Re: [Xen-devel] [PATCH v12 2/3] x86/mem_sharing: reset a fork

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Mar 23, 2020 at 10:04:36AM -0700, Tamas K Lengyel wrote:
> Implement hypercall that allows a fork to shed all memory that got allocated
> for it during its execution and re-load its vCPU context from the parent VM.
> This allows the forked VM to reset into the same state the parent VM is in a
> faster way then creating a new fork would be. Measurements show about a 2x
> speedup during normal fuzzing operations. Performance may vary depending how
> much memory got allocated for the forked VM. If it has been completely
> deduplicated from the parent VM then creating a new fork would likely be more
> performant.
> 
> Signed-off-by: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>

LGTM:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

One minor nit below.

> ---
> v12: remove continuation & add comment back
>      address style issues pointed out by Jan
> ---
>  xen/arch/x86/mm/mem_sharing.c | 77 +++++++++++++++++++++++++++++++++++
>  xen/include/public/memory.h   |  1 +
>  2 files changed, 78 insertions(+)
> 
> diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
> index 23deeddff2..930a5f58ef 100644
> --- a/xen/arch/x86/mm/mem_sharing.c
> +++ b/xen/arch/x86/mm/mem_sharing.c
> @@ -1775,6 +1775,60 @@ static int fork(struct domain *cd, struct domain *d)
>      return rc;
>  }
>  
> +/*
> + * The fork reset operation is intended to be used on short-lived forks only.
> + * There is no hypercall continuation operation implemented for this reason.
> + * For forks that obtain a larger memory footprint it is likely going to be
> + * more performant to create a new fork instead of resetting an existing one.
> + *
> + * TODO: In case this hypercall would become useful on forks with larger 
> memory
> + * footprints the hypercall continuation should be implemented (or if this
> + * feature needs to be become "stable").
> + */
> +static int mem_sharing_fork_reset(struct domain *d, struct domain *pd)
> +{
> +    int rc;
> +    struct p2m_domain *p2m = p2m_get_hostp2m(d);
> +    struct page_info *page, *tmp;
> +
> +    spin_lock(&d->page_alloc_lock);
> +    domain_pause(d);
> +
> +    page_list_for_each_safe(page, tmp, &d->page_list)
> +    {
> +        p2m_type_t p2mt;
> +        p2m_access_t p2ma;
> +        mfn_t mfn = page_to_mfn(page);
> +        gfn_t gfn = mfn_to_gfn(d, mfn);
> +
> +        mfn = __get_gfn_type_access(p2m, gfn_x(gfn), &p2mt, &p2ma,
> +                                    0, NULL, false);
> +
> +        /* only reset pages that are sharable */
> +        if ( !p2m_is_sharable(p2mt) )
> +            continue;
> +
> +        /* take an extra reference or just skip if can't for whatever reason 
> */
> +        if ( !get_page(page, d) )
> +            continue;

You can join both conditions above into a single one, if both just
need to perform a continue.

Thanks, Roger.