Re: [PATCH v3 3/3] xen/mm: limit non-scrubbed allocations to a specific order

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.01.2026 18:38, Roger Pau Monne wrote:
> The current logic allows for up to 1G pages to be scrubbed in place, which
> can cause the watchdog to trigger in practice.  Reduce the limit for
> in-place scrubbed allocations to a newly introduced define:
> CONFIG_DIRTY_MAX_ORDER.  This currently defaults to CONFIG_PTDOM_MAX_ORDER
> on all architectures.  Also introduce a command line option to set the
> value.
> 
> Fixes: 74d2e11ccfd2 ("mm: Scrub pages in alloc_heap_pages() if needed")
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Apart from a nit (see below) looks technically okay to me now. Still I have
an uneasy feeling about introducing such a restriction, so I'm (still)
hesitant to ack the change.

> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -267,6 +267,13 @@ static PAGE_LIST_HEAD(page_offlined_list);
>  /* Broken page list, protected by heap_lock. */
>  static PAGE_LIST_HEAD(page_broken_list);
>  
> +/* Maximum order allowed for allocations with MEMF_no_scrub. */
> +#ifndef CONFIG_DIRTY_MAX_ORDER
> +# define CONFIG_DIRTY_MAX_ORDER CONFIG_PTDOM_MAX_ORDER
> +#endif
> +static unsigned int __ro_after_init dirty_max_order = CONFIG_DIRTY_MAX_ORDER;
> +integer_param("max-order-dirty", dirty_max_order);

The comment may want to mention "post-boot", to account for ...

> @@ -1008,7 +1015,13 @@ static struct page_info *alloc_heap_pages(
>  
>      pg = get_free_buddy(zone_lo, zone_hi, order, memflags, d);
>      /* Try getting a dirty buddy if we couldn't get a clean one. */
> -    if ( !pg && !(memflags & MEMF_no_scrub) )
> +    if ( !pg && !(memflags & MEMF_no_scrub) &&
> +         /*
> +          * Allow any order unscrubbed allocations during boot time, we
> +          * compensate by processing softirqs in the scrubbing loop below 
> once
> +          * irqs are enabled.
> +          */
> +         (order <= dirty_max_order || system_state < SYS_STATE_active) )

... the system_state check here.

Jan