Re: [PATCH 3/3] fs, mm: account buffer_head to kmemcg

Johannes Weiner
2018-06-19T16:25:24Z

On Mon, Jun 18, 2018 at 10:13:27PM -0700, Shakeel Butt wrote:
> The buffer_head can consume a significant amount of system memory and
> is directly related to the amount of page cache. In our production
> environment we have observed that a lot of machines are spending a
> significant amount of memory as buffer_head and can not be left as
> system memory overhead.
>
> Charging buffer_head is not as simple as adding __GFP_ACCOUNT to the
> allocation. The buffer_heads can be allocated in a memcg different from
> the memcg of the page for which buffer_heads are being allocated. One
> concrete example is memory reclaim. The reclaim can trigger I/O of pages
> of any memcg on the system. So, the right way to charge buffer_head is
> to extract the memcg from the page for which buffer_heads are being
> allocated and then use targeted memcg charging API.
>
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Greg Thelen <gthelen@google.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> fs/buffer.c | 14 +++++++++++++-
> include/linux/memcontrol.h | 7 +++++++
> mm/memcontrol.c | 21 +++++++++++++++++++++
> 3 files changed, 41 insertions(+), 1 deletion(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 8194e3049fc5..26389b7a3cab 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -815,10 +815,17 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> struct buffer_head *bh, *head;
> gfp_t gfp = GFP_NOFS;
> long offset;
> + struct mem_cgroup *old_memcg;
> + struct mem_cgroup *memcg = get_mem_cgroup_from_page(page);
>
> if (retry)
> gfp |= __GFP_NOFAIL;
>
> + if (memcg) {
> + gfp |= __GFP_ACCOUNT;
> + old_memcg = memalloc_memcg_save(memcg);
> + }

Please move the get_mem_cgroup_from_page() call out of the
declarations and down to right before the if (memcg) branch.

> head = NULL;
> offset = PAGE_SIZE;
> while ((offset -= size) >= 0) {
> @@ -835,6 +842,11 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
> /* Link the buffer to its page */
> set_bh_page(bh, page, offset);
> }
> +out:
> + if (memcg) {
> + memalloc_memcg_restore(old_memcg);
> +#ifdef CONFIG_MEMCG
> + css_put(&memcg->css);
> +#endif

Please add a put_mem_cgroup() ;)

Otherwise this looks fine to me.