Re: [PATCH v1 2/5] mm/page_alloc: place pages to tail in __putback_isolated_page()

[Michal Hocko]

On Mon 28-09-20 20:21:07, David Hildenbrand wrote:
> __putback_isolated_page() already documents that pages will be placed to
> the tail of the freelist - this is, however, not the case for
> "order >= MAX_ORDER - 2" (see buddy_merge_likely()) - which should be
> the case for all existing users.
> 
> This change affects two users:
> - free page reporting
> - page isolation, when undoing the isolation (including memory onlining).
> 
> This behavior is desireable for pages that haven't really been touched
> lately, so exactly the two users that don't actually read/write page
> content, but rather move untouched pages.
> 
> The new behavior is especially desirable for memory onlining, where we
> allow allocation of newly onlined pages via undo_isolate_page_range()
> in online_pages(). Right now, we always place them to the head of the
> free list, resulting in undesireable behavior: Assume we add
> individual memory chunks via add_memory() and online them right away to
> the NORMAL zone. We create a dependency chain of unmovable allocations
> e.g., via the memmap. The memmap of the next chunk will be placed onto
> previous chunks - if the last block cannot get offlined+removed, all
> dependent ones cannot get offlined+removed. While this can already be
> observed with individual DIMMs, it's more of an issue for virtio-mem
> (and I suspect also ppc DLPAR).
> 
> Document that this should only be used for optimizations, and no code
> should realy on this for correction (if the order of freepage lists
> ever changes).
> 
> We won't care about page shuffling: memory onlining already properly
> shuffles after onlining. free page reporting doesn't care about
> physically contiguous ranges, and there are already cases where page
> isolation will simply move (physically close) free pages to (currently)
> the head of the freelists via move_freepages_block() instead of
> shuffling. If this becomes ever relevant, we should shuffle the whole
> zone when undoing isolation of larger ranges, and after
> free_contig_range().
> 
> Reviewed-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> Cc: Scott Cheloha <cheloha@xxxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/page_alloc.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index daab90e960fe..9e3ed4a6f69a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -89,6 +89,18 @@ typedef int __bitwise fop_t;
>   */
>  #define FOP_SKIP_REPORT_NOTIFY	((__force fop_t)BIT(0))
>  
> +/*
> + * Place the (possibly merged) page to the tail of the freelist. Will ignore
> + * page shuffling (relevant code - e.g., memory onlining - is expected to
> + * shuffle the whole zone).
> + *
> + * Note: No code should rely onto this flag for correctness - it's purely
> + *       to allow for optimizations when handing back either fresh pages
> + *       (memory onlining) or untouched pages (page isolation, free page
> + *       reporting).
> + */
> +#define FOP_TO_TAIL		((__force fop_t)BIT(1))
> +
>  /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
>  static DEFINE_MUTEX(pcp_batch_high_lock);
>  #define MIN_PERCPU_PAGELIST_FRACTION	(8)
> @@ -1038,7 +1050,9 @@ static inline void __free_one_page(struct page *page, unsigned long pfn,
>  done_merging:
>  	set_page_order(page, order);
>  
> -	if (is_shuffle_order(order))
> +	if (fop_flags & FOP_TO_TAIL)
> +		to_tail = true;
> +	else if (is_shuffle_order(order))
>  		to_tail = shuffle_pick_tail();
>  	else
>  		to_tail = buddy_merge_likely(pfn, buddy_pfn, page, order);
> @@ -3300,7 +3314,7 @@ void __putback_isolated_page(struct page *page, unsigned int order, int mt)
>  
>  	/* Return isolated page to tail of freelist. */
>  	__free_one_page(page, page_to_pfn(page), zone, order, mt,
> -			FOP_SKIP_REPORT_NOTIFY);
> +			FOP_SKIP_REPORT_NOTIFY | FOP_TO_TAIL);
>  }
>  
>  /*
> -- 
> 2.26.2

-- 
Michal Hocko
SUSE Labs