Re: [PATCH] migration/block-dirty-bitmap: fix larger granularity bitmaps

[Stefan Reiter]

On 10/21/20 5:17 PM, Vladimir Sementsov-Ogievskiy wrote:
> 21.10.2020 17:44, Stefan Reiter wrote:
> > sectors_per_chunk is a 64 bit integer, but the calculation is done in 32
> > bits, leading to an overflow for coarse bitmap granularities.
> >
> > If that results in the value 0, it leads to a hang where no progress is
> > made but send_bitmap_bits is constantly called with nr_sectors being 0.
> >
> > Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
> > ---
> >   migration/block-dirty-bitmap.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/migration/block-dirty-bitmap.c 
> > b/migration/block-dirty-bitmap.c
> > index 5bef793ac0..5398869e2b 100644
> > --- a/migration/block-dirty-bitmap.c
> > +++ b/migration/block-dirty-bitmap.c
> > @@ -562,8 +562,9 @@ static int add_bitmaps_to_list(DBMSaveState *s, 
> > BlockDriverState *bs,
> >           dbms->bitmap_alias = g_strdup(bitmap_alias);
> >           dbms->bitmap = bitmap;
> >           dbms->total_sectors = bdrv_nb_sectors(bs);
> > -        dbms->sectors_per_chunk = CHUNK_SIZE * 8 *
> > +        dbms->sectors_per_chunk = CHUNK_SIZE * 8lu *
>
> I'd prefer 8llu for absolute safety.
>
> >               bdrv_dirty_bitmap_granularity(bitmap) >> BDRV_SECTOR_BITS;
> > +        assert(dbms->sectors_per_chunk != 0);
>
> I doubt that we need this assertion. Bug fixed, and it's obviously 
> impossible.
> And if we really want to assert that there is no overflow (assuming 
> future changes),
> it should look like this:
>
>    assert(bdrv_dirty_bitmap_granularity(bitmap) < (1ull << 63) / 
> CHUNK_SIZE / 8 >> BDRV_SECTOR_BITS);
>
> to cover not only corner case but any overflow.. And of course we should 
> modify original expression
> to do ">> BDRV_SECTOR_BITS" earlier than all multiplies, like
>
>    dbms->sectors_per_chunk = CHUNK_SIZE * 8llu * 
> (bdrv_dirty_bitmap_granularity(bitmap) >> BDRV_SECTOR_BITS);
>
>
> But I think that only s/8/8ull/ change is enough.

I agree, and I wouldn't mind removing the assert, but just to clarify it 
was mostly meant to prevent the case where the migration gets stuck 
entirely. Even if the calculation is wrong, it would at least do 
_something_ instead of endlessly looping.

Maybe an

    assert(nr_sectors != 0);

in send_bitmap_bits instead for that?

> >           if (bdrv_dirty_bitmap_enabled(bitmap)) {
> >               dbms->flags |= DIRTY_BITMAP_MIG_START_FLAG_ENABLED;
> >           }
>
>
> With 8llu and with or without assertion:
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>