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Date:   Mon, 27 Nov 2023 13:42:59 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Nhat Pham <nphamcs@gmail.com>
Cc:     hannes@cmpxchg.org, cerasuolodomenico@gmail.com,
        yosryahmed@google.com, sjenning@redhat.com, ddstreet@ieee.org,
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        roman.gushchin@linux.dev, shakeelb@google.com,
        muchun.song@linux.dev, chrisl@kernel.org, linux-mm@kvack.org,
        kernel-team@meta.com, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org, shuah@kernel.org
Subject: Re: [PATCH v6 2/6] memcontrol: allows mem_cgroup_iter() to check
 for onlineness
Message-Id: <20231127134259.67b69ab47f4f88c9751e5222@linux-foundation.org>
In-Reply-To: <20231127193703.1980089-3-nphamcs@gmail.com>
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        <20231127193703.1980089-3-nphamcs@gmail.com>
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On Mon, 27 Nov 2023 11:36:59 -0800 Nhat Pham <nphamcs@gmail.com> wrote:

> The new zswap writeback scheme requires an online-only memcg hierarchy
> traversal. Add a new parameter to mem_cgroup_iter() to check for
> onlineness before returning.

I get a few build errors, perhaps because of patch timing issues...

mm/shrinker_debug.c: In function 'shrinker_debugfs_count_show':
mm/shrinker_debug.c:64:17: error: too few arguments to function 'mem_cgroup_iter'
   64 |         memcg = mem_cgroup_iter(NULL, NULL, NULL);
      |                 ^~~~~~~~~~~~~~~
In file included from mm/shrinker_debug.c:7:
./include/linux/memcontrol.h:833:20: note: declared here
  833 | struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
      |                    ^~~~~~~~~~~~~~~
mm/shrinker_debug.c:89:27: error: too few arguments to function 'mem_cgroup_iter'
   89 |         } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
      |                           ^~~~~~~~~~~~~~~
./include/linux/memcontrol.h:833:20: note: declared here
  833 | struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
      |                    ^~~~~~~~~~~~~~~
mm/damon/sysfs-schemes.c: In function 'damon_sysfs_memcg_path_to_id':
mm/damon/sysfs-schemes.c:1594:22: error: too few arguments to function 'mem_cgroup_iter'
 1594 |         for (memcg = mem_cgroup_iter(NULL, NULL, NULL); memcg;
      |                      ^~~~~~~~~~~~~~~
In file included from ./include/linux/damon.h:11,
                 from mm/damon/sysfs-common.h:8,
                 from mm/damon/sysfs-schemes.c:10:
./include/linux/memcontrol.h:833:20: note: declared here
  833 | struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
      |                    ^~~~~~~~~~~~~~~
mm/damon/sysfs-schemes.c:1595:33: error: too few arguments to function 'mem_cgroup_iter'
 1595 |                         memcg = mem_cgroup_iter(NULL, memcg, NULL)) {
      |                                 ^~~~~~~~~~~~~~~
./include/linux/memcontrol.h:833:20: note: declared here
  833 | struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
      |                    ^~~~~~~~~~~~~~~

> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -832,7 +832,7 @@ static inline void mem_cgroup_put(struct mem_cgroup *memcg)
>  
>  struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *,
>  				   struct mem_cgroup *,
> -				   struct mem_cgroup_reclaim_cookie *);
> +				   struct mem_cgroup_reclaim_cookie *, bool online);

How many callsites do we expect to utilize the new `online' argument? 
Few, I suspect.

How about we fix the above and simplify the patch by adding a new
mem_cgroup_iter_online() and make mem_cgroup_iter() a one-line wrapper
which calls that and adds the online=false argument?

I also saw this, didn't investigate.

drivers/android/binder_alloc.c: In function 'binder_update_page_range':
drivers/android/binder_alloc.c:237:34: error: too few arguments to function 'list_lru_del'
  237 |                         on_lru = list_lru_del(&binder_alloc_lru, &page->lru);