Re: [Xen-devel] [PATCH v3 2/9] mm: Place unscrubbed pages at the end of pagelist

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 14/04/17 16:37, Boris Ostrovsky wrote:
> . so that it's easy to find pages that need to be scrubbed (those pages are
> now marked with _PGC_need_scrub bit).
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> ---
> Changes in v3:
> * Keep dirty bit per page, add dirty_head to page_info that indicates whether
>   the buddy has dirty pages.
> * Make page_list_add_scrub() set buddy's page order
> * Data type adjustments (int -> unsigned)
> 
>  xen/common/page_alloc.c  |  119 +++++++++++++++++++++++++++++++++++++--------
>  xen/include/asm-arm/mm.h |    6 ++
>  xen/include/asm-x86/mm.h |    6 ++
>  3 files changed, 110 insertions(+), 21 deletions(-)
> 
> diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
> index 6fe55ee..9dcf6ee 100644
> --- a/xen/common/page_alloc.c
> +++ b/xen/common/page_alloc.c
> @@ -383,6 +383,8 @@ typedef struct page_list_head 
> heap_by_zone_and_order_t[NR_ZONES][MAX_ORDER+1];
>  static heap_by_zone_and_order_t *_heap[MAX_NUMNODES];
>  #define heap(node, zone, order) ((*_heap[node])[zone][order])
>  
> +static unsigned long node_need_scrub[MAX_NUMNODES];
> +
>  static unsigned long *avail[MAX_NUMNODES];
>  static long total_avail_pages;
>  
> @@ -678,6 +680,20 @@ static void check_low_mem_virq(void)
>      }
>  }
>  
> +/* Pages that need scrub are added to tail, otherwise to head. */
> +static void page_list_add_scrub(struct page_info *pg, unsigned int node,
> +                                unsigned int zone, unsigned int order,
> +                                bool need_scrub)
> +{
> +    PFN_ORDER(pg) = order;
> +    pg->u.free.dirty_head = need_scrub;
> +
> +    if ( need_scrub )
> +        page_list_add_tail(pg, &heap(node, zone, order));
> +    else
> +        page_list_add(pg, &heap(node, zone, order));
> +}
> +
>  /* Allocate 2^@order contiguous pages. */
>  static struct page_info *alloc_heap_pages(
>      unsigned int zone_lo, unsigned int zone_hi,
> @@ -802,7 +818,7 @@ static struct page_info *alloc_heap_pages(
>      while ( j != order )
>      {
>          PFN_ORDER(pg) = --j;
> -        page_list_add_tail(pg, &heap(node, zone, j));
> +        page_list_add(pg, &heap(node, zone, j));
>          pg += 1 << j;
>      }
>  
> @@ -851,11 +867,14 @@ static int reserve_offlined_page(struct page_info *head)
>      int zone = page_to_zone(head), i, head_order = PFN_ORDER(head), count = 
> 0;
>      struct page_info *cur_head;
>      int cur_order;
> +    bool need_scrub;
>  
>      ASSERT(spin_is_locked(&heap_lock));
>  
>      cur_head = head;
>  
> +    head->u.free.dirty_head = false;
> +
>      page_list_del(head, &heap(node, zone, head_order));
>  
>      while ( cur_head < (head + (1 << head_order)) )
> @@ -892,8 +911,16 @@ static int reserve_offlined_page(struct page_info *head)
>              {
>              merge:
>                  /* We don't consider merging outside the head_order. */
> -                page_list_add_tail(cur_head, &heap(node, zone, cur_order));
> -                PFN_ORDER(cur_head) = cur_order;
> +
> +                /* See if any of the pages need scrubbing. */
> +                need_scrub = false;
> +                for ( i = 0; i < (1 << cur_order); i++ )
> +                    if ( test_bit(_PGC_need_scrub, &cur_head[i].count_info) )
> +                    {
> +                        need_scrub = true;
> +                        break;
> +                    }
> +                page_list_add_scrub(cur_head, node, zone, cur_order, 
> need_scrub);

This thing with clearing dirty_head, then setting it again in
page_list_add_scrub() could use some explanation -- either near one of
these loops, or in mm.h preferrably.


>                  cur_head += (1 << cur_order);
>                  break;
>              }
> @@ -922,10 +949,13 @@ static int reserve_offlined_page(struct page_info *head)
>  /* Returns new buddy head. */
>  static struct page_info *
>  merge_and_free_buddy(struct page_info *pg, unsigned int node,
> -                     unsigned int zone, unsigned int order)
> +                     unsigned int zone, unsigned int order,
> +                     bool need_scrub)

What is the meaning of "need_scrub" here?  Does this mean that pg needs
to be scrubbed?

 -George


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