Re: [Xen-devel] [PATCH 2/2] xen/mm: Introduce PG_state_uninitialised

[Jan Beulich <jbeulich@suse.com>]

On 07.02.2020 19:04, David Woodhouse wrote:
> --- a/xen/arch/x86/mm.c
> +++ b/xen/arch/x86/mm.c
> @@ -488,7 +488,8 @@ void share_xen_page_with_guest(struct page_info *page, struct domain *d,
>  
>      page_set_owner(page, d);
>      smp_wmb(); /* install valid domain ptr before updating refcnt. */
> -    ASSERT((page->count_info & ~PGC_xen_heap) == 0);
> +    ASSERT((page->count_info & ~PGC_xen_heap) == PGC_state_inuse ||
> +           (page->count_info & ~PGC_xen_heap) == PGC_state_uninitialised);

Can uninitialized pages really make it here?

> @@ -1389,6 +1391,16 @@ static void free_heap_pages(
>      ASSERT(order <= MAX_ORDER);
>      ASSERT(node >= 0);
>  
> +    if ( page_state_is(pg, uninitialised) )
> +    {
> +        init_heap_pages(pg, 1 << order, need_scrub);
> +        /*
> +         * init_heap_pages() will call back into free_heap_pages() for
> +         * each page but cannot keep recursing because each page will
> +         * be set to PGC_state_inuse first.
> +         */
> +        return;
> +    }
>      spin_lock(&heap_lock);

Can you also add a blank line above here please?

> @@ -1780,11 +1792,10 @@ int query_page_offline(mfn_t mfn, uint32_t *status)
>   * latter is not on a MAX_ORDER boundary, then we reserve the page by
>   * not freeing it to the buddy allocator.
>   */
> -static void init_heap_pages(
> -    struct page_info *pg, unsigned long nr_pages)
> +static void init_heap_pages(struct page_info *pg, unsigned long nr_pages,
> +                            bool scrub)

Is this new parameter strictly needed, i.e. can free_heap_pages()
be called with uninitialized pages which need scrubbing? (The
code change is simple enough, and hence may warrant keeping, but
then the commit message could indicate so in case this isn't a
strict requirement.)

> @@ -2301,10 +2316,11 @@ int assign_pages(
>      for ( i = 0; i < (1 << order); i++ )
>      {
>          ASSERT(page_get_owner(&pg[i]) == NULL);
> -        ASSERT(!pg[i].count_info);
> +        ASSERT(pg[i].count_info == PGC_state_inuse ||
> +               pg[i].count_info == PGC_state_uninitialised);

Same question here: Can uninitialized pages make it here? If
so, wouldn't it be better to correct this, rather than having
the more permissive assertion?

>          page_set_owner(&pg[i], d);
>          smp_wmb(); /* Domain pointer must be visible before updating refcnt. */
> -        pg[i].count_info = PGC_allocated | 1;
> +        pg[i].count_info |= PGC_allocated | 1;

This is too relaxed for my taste: I understand you want to
retain page state, but I suppose other bits would want clearing
nevertheless.

> --- a/xen/include/asm-x86/mm.h
> +++ b/xen/include/asm-x86/mm.h
> @@ -72,12 +72,13 @@
>    * { inuse, offlining, offlined, free, broken_offlining, broken }
>    */
>  #define PGC_state                  PG_mask(7, 9)
> -#define PGC_state_inuse            PG_mask(0, 9)
> +#define PGC_state_uninitialised    PG_mask(0, 9)
>  #define PGC_state_offlining        PG_mask(1, 9)
>  #define PGC_state_offlined         PG_mask(2, 9)
>  #define PGC_state_free             PG_mask(3, 9)
>  #define PGC_state_broken_offlining PG_mask(4, 9)
>  #define PGC_state_broken           PG_mask(5, 9)
> +#define PGC_state_inuse            PG_mask(6, 9)

Would imo be nice if this most common state was actually
either 1 or 7, for easy recognition. But the most suitable
value to pick may also depend on the outcome of one of the
comments on patch 1.

Jan

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