Re: [PATCH 8/8] shmem,percpu_counter: add _limited_add(fbc, limit, amount)

Jan Kara
2023-10-04T15:32:37Z

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@google.com>
> Cc: Tim Chen <tim.c.chen@intel.com>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: Darrick J. Wong <djwong@kernel.org>

Looks good to me. Feel free to add:

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

Honza

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