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SubjectRe: [PATCH] mm/memcg: avoid page count check for zone device
On Thu 14-09-17 15:00:11, jglisse@redhat.com wrote:
> From: Jérôme Glisse <jglisse@redhat.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@redhat.com>
> Reported-by: Evgeny Baskakov <ebaskakov@nvidia.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>

Acked-by: Michal Hocko <mhocko@suse.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

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