Re: [Qemu-devel] [PATCH RFC kernel] balloon: speed up inflating/deflating process

[Li, Liang Z]

> > > > > > > This is grossly inefficient if you only requested a single page.
> > > > > > > And it's also allocating memory very aggressively without
> > > > > > > ever telling the host what is going on.
> > > > > >
> > > > > > If only requested a single page, there is no need  to send the
> > > > > > entire page bitmap, This RFC patch has already considered about
> this.
> > > > >
> > > > > where's that addressed in code?
> > > > >
> > > >
> > > > By record the start_pfn and end_pfn.
> > > >
> > > > The start_pfn & end_pfn will be updated in set_page_bitmap() and
> > > > will be used in the function tell_host():
> > > >
> > > > ------------------------------------------------------------------
> > > > ----
> > > > -----------
> > > > +static void set_page_bitmap(struct virtio_balloon *vb, struct
> > > > +page
> > > > +*page) {
> > > > +       unsigned int i;
> > > > +       unsigned long *bitmap = vb->page_bitmap;
> > > > +       unsigned long balloon_pfn = page_to_balloon_pfn(page);
> > > > +
> > > > +       for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
> > > > +               set_bit(balloon_pfn + i, bitmap);
> > >
> > > BTW, there's a page size value in header so there is no longer need
> > > to set multiple bits per page.
> >
> > Yes, you are right.
> >
> > >
> > > > +       if (balloon_pfn < vb->start_pfn)
> > > > +               vb->start_pfn = balloon_pfn;
> > > > +       if (balloon_pfn > vb->end_pfn)
> > > > +               vb->end_pfn = balloon_pfn;
> > > > +}
> > >
> > > Sounds good, but you also need to limit by allocated bitmap size.
> >
> > Why should we limit the page bitmap size? Is it no good to send a large
> page bitmap?
> > or to save the memory used for page bitmap? Or some other reason?
> 
> To save memory. First allocating a large bitmap can fail, second this is 
> pinned
> memory that is wasted - it's unused most of the time while guest is running.

Make sense.

> 
> > >
> > > >
> > > > +               unsigned long start_pfn, end_pfn, flags = 0, bmap_len;
> > > > +               struct scatterlist sg[5];
> > > > +
> > > > +               start_pfn = rounddown(vb->start_pfn, BITS_PER_LONG);
> > > > +               end_pfn = roundup(vb->end_pfn, BITS_PER_LONG);
> > > > +               bmap_len = (end_pfn - start_pfn) / BITS_PER_LONG *
> > > sizeof(long);
> > > > +
> > > > +               sg_init_table(sg, 5);
> > > > +               sg_set_buf(&sg[0], &flags, sizeof(flags));
> > > > +               sg_set_buf(&sg[1], &start_pfn, sizeof(start_pfn));
> > > > +               sg_set_buf(&sg[2], &page_shift, sizeof(page_shift));
> > > > +               sg_set_buf(&sg[3], &bmap_len, sizeof(bmap_len));
> > > > +               sg_set_buf(&sg[4], vb->page_bitmap +
> > > > +                                (start_pfn / BITS_PER_LONG), bmap_len);
> > >
> > > Looks wrong. start_pfn should start at offset 0 I think ...
> >
> > I don't know what is wrong here, could you tell me why?
> >
> > Thanks!
> >
> > Liang
> 
> start_pfn should mean "bit 0 in bitmap refers to pfn X".
> So it does not make sense to also add it as offset within bitmap.

I got it, but in my code, it's correct, because the page_bitmap is
range from pfn 0 to max_pfn. 
It should be changed if we are going to use a small page bitmap.

Thanks!

Liang


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