Subject: Re: [PATCH 20/20] iscsi: support for swapping over iSCSI.

From: Mike Christie <[email-masked]>
Date: Wed, 13 Sep 2006 15:50:30 -0500

Peter Zijlstra wrote:
> Implement sht->swapdev() for iSCSI. This method takes care of reserving
> the extra memory needed and marking all relevant sockets with SOCK_VMIO.
> 
> When used for swapping, TCP socket creation is done under GFP_MEMALLOC and
> the TCP connect is done with SOCK_VMIO to ensure their success. Also the
> netlink userspace interface is marked SOCK_VMIO, this will ensure that even
> under pressure we can still communicate with the daemon (which runs as
> mlockall() and needs no additional memory to operate).
> 
> Netlink requests are handled under the new PF_MEM_NOWAIT when a swapper is
> present. This ensures that the netlink socket will not block. User-space will
> need to retry failed requests.
> 
> The TCP receive path is handled under PF_MEMALLOC for SOCK_VMIO sockets.
> This makes sure we do not block the critical socket, and that we do not
> fail to process incomming data.
> 
> Signed-off-by: Peter Zijlstra <[email-masked]>
> CC: Mike Christie <[email-masked]>
> ---
>  drivers/scsi/iscsi_tcp.c            |  103 +++++++++++++++++++++++++++++++-----
>  drivers/scsi/scsi_transport_iscsi.c |   23 +++++++-
>  include/scsi/libiscsi.h             |    1 
>  include/scsi/scsi_transport_iscsi.h |    2 
>  4 files changed, 113 insertions(+), 16 deletions(-)
> 
> Index: linux-2.6/drivers/scsi/iscsi_tcp.c
> ===================================================================
> --- linux-2.6.orig/drivers/scsi/iscsi_tcp.c
> +++ linux-2.6/drivers/scsi/iscsi_tcp.c
> @@ -42,6 +42,7 @@
>  #include <scsi/scsi_host.h>
>  #include <scsi/scsi.h>
>  #include <scsi/scsi_transport_iscsi.h>
> +#include <scsi/scsi_device.h>
>  
>  #include "iscsi_tcp.h"
>  
> @@ -845,9 +846,13 @@ iscsi_tcp_data_recv(read_descriptor_t *r
>  	int rc;
>  	struct iscsi_conn *conn = rd_desc->arg.data;
>  	struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
> -	int processed;
> +	int processed = 0;
>  	char pad[ISCSI_PAD_LEN];
>  	struct scatterlist sg;
> +	unsigned long pflags = current->flags;
> +
> +	if (sk_has_vmio(tcp_conn->sock->sk))
> +		current->flags |= PF_MEMALLOC;
>  

Is this too late or not needed or what is it for? This function gets run
from the network layer's softirq and at this point we have a skbuff with
data that we want to process. The iscsi layer also does not allocate
memory for read or write IO in this path.

I think we would want to set this flag at a lower level. Something
closer to where the skbuf is allocated?