linux-kernel - Re: [PATCH] epoll: account epitem and eppoll

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Date:   Wed, 4 Oct 2017 15:17:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] epoll: account epitem and eppoll_entry to kmemcg

On Mon 02-10-17 19:15:19, Shakeel Butt wrote:
> The user space application can directly trigger the allocations
> from eventpoll_epi and eventpoll_pwq slabs. A buggy or malicious
> application can consume a significant amount of system memory by
> triggering such allocations. Indeed we have seen in production
> where a buggy application was leaking the epoll references and
> causing a burst of eventpoll_epi and eventpoll_pwq slab
> allocations. This patch opt-in the charging of eventpoll_epi
> and eventpoll_pwq slabs.

I am not objecting to the patch I would just like to understand the
runaway case. ep_insert seems to limit the maximum number of watches to
max_user_watches which should be ~4% of lowmem if I am following the
code properly. pwq_cache should be bound by the number of watches as
well, or am I misunderstanding the code?

> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> ---
>  fs/eventpoll.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 2fabd19cdeea..a45360444895 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -2329,11 +2329,11 @@ static int __init eventpoll_init(void)
>  
>  	/* Allocates slab cache used to allocate "struct epitem" items */
>  	epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
> -			0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);
> +			0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);
>  
>  	/* Allocates slab cache used to allocate "struct eppoll_entry" */
>  	pwq_cache = kmem_cache_create("eventpoll_pwq",
> -			sizeof(struct eppoll_entry), 0, SLAB_PANIC, NULL);
> +		sizeof(struct eppoll_entry), 0, SLAB_PANIC|SLAB_ACCOUNT, NULL);
>  
>  	return 0;
>  }
> -- 
> 2.14.2.822.g60be5d43e6-goog

-- 
Michal Hocko
SUSE Labs