Re: [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.02.2020 16:14, Stewart Hildebrand wrote:
> From: Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>
> 
> The Xen heap is split up into nodes and zones. Each node + zone is
> managed as a separate pool of memory.
> 
> When returning pages to the heap, free_heap_pages will check adjacent
> blocks to see if they can be combined into a larger block. However, the
> zone of the adjacent block is not checked. This results in blocks that
> migrate from one zone to another.
> 
> When a block migrates to the adjacent zone, the avail counters for the
> old and new node + zone is not updated accordingly. The avail counter
> is used when allocating pages to determine whether to skip over a zone.
> With this behavior, it is possible for free pages to collect in a zone
> with the avail counter smaller than the actual page count, resulting
> in free pages that are not allocable.

"When a block migrates" - fine. But is this situation possible to
occur, without "xen/page_alloc: Keep away MFN 0 from the buddy
allocator" reverted? If not, there's no bug, no need for a change,
and even less so ...

> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -1462,6 +1462,7 @@ static void free_heap_pages(
>              if ( !mfn_valid(page_to_mfn(predecessor)) ||
>                   !page_state_is(predecessor, free) ||
>                   (PFN_ORDER(predecessor) != order) ||
> +                 (page_to_zone(predecessor) != zone) ||
>                   (phys_to_nid(page_to_maddr(predecessor)) != node) )
>                  break;
>  
> @@ -1485,6 +1486,7 @@ static void free_heap_pages(
>              if ( !mfn_valid(page_to_mfn(successor)) ||
>                   !page_state_is(successor, free) ||
>                   (PFN_ORDER(successor) != order) ||
> +                 (page_to_zone(successor) != zone) ||
>                   (phys_to_nid(page_to_maddr(successor)) != node) )
>                  break;

... for one that slows down many free operations, even if just
slightly. IOW afaict either the change is not needed, or its
description needs updating.

Jan

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