Re: [PATCH] mm/memcg: avoid page count check for zone device

[Michal Hocko]

On Thu 14-09-17 15:00:11, jglisse@xxxxxxxxxx wrote:
> From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> 
> Fix for 4.14, zone device page always have an elevated refcount
> of one and thus page count sanity check in uncharge_page() is
> inappropriate for them.
> 
> Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> Reported-by: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Side note. Wouldn't it be better to re-organize the check a bit? It is
true that this is VM_BUG so it is not usually compiled in but when it
preferably checks for unlikely cases first while the ref count will be
0 in the prevailing cases. So can we have
	VM_BUG_ON_PAGE(page_count(page) && !is_zone_device_page(page) &&
			!PageHWPoison(page), page);

I would simply fold this nano optimization into the patch as you are
touching it already. Not sure it is worth a separate commit.

> ---
>  mm/memcontrol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 15af3da5af02..d51d3e1f49c9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5648,7 +5648,8 @@ static void uncharge_batch(const struct uncharge_gather *ug)
>  static void uncharge_page(struct page *page, struct uncharge_gather *ug)
>  {
>  	VM_BUG_ON_PAGE(PageLRU(page), page);
> -	VM_BUG_ON_PAGE(!PageHWPoison(page) && page_count(page), page);
> +	VM_BUG_ON_PAGE(!PageHWPoison(page) && !is_zone_device_page(page) &&
> +			page_count(page), page);
>  
>  	if (!page->mem_cgroup)
>  		return;
> -- 
> 2.13.5

-- 
Michal Hocko
SUSE Labs