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

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Date:   Fri, 15 Sep 2017 09:01:00 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     jglisse@...hat.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH] mm/memcg: avoid page count check for zone device

On Thu 14-09-17 15:00:11, jglisse@...hat.com wrote:
> From: Jérôme Glisse <jglisse@...hat.com>
> 
> 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@...hat.com>
> Reported-by: Evgeny Baskakov <ebaskakov@...dia.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: Vladimir Davydov <vdavydov.dev@...il.com>

Acked-by: Michal Hocko <mhocko@...e.com>

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