Re: [Xen-devel] [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Fri, Mar 15, 2019 at 06:14:09PM +0000, Anthony PERARD wrote:
> On Fri, Mar 15, 2019 at 09:58:47AM +0100, Roger Pau Monne wrote:
> > Or if it's not possible to honor the hinted address an error is returned
> > instead. This makes it easier to spot the actual failure, instead of
> > failing later on when the caller of xen_remap_bucket realizes the
> > mapping has not been created at the requested address.
> > 
> > Also note that at least on FreeBSD using MAP_FIXED will cause mmap to
> > try harder to honor the passed address.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> The patch looks fine, and MAP_FIXED seems to be the expected behavior
> even on Linux.
> 
> Acked-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> 
> > diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> > @@ -185,8 +185,14 @@ static void xen_remap_bucket(MapCacheEntry *entry,
> >      }
> >  
> >      if (!dummy) {
> > +        /*
> > +         * If the caller has requested the mapping at a specific address 
> > use
> > +         * MAP_FIXED to make sure it's honored. Note that with MAP_FIXED at
> > +         * least FreeBSD will try harder to honor the passed address.
> > +         */
> >          vaddr_base = xenforeignmemory_map2(xen_fmem, xen_domid, vaddr,
> > -                                           PROT_READ | PROT_WRITE, 0,
> > +                                           PROT_READ | PROT_WRITE,
> > +                                           vaddr ? MAP_FIXED : 0,
> >                                             nb_pfn, pfns, err);

I've noticed that there's a mmap call just after which has also vaddr as
hint, should that second call also have MAP_FIXED as flags?

On the other hand, it seems that when vaddr!=NULL, dummy==false, so that
second mmap call will never be called with vaddr!=NULL. But it might be
worse fixing this in the same patch.

>  } else {
>      /*
>       * We create dummy mappings where we are unable to create a foreign
>       * mapping immediately due to certain circumstances (i.e. on resume now)
>       */
>      vaddr_base = mmap(vaddr, size, PROT_READ | PROT_WRITE,
>                        MAP_ANON | MAP_SHARED, -1, 0);

-- 
Anthony PERARD

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