linux-kernel - Re: [PATCH mm-unstable RFC 0/5] cgroup: eliminate atomic rstat

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: <f26eed88dddcf6c8d2a127f4cdc6aaf740bbfb7d.camel@linux.intel.com>
Date:   Thu, 06 Apr 2023 11:23:47 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-unstable RFC 0/5] cgroup: eliminate atomic rstat

On Mon, 2023-04-03 at 22:03 +0000, Yosry Ahmed wrote:
> A previous patch series ([1] currently in mm-unstable) changed most

Can you include the link to [1]?

Thanks.

Tim

> atomic rstat flushing contexts to become non-atomic. This was done to
> avoid an expensive operation that scales with # cgroups and # cpus to
> happen with irqs disabled and scheduling not permitted. There were two
> remaining atomic flushing contexts after that series. This series tries
> to eliminate them as well, eliminating atomic rstat flushing completely.
> 
> The two remaining atomic flushing contexts are:
> (a) wb_over_bg_thresh()->mem_cgroup_wb_stats()
> (b) mem_cgroup_threshold()->mem_cgroup_usage()
> 
> For (a), flushing needs to be atomic as wb_writeback() calls
> wb_over_bg_thresh() with a spinlock held. However, it seems like the
> call to wb_over_bg_thresh() doesn't need to be protected by that
> spinlock, so this series proposes a refactoring that moves the call
> outside the lock criticial section and makes the stats flushing
> in mem_cgroup_wb_stats() non-atomic.
> 
> For (b), flushing needs to be atomic as mem_cgroup_threshold() is called
> with irqs disabled. We only flush the stats when calculating the root
> usage, as it is approximated as the sum of some memcg stats (file, anon,
> and optionally swap) instead of the conventional page counter. This
> series proposes changing this calculation to use the global stats
> instead, eliminating the need for a memcg stat flush.
> 
> After these 2 contexts are eliminated, we no longer need
> mem_cgroup_flush_stats_atomic() or cgroup_rstat_flush_atomic(). We can
> remove them and simplify the code.
> 
> Yosry Ahmed (5):
>   writeback: move wb_over_bg_thresh() call outside lock section
>   memcg: flush stats non-atomically in mem_cgroup_wb_stats()
>   memcg: calculate root usage from global state
>   memcg: remove mem_cgroup_flush_stats_atomic()
>   cgroup: remove cgroup_rstat_flush_atomic()
> 
>  fs/fs-writeback.c          | 16 +++++++----
>  include/linux/cgroup.h     |  1 -
>  include/linux/memcontrol.h |  5 ----
>  kernel/cgroup/rstat.c      | 26 ++++--------------
>  mm/memcontrol.c            | 54 ++++++++------------------------------
>  5 files changed, 27 insertions(+), 75 deletions(-)
>