Re: [Qemu-devel] [PATCH 1/1] Count used RAMBlock pages for migration_dirty_pages

[Dr. David Alan Gilbert]

* ???? (address@hidden) wrote:
> 
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > This is a fix for a bug* triggered by a migration after hot unplugging
> > a few virtio-net NICs, that caused migration never to converge, because
> > 'migration_dirty_pages' is incorrectly initialised.
> > 
> > 'migration_dirty_pages' is used as a tally of the number of outstanding
> > dirty pages, to give the migration code an idea of how much more data
> > will need to be transferred, and thus whether it can end the iterative
> > phase.
> > 
> > It was initialised to the total size of the RAMBlock address space,
> > however hotunplug can leave this space sparse, and hence
> > migration_dirty_pages ended up too large.
> > 
> > Note that the code tries to be careful when counting to deal with
> > RAMBlocks that share the same end/start page - I don't know
> > if this is actually possible and it does complicate the code,
> > but since there was other code that dealt with unaligned RAMBlocks
> > it seemed possible.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > 
> > (* https://bugzilla.redhat.com/show_bug.cgi?id=1074913 )
> > ---
> > arch_init.c | 41 +++++++++++++++++++++++++++++++++++++----
> > 1 file changed, 37 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch_init.c b/arch_init.c
> > index f18f42e..ef0e98d 100644
> > --- a/arch_init.c
> > +++ b/arch_init.c
> > @@ -727,11 +727,8 @@ static void reset_ram_globals(void)
> > static int ram_save_setup(QEMUFile *f, void *opaque)
> > {
> >     RAMBlock *block;
> > -    int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;
> > +    int64_t ram_bitmap_pages;
> > 
> > -    migration_bitmap = bitmap_new(ram_pages);
> > -    bitmap_set(migration_bitmap, 0, ram_pages);
> > -    migration_dirty_pages = ram_pages;
> >     mig_throttle_on = false;
> >     dirty_rate_high_cnt = 0;
> > 
> > @@ -770,6 +767,42 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
> >     bytes_transferred = 0;
> >     reset_ram_globals();
> > 
> > +    ram_bitmap_pages = last_ram_offset() >> TARGET_PAGE_BITS;
> > +    migration_bitmap = bitmap_new(ram_bitmap_pages);
> > +    bitmap_set(migration_bitmap, 0, ram_bitmap_pages);
> > +    /*
> > +     * Count the total number of pages used by ram blocks. We clear the 
> > dirty
> > +     * bit for the start/end of each ramblock as we go so that we don't 
> > double
> > +     * count ramblocks that have overlapping pages - at entry the whole 
> > dirty
> > +     * bitmap is set.
> > +     */
> > +    migration_dirty_pages = 0;
> > +    QTAILQ_FOREACH(block, &ram_list.blocks, next) {
> > +        uint64_t block_pages = 0;
> > +        ram_addr_t saddr, eaddr;
> > +
> > +        saddr = block->mr->ram_addr;
> > +        eaddr = saddr + block->length - 1;
> > +        saddr /= TARGET_PAGE_SIZE;
> > +        eaddr /= TARGET_PAGE_SIZE;
> > +
> > +        /* Do the end pages of the range, that might be shared with others 
> > */
> > +        block_pages += test_and_clear_bit(saddr, migration_bitmap);
> > +        block_pages += test_and_clear_bit(eaddr, migration_bitmap);
> > +
> > +        if ((saddr + 1) < eaddr) {
> > +            block_pages += eaddr - (saddr + 1);
> > +        }
> > +        migration_dirty_pages += block_pages;
> > +        /*fprintf(stderr, "ram_save_setup: %s s/e=%zx/%zx page s/e=%zx/%zx"
> > +                " bp=%zu\n",
> > +                block->idstr, block->mr->ram_addr,
> > +                block->mr->ram_addr+block->length-1,
> > +                saddr, eaddr, block_pages); */
> > +    }
> > +    /* and set the bits again that got used for our overlap check */
> > +    bitmap_set(migration_bitmap, 0, ram_bitmap_pages);
> 
> maybe it should like this:
> bitmap_set(migration_bitmap, 0, last_ram_offset() >> TARGET_PAGE_BITS);
> Because there are some space sparse.

No, ram_bitmap_pages is already equal to last_ram_offset() >> TARGET_PAGE_BITS
The loop has been calculating migration_dirty_pages and hasn't touched 
ram_bitmap_pages.

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK