Re: [PATCH 0/4] nbd: replace socks pointer array with xarray to eliminate queue freeze

[Long Li]

On Fri, Mar 27, 2026 at 05:12:19PM +0800, leo.lilong@xxxxxxxxxxxxxxx wrote:
> From: Long Li <leo.lilong@xxxxxxxxxx>
> 
> Hi,
> 
> Commit b98e762e3d ("nbd: freeze the queue while we're adding
> connections") introduced blk_mq_freeze_queue() in the add-socket path
> to prevent use-after-free when krealloc() relocates the config->socks
> array while I/O is in flight. However, freezing the queue on every
> connection setup introduces significant latency when establishing a
> large number of connections.
> 
> This series eliminates the queue freeze by replacing the
> krealloc-based struct nbd_sock **socks array with a struct xarray.
> The xarray provides RCU-safe pointer publishing: each nbd_sock is
> fully initialized before being stored via xa_store(), and concurrent
> readers access individual entries through xa_load() without ever
> holding a reference to the array itself. This removes the possibility
> of UAF on array reallocation, making the queue freeze unnecessary.
> 
> The following test was performed with 256 connections on a local
> nbd-server:
> 
>   nbd-server -M 256 -C /etc/nbd-server/config
>   time nbd-client 127.0.0.1 10809 /dev/nbd0 -N myexport -C 256
> 
>   Before:  real 4.510s  user 0.004s  sys 0.038s
>   After:   real 0.263s  user 0.009s  sys 0.032s
> 
> Connection setup time is reduced by ~94%.
> 
> Long Li (4):
>   nbd: simplify find_fallback() by removing redundant logic
>   nbd: replace socks pointer array with xarray
>   nbd: remove redundant num_connections boundary checks
>   nbd: remove queue freeze in nbd_add_socket
> 
>  drivers/block/nbd.c | 201 +++++++++++++++++++++++---------------------
>  1 file changed, 106 insertions(+), 95 deletions(-)
> 
> -- 
> 2.39.2
> 

Friendly ping ...