Re: [PATCH] mm/memcg: remove useless check on page->mem_cgroup

Michal Hocko
2020-08-10T09:55:05Z

On Wed 05-08-20 20:28:33, Alex Shi wrote:
[...]
> >From 2ca3e87fd3878ab729551682ad083a70f15bb3fc Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@linux.alibaba.com>
> Date: Sat, 1 Aug 2020 10:43:55 +0800
> Subject: [PATCH v3] mm/memcg: warning on !memcg after readahead page charged
>
> Since readahead page is charged on memcg too, in theory we don't have to
> check this exception now. Before safely remove them all, add a warning
> for the unexpected !memcg.
>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org

Looks good to me. I am not familiar with the section tweaks but that
should be ok.

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

Once you collect more feedback, please send both patches so that they do
not get lost in this thread.

Thanks!

> ---
> include/linux/mmdebug.h | 13 +++++++++++++
> mm/memcontrol.c | 15 ++++++++-------
> 2 files changed, 21 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h
> index 2ad72d2c8cc5..4ed52879ce55 100644
> --- a/include/linux/mmdebug.h
> +++ b/include/linux/mmdebug.h
> @@ -37,6 +37,18 @@
> BUG(); \
> } \
> } while (0)
> +#define VM_WARN_ON_ONCE_PAGE(cond, page) ({ \
> + static bool __section(.data.once) __warned; \
> + int __ret_warn_once = !!(cond); \
> + \
> + if (unlikely(__ret_warn_once && !__warned)) { \
> + dump_page(page, "VM_WARN_ON_ONCE_PAGE(" __stringify(cond)")");\
> + __warned = true; \
> + WARN_ON(1); \
> + } \
> + unlikely(__ret_warn_once); \
> +})
> +
> #define VM_WARN_ON(cond) (void)WARN_ON(cond)
> #define VM_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
> #define VM_WARN_ONCE(cond, format...) (void)WARN_ONCE(cond, format)
> @@ -48,6 +60,7 @@
> #define VM_BUG_ON_MM(cond, mm) VM_BUG_ON(cond)
> #define VM_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
> #define VM_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
> +#define VM_WARN_ON_ONCE_PAGE(cond, page) BUILD_BUG_ON_INVALID(cond)
> #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
> #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
> #endif
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 130093bdf74b..299382fc55a9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1322,10 +1322,8 @@ struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgd
> }
>
> memcg = page->mem_cgroup;
> - /*
> - * Swapcache readahead pages are added to the LRU - and
> - * possibly migrated - before they are charged.
> - */
> + /* Readahead page is charged too, to see if other page uncharged */
> + VM_WARN_ON_ONCE_PAGE(!memcg, page);
> if (!memcg)
> memcg = root_mem_cgroup;
>
> @@ -6906,8 +6904,9 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
> if (newpage->mem_cgroup)
> return;
>
> - /* Swapcache readahead pages can get replaced before being charged */
> memcg = oldpage->mem_cgroup;
> + /* Readahead page is charged too, to see if other page uncharged */
> + VM_WARN_ON_ONCE_PAGE(!memcg, oldpage);
> if (!memcg)
> return;
>
> @@ -7104,7 +7103,8 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>
> memcg = page->mem_cgroup;
>
> - /* Readahead page, never charged */
> + /* Readahead page is charged too, to see if other page uncharged */
> + VM_WARN_ON_ONCE_PAGE(!memcg, page);
> if (!memcg)
> return;
>
> @@ -7168,7 +7168,8 @@ int mem_cgroup_try_charge_swap(struct page *page, swp_entry_t entry)
>
> memcg = page->mem_cgroup;
>
> - /* Readahead page, never charged */
> + /* Readahead page is charged too, to see if other page uncharged */
> + VM_WARN_ON_ONCE_PAGE(!memcg, page);
> if (!memcg)
> return 0;
>
> --
> 1.8.3.1

--
Michal Hocko
SUSE Labs