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    SubjectRe: [PATCH 20/20] iscsi: support for swapping over iSCSI.
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    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@chello.nl>
    > > CC: Mike Christie <michaelc@cs.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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