further work they can be reallocated for other
purposes.
Agree that we need to set ext_off to zero.
> It should also be noted, that on any QEMU crash
> appropriate format extensions are to be properly
> treated. We could not make them RW until this would
> not be addressed as we could easily mess up with
> trashed metadata.
If QEMU crashes after extensions loading there will be
zero in the ext_off field and an inappropriate dirty bitmap
will be ignored.

--
Best regards,
Alexander Ivanov