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);
> + }
> }
>
> /*
