Re: [PATCH] io-wq: fix handling of NUMA node IDs

[Jens Axboe <axboe@kernel.dk>]

On 11/26/19 11:10 AM, Jann Horn wrote:
> There are several things that can go wrong in the current code on NUMA
> systems, especially if not all nodes are online all the time:
> 
>   - If the identifiers of the online nodes do not form a single contiguous
>     block starting at zero, wq->wqes will be too small, and OOB memory
>     accesses will occur e.g. in the loop in io_wq_create().
>   - If a node comes online between the call to num_online_nodes() and the
>     for_each_node() loop in io_wq_create(), an OOB write will occur.
>   - If a node comes online between io_wq_create() and io_wq_enqueue(), a
>     lookup is performed for an element that doesn't exist, and an OOB read
>     will probably occur.
> 
> Fix it by:
> 
>   - using nr_node_ids instead of num_online_nodes() for the allocation size;
>     nr_node_ids is calculated by setup_nr_node_ids() to be bigger than the
>     highest node ID that could possibly come online at some point, even if
>     those nodes' identifiers are not a contiguous block
>   - creating workers for all possible CPUs, not just all online ones
> 
> This is basically what the normal workqueue code also does, as far as I can
> tell.
> 
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> 
> Notes:
>      compile-tested only.
>      
>      While I think I probably got this stuff right, it might be good if
>      someone more familiar with the NUMA logic could give an opinion on this.
>      
>      An alternative might be to only allocate workers for online nodes, but
>      then we'd have to either fiddle together logic to create more workers
>      on demand or punt requests on newly-onlined nodes over to older nodes.
>      Both of those don't seem very nice to me.

I don't think caring about not-online nodes in terms of savings is worth
the trouble. I'll run this through the regular testing I have with no
and 2 nodes, thanks.

-- 
Jens Axboe