linux-kernel - Re: [PATCH 8/8] shmem,percpu_counter: add _limited

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]

Date:   Wed, 4 Oct 2023 17:32:31 +0200
From:   Jan Kara <jack@...e.cz>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...el.com>,
        Dave Chinner <dchinner@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Carlos Maiolino <cem@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit,
 amount)

On Fri 29-09-23 20:42:45, Hugh Dickins wrote:
> Percpu counter's compare and add are separate functions: without locking
> around them (which would defeat their purpose), it has been possible to
> overflow the intended limit.  Imagine all the other CPUs fallocating
> tmpfs huge pages to the limit, in between this CPU's compare and its add.
> 
> I have not seen reports of that happening; but tmpfs's recent addition
> of dquot_alloc_block_nodirty() in between the compare and the add makes
> it even more likely, and I'd be uncomfortable to leave it unfixed.
> 
> Introduce percpu_counter_limited_add(fbc, limit, amount) to prevent it.
> 
> I believe this implementation is correct, and slightly more efficient
> than the combination of compare and add (taking the lock once rather
> than twice when nearing full - the last 128MiB of a tmpfs volume on a
> machine with 128 CPUs and 4KiB pages); but it does beg for a better
> design - when nearing full, there is no new batching, but the costly
> percpu counter sum across CPUs still has to be done, while locked.
> 
> Follow __percpu_counter_sum()'s example, including cpu_dying_mask as
> well as cpu_online_mask: but shouldn't __percpu_counter_compare() and
> __percpu_counter_limited_add() then be adding a num_dying_cpus() to
> num_online_cpus(), when they calculate the maximum which could be held
> across CPUs?  But the times when it matters would be vanishingly rare.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: Tim Chen <tim.c.chen@...el.com>
> Cc: Dave Chinner <dchinner@...hat.com>
> Cc: Darrick J. Wong <djwong@...nel.org>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR