Re: [Qemu-devel] [PATCH 1/4] block: Fix dirty bitmap in bdrv_co_discard

[Fam Zheng]

On Tue, 05/05 15:06, Paolo Bonzini wrote:
> 
> 
> On 05/05/2015 14:46, Fam Zheng wrote:
> > Unsetting dirty globally with discard is not very correct. The discard may 
> > zero
> > out sectors (depending on can_write_zeroes_with_unmap), we should replicate
> > this change to destinition side to make sure that the guest sees the same 
> > data.
> > 
> > Calling bdrv_reset_dirty also troubles mirror job because the hbitmap 
> > iterator
> > doesn't expect unsetting of bits after current position.
> > 
> > So let's do it the opposite way which fixes both problems: set the dirty 
> > bits
> > if we are to discard it.
> 
> This is not enough, you also have to do the discard in block/mirror.c,
> otherwise the destination image could even become fully provisioned!

I wasn't sure what if src and dest have different can_write_zeroes_with_unmap
value, but your argument is stronger.

I will add discard in mirror.  Besides that, do we need to compare the
can_write_zeroes_with_unmap?

Fam

> 
> Paolo
> 
> > Reported-by: address@hidden
> > Signed-off-by: Fam Zheng <address@hidden>
> > ---
> >  block/io.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/block/io.c b/block/io.c
> > index 1ce62c4..809688b 100644
> > --- a/block/io.c
> > +++ b/block/io.c
> > @@ -2343,8 +2343,6 @@ int coroutine_fn bdrv_co_discard(BlockDriverState 
> > *bs, int64_t sector_num,
> >          return -EROFS;
> >      }
> >  
> > -    bdrv_reset_dirty(bs, sector_num, nb_sectors);
> > -
> >      /* Do nothing if disabled.  */
> >      if (!(bs->open_flags & BDRV_O_UNMAP)) {
> >          return 0;
> > @@ -2354,6 +2352,8 @@ int coroutine_fn bdrv_co_discard(BlockDriverState 
> > *bs, int64_t sector_num,
> >          return 0;
> >      }
> >  
> > +    bdrv_set_dirty(bs, sector_num, nb_sectors);
> > +
> >      max_discard = MIN_NON_ZERO(bs->bl.max_discard, 
> > BDRV_REQUEST_MAX_SECTORS);
> >      while (nb_sectors > 0) {
> >          int ret;
> >