linux-kernel - Re: [PATCH mm-unstable v2 4/5] mm: restart if multiple traversals raced

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <zh4ccaje54qbi6a62rvlhclysyaymw76bona4qtd53k4ogjuv7@tppv2q4zgyjk>
Date: Wed, 14 Aug 2024 11:00:21 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Kinsey Ho <kinseyho@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, Yosry Ahmed <yosryahmed@...gle.com>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Muchun Song <muchun.song@...ux.dev>, Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>
Subject: Re: [PATCH mm-unstable v2 4/5] mm: restart if multiple traversals
 raced

On Tue, Aug 13, 2024 at 08:47:14PM GMT, Kinsey Ho <kinseyho@...gle.com> wrote:
> @@ -1072,21 +1073,26 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  		 * and kicking, and don't take an extra reference.
>  		 */
>  		if (css == &root->css || css_tryget(css)) {
> -			memcg = mem_cgroup_from_css(css);
>  			break;
>  		}
>  	}
>  
> +	memcg = mem_cgroup_from_css(css);
> +
>  	if (reclaim) {
>  		/*
>  		 * The position could have already been updated by a competing
>  		 * thread, so check that the value hasn't changed since we read
>  		 * it to avoid reclaiming from the same cgroup twice.
>  		 */
> -		(void)cmpxchg(&iter->position, pos, memcg);
> +		if (cmpxchg(&iter->position, pos, memcg) != pos) {
> +			if (css && css != &root->css)
> +				css_put(css);
> +			goto restart;
> +		}

I may be missing (literal) context but I'd suggest not moving the memcg
assignment and leverage
	if (memcg != NULL)
		css_put(memcg->css)
so that the is-root comparison needn't be repeated.

Thanks,
Michal

Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)