linux-kernel - Re: [PATCH 20/20] iscsi: support for swapping over iSCSI.

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Message-Id: <1158214650.13665.27.camel@twins>
Date:	Thu, 14 Sep 2006 08:17:30 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mike Christie <michaelc@...wisc.edu>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	David Miller <davem@...emloft.net>,
	Rik van Riel <riel@...hat.com>,
	Daniel Phillips <phillips@...gle.com>
Subject: Re: [PATCH 20/20] iscsi: support for swapping over iSCSI.

On Wed, 2006-09-13 at 15:50 -0500, Mike Christie wrote:
> 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 <a.p.zijlstra@...llo.nl>
> > CC: Mike Christie <michaelc@...wisc.edu>
> > ---
> >  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 thought I found allocations in that path, lemme search...
found this:

iscsi_tcp_data_recv()
  iscsi_data_rescv()
    iscsi_complete_pdu()
      __iscsi_complete_pdu()
        iscsi_recv_pdu()
          alloc_skb( GFP_ATOMIC);

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

Is that the skbuff you were talking about? If so, I'd need to carve a
path to pass the swapper information. I had that in a previous patch,
but that was large and ugly. I had to go carrying gfp_t flags all
through that call chain.

I could try again if you prefer that.

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