Re: [Xen-devel] [PATCH v4 13/18] x86/mem_sharing: Skip xen heap pages in memshr nominate

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Jan 20, 2020 at 9:23 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 08.01.2020 18:14, Tamas K Lengyel wrote:
> > Trying to share these would fail anyway, better to skip them early.
> >
> > Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>
>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> albeit I wonder if this couldn't be further generalized by ...
>
> > --- a/xen/arch/x86/mm/mem_sharing.c
> > +++ b/xen/arch/x86/mm/mem_sharing.c
> > @@ -852,6 +852,11 @@ static int nominate_page(struct domain *d, gfn_t gfn,
> >      if ( !p2m_is_sharable(p2mt) )
> >          goto out;
> >
> > +    /* Skip xen heap pages */
> > +    page = mfn_to_page(mfn);
> > +    if ( !page || is_xen_heap_page(page) )
> > +        goto out;
>
> ... checking for a zero type ref count (the only means to permit
> a type change) here, and maybe also ->count_info to fit what
> page_make_sharable() expects.

Not sure I follow you, type count is checked by page_make_sharable but
it has to be exactly 1:

    /* Check if page is already typed and bail early if it is */
    if ( (page->u.inuse.type_info & PGT_count_mask) != 1 )
    {
        spin_unlock(&d->page_alloc_lock);
        return -EEXIST;
    }

I specifically want to avoid calling page_make_sharable on xen heap
pages because they end up printing an error to the console which is
very annoying.

Tamas

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