Re: [Xen-devel] [PATCH 1 of 4] Prevent low values of max_pages for domains doing sharing or paging

["Jan Beulich" <JBeulich@xxxxxxxx>]

 >>> On 16.02.12 at 04:57, Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> wrote:
> xen/common/domctl.c |  8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
> 
> 
> Apparently, setting d->max_pages to something lower than d->tot_pages is
> used as a mechanism for controling a domain's footprint. It will result
> in all new page allocations failing.
> 
> This is a really bad idea with paging or sharing, as regular guest memory
> accesses may need to be satisfied by allocating new memory (either to
> page in or to unshare).
> 
> Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> 
> diff -r 62b1fe67b8d1 -r 11fd4e0a1e1a xen/common/domctl.c
> --- a/xen/common/domctl.c
> +++ b/xen/common/domctl.c
> @@ -813,8 +813,14 @@ long do_domctl(XEN_GUEST_HANDLE(xen_domc
>           * NB. We removed a check that new_max >= current tot_pages; this 
> means
>           * that the domain will now be allowed to "ratchet" down to new_max. 
> In
>           * the meantime, while tot > max, all new allocations are disallowed.
> +         *
> +         * Except that this is not a great idea for domains doing sharing or 
> +         * paging, as they need to perform new allocations on the fly.
>           */
> -        d->max_pages = new_max;
> +        if ( (new_max > d->max_pages) ||
> +             !((d->mem_event->paging.ring_page != NULL) ||
> +                d->arch.hvm_domain.mem_sharing_enabled) )
> +            d->max_pages = new_max;

I don't think this is appropriate here, as it will allow not only
paging/sharing allocations to succeed, but also such that aren't
intended to. I think that *if* an override to the limit enforcement
is needed, it ought to be implemented by passing a special flag
to the page allocator to have it make an exception. But that's
questionable in the first place - if a domain is told to shrink, it
ought to follow that directive (and the paging/sharing code then
probably should obtain a page from inside the guest; I realize
that might be problematic when sharing is enabled but paging
isn't).

Jan

>          ret = 0;
>          spin_unlock(&d->page_alloc_lock);
>  
> 
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