linux-kernel - Re: [PATCH 3/6] cifs: smbd: Avoid allocating iov on the stack

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Date:   Tue, 17 Apr 2018 09:29:32 +0200
From:   Greg KH <greg@...ah.com>
To:     longli@...rosoft.com
Cc:     Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 3/6] cifs: smbd: Avoid allocating iov on the stack

On Mon, Apr 16, 2018 at 05:49:15PM -0700, Long Li wrote:
> From: Long Li <longli@...rosoft.com>
> 
> It's not necessary to allocate another iov when going through the buffers
> in smbd_send() through RDMA send.
> 
> Remove it to reduce stack size.
> 
> Signed-off-by: Long Li <longli@...rosoft.com>
> ---
>  fs/cifs/smbdirect.c | 36 ++++++++++++------------------------
>  1 file changed, 12 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/cifs/smbdirect.c b/fs/cifs/smbdirect.c
> index b5c6c0d..f575e9a 100644
> --- a/fs/cifs/smbdirect.c
> +++ b/fs/cifs/smbdirect.c
> @@ -2088,7 +2088,7 @@ int smbd_send(struct smbd_connection *info, struct smb_rqst *rqst)
>  	int start, i, j;
>  	int max_iov_size =
>  		info->max_send_size - sizeof(struct smbd_data_transfer);
> -	struct kvec iov[SMBDIRECT_MAX_SGE];
> +	struct kvec *iov;
>  	int rc;
>  
>  	info->smbd_send_pending++;
> @@ -2099,32 +2099,20 @@ int smbd_send(struct smbd_connection *info, struct smb_rqst *rqst)
>  	}
>  
>  	/*
> -	 * This usually means a configuration error
> -	 * We use RDMA read/write for packet size > rdma_readwrite_threshold
> -	 * as long as it's properly configured we should never get into this
> -	 * situation
> -	 */
> -	if (rqst->rq_nvec + rqst->rq_npages > SMBDIRECT_MAX_SGE) {
> -		log_write(ERR, "maximum send segment %x exceeding %x\n",
> -			 rqst->rq_nvec + rqst->rq_npages, SMBDIRECT_MAX_SGE);
> -		rc = -EINVAL;
> -		goto done;
> -	}
> -
> -	/*
> -	 * Remove the RFC1002 length defined in MS-SMB2 section 2.1
> -	 * It is used only for TCP transport
> +	 * Skip the RFC1002 length defined in MS-SMB2 section 2.1
> +	 * It is used only for TCP transport in the iov[0]
>  	 * In future we may want to add a transport layer under protocol
>  	 * layer so this will only be issued to TCP transport
>  	 */
> -	iov[0].iov_base = (char *)rqst->rq_iov[0].iov_base + 4;
> -	iov[0].iov_len = rqst->rq_iov[0].iov_len - 4;
> -	buflen += iov[0].iov_len;
> +
> +	if (rqst->rq_iov[0].iov_len != 4) {
> +		log_write(ERR, "expected the pdu length in 1st iov, but got 0x%lu\n", rqst->rq_iov[0].iov_len);
> +		return -EINVAL;
> +	}
> +	iov = &rqst->rq_iov[1];
>  
>  	/* total up iov array first */
> -	for (i = 1; i < rqst->rq_nvec; i++) {
> -		iov[i].iov_base = rqst->rq_iov[i].iov_base;
> -		iov[i].iov_len = rqst->rq_iov[i].iov_len;
> +	for (i = 0; i < rqst->rq_nvec-1; i++) {
>  		buflen += iov[i].iov_len;
>  	}
>  
> @@ -2197,14 +2185,14 @@ int smbd_send(struct smbd_connection *info, struct smb_rqst *rqst)
>  						goto done;
>  				}
>  				i++;
> -				if (i == rqst->rq_nvec)
> +				if (i == rqst->rq_nvec-1)
>  					break;
>  			}
>  			start = i;
>  			buflen = 0;
>  		} else {
>  			i++;
> -			if (i == rqst->rq_nvec) {
> +			if (i == rqst->rq_nvec-1) {
>  				/* send out all remaining vecs */
>  				remaining_data_length -= buflen;
>  				log_write(INFO,
> -- 
> 2.7.4

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>