Re: [Xen-devel] [PATCH 2/3] x86/mem_sharing: replace use of page_lock/unlock with our own lock

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Apr 25, 2019 at 12:58 PM Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 25/04/2019 16:32, Tamas K Lengyel wrote:
> > diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
> > index 6faa563167..594de6148f 100644
> > --- a/xen/include/asm-x86/mm.h
> > +++ b/xen/include/asm-x86/mm.h
> > @@ -133,7 +133,10 @@ struct page_info
> >           * of sharing share the version they expect to.
> >           * This list is allocated and freed when a page is shared/unshared.
> >           */
> > -        struct page_sharing_info *sharing;
> > +        struct {
> > +            struct page_sharing_info *info;
> > +            rwlock_t lock;
> > +        } sharing;
> >      };
> >
> >      /* Reference count and various PGC_xxx flags and fields. */
>
> I'm afraid this is a no-go, but for some reasons which are rather more
> subtle that they appear here.
>
> There is one struct page_info per 4k frame, and you've added an extra 16
> bytes, taking it from 32 bytes to 48 bytes.
>
> Other than making it a non-power-of-2 (net diffstat grow/shrink: 256/27
> up/down: 7750/-5696 (2054) due to less efficient arithmetic[1]), the
> framtable has a fixed virtual size (128G by default), so you've ended up
> dropping Xen's memory limit (16TB by default) by 1/3.

Interesting, I'm not sure I completely follow but does this mean that
this structure is now not allowed to grow, like ever, or that I just
need to add padding to fix up its alignment?

Tamas

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