Re: [Qemu-devel] [PATCH v2] cpu_physical_memory_sync_dirty_bitmap: Fix alignment check

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> On 24/07/2017 18:51, Dr. David Alan Gilbert (git) wrote:
> > From: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > 
> > This code has an optimised, word aligned version, and a boring
> > unaligned version.  Recently 084140bd498909 fixed a missing offset
> > addition from the core of both versions.  However, the offset isn't
> > necessarily aligned and thus the choice between the two versions
> > needs fixing up to also include the offset.
> > 
> > Symptom:
> >   A few stuck unsent pages during migration; not normally noticed
> > unless under very low bandwidth in which case the migration may get
> > stuck never ending and never performing a 2nd sync; noticed by
> > a hanging postcopy-test on a very heavily loaded system.
> > 
> > Fixes: 084140bd498909
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Reported-by: Alex Benneé <alex.benee@linaro.org>
> > Tested-by: Alex Benneé <alex.benee@linaro.org>
> > 
> > --
> > v2
> >   Move 'page' inside the if (Comment from Paolo)
> > ---
> >  include/exec/ram_addr.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> > index c04f4f67f6..d017639f7e 100644
> > --- a/include/exec/ram_addr.h
> > +++ b/include/exec/ram_addr.h
> > @@ -377,19 +377,20 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
> >                                                 uint64_t *real_dirty_pages)
> >  {
> >      ram_addr_t addr;
> > -    unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
> > +    unsigned long word = BIT_WORD((start + rb->offset) >> TARGET_PAGE_BITS);
> >      uint64_t num_dirty = 0;
> >      unsigned long *dest = rb->bmap;
> >  
> >      /* start address is aligned at the start of a word? */
> > -    if (((page * BITS_PER_LONG) << TARGET_PAGE_BITS) == start) {
> > +    if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) ==
> > +         (start + rb->offset)) {
> >          int k;
> >          int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
> >          unsigned long * const *src;
> > -        unsigned long word = BIT_WORD((start + rb->offset) >> TARGET_PAGE_BITS);
> >          unsigned long idx = (word * BITS_PER_LONG) / DIRTY_MEMORY_BLOCK_SIZE;
> >          unsigned long offset = BIT_WORD((word * BITS_PER_LONG) %
> >                                          DIRTY_MEMORY_BLOCK_SIZE);
> > +        unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
> >  
> >          rcu_read_lock();
> >  
> > 
> 
> 
> Thanks, do you want me to queue this patch?

Yes please.

Dave

> Paolo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK