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    Subject[PATCH 3/4] nbd: deadlock prevention for NBD

    Use sk_set_vmio() on the nbd socket.

    Limit each request to 1 page, so that the request throttling also limits the
    number of in-flight pages and force the IO scheduler to NOOP as anything else
    doesn't make sense anyway.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: Daniel Phillips <phillips@google.com>
    ---
    drivers/block/nbd.c | 18 ++++++++++++++++--
    1 file changed, 16 insertions(+), 2 deletions(-)

    Index: linux-2.6/drivers/block/nbd.c
    ===================================================================
    --- linux-2.6.orig/drivers/block/nbd.c
    +++ linux-2.6/drivers/block/nbd.c
    @@ -135,7 +135,6 @@ static int sock_xmit(struct socket *sock
    spin_unlock_irqrestore(&current->sighand->siglock, flags);

    do {
    - sock->sk->sk_allocation = GFP_NOIO;
    iov.iov_base = buf;
    iov.iov_len = size;
    msg.msg_name = NULL;
    @@ -361,8 +360,16 @@ static void nbd_do_it(struct nbd_device

    BUG_ON(lo->magic != LO_MAGIC);

    + sk_adjust_memalloc(0, 1);
    + if (sk_set_vmio(lo->sock->sk))
    + printk(KERN_WARNING
    + "failed to set SOCK_VMIO on NBD socket\n");
    +
    while ((req = nbd_read_stat(lo)) != NULL)
    nbd_end_request(req);
    +
    + sk_adjust_memalloc(0, -1);
    +
    return;
    }

    @@ -525,6 +533,7 @@ static int nbd_ioctl(struct inode *inode
    if (S_ISSOCK(inode->i_mode)) {
    lo->file = file;
    lo->sock = SOCKET_I(inode);
    + lo->sock->sk->sk_allocation = GFP_NOIO;
    error = 0;
    } else {
    fput(file);
    @@ -628,11 +637,16 @@ static int __init nbd_init(void)
    * every gendisk to have its very own request_queue struct.
    * These structs are big so we dynamically allocate them.
    */
    - disk->queue = blk_init_queue(do_nbd_request, &nbd_lock);
    + disk->queue = blk_init_queue_node_elv(do_nbd_request,
    + &nbd_lock, -1, "noop");
    if (!disk->queue) {
    put_disk(disk);
    goto out;
    }
    + blk_queue_pin_elevator(disk->queue);
    + blk_queue_max_segment_size(disk->queue, PAGE_SIZE);
    + blk_queue_max_hw_segments(disk->queue, 1);
    + blk_queue_max_phys_segments(disk->queue, 1);
    }

    if (register_blkdev(NBD_MAJOR, "nbd")) {
    -
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