Re: [PATCH 2/6] mm: page_alloc: fix up block types when merging compatible blocks

[Vlastimil Babka]

On 9/11/23 21:41, Johannes Weiner wrote:
> The buddy allocator coalesces compatible blocks during freeing, but it
> doesn't update the types of the subblocks to match. When an allocation
> later breaks the chunk down again, its pieces will be put on freelists
> of the wrong type. This encourages incompatible page mixing (ask for
> one type, get another), and thus long-term fragmentation.

Yeah why not. Sould be pretty rare as this only affects >=pageblock_order,
but then also the overhead in the otherwise hot function is limited to its
colder part.

> Update the subblocks when merging a larger chunk, such that a later
> expand() will maintain freelist type hygiene.
> 
> v2:
> - remove spurious change_pageblock_range() move (Zi Yan)
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/page_alloc.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e3f1c777feed..3db405414174 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -783,10 +783,17 @@ static inline void __free_one_page(struct page *page,
>  			 */
>  			int buddy_mt = get_pfnblock_migratetype(buddy, buddy_pfn);
>  
> -			if (migratetype != buddy_mt
> -					&& (!migratetype_is_mergeable(migratetype) ||
> -						!migratetype_is_mergeable(buddy_mt)))
> -				goto done_merging;
> +			if (migratetype != buddy_mt) {
> +				if (!migratetype_is_mergeable(migratetype) ||
> +				    !migratetype_is_mergeable(buddy_mt))
> +					goto done_merging;
> +				/*
> +				 * Match buddy type. This ensures that
> +				 * an expand() down the line puts the
> +				 * sub-blocks on the right freelists.
> +				 */
> +				set_pageblock_migratetype(buddy, migratetype);
> +			}
>  		}
>  
>  		/*