2008-03-25 16:12:35

by Tom Tucker

[permalink] [raw]
Subject: [PATCH] SVCRDMA: Check num_sge when setting LAST_CTXT bit

SVCRDMA: Check num_sge when setting LAST_CTXT bit

The RDMACTXT_F_LAST_CTXT bit was getting set incorrectly
when the last chunk in the read-list spanned multiple pages. This
resulted in a kernel panic when the wrong context was used to
build the RPC iovec page list.

RDMA_READ is used to fetch RPC data from the client for
NFS_WRITE requests. A scatter-gather is used to map the
advertised client side buffer to the server-side iovec and
associated page list.

WR contexts are used to convey which scatter-gather entries are
handled by each WR. When the write data is large, a single RPC may
require multiple RDMA_READ requests so the contexts for a single RPC
are chained together in a linked list. The last context in this list
is marked with a bit RDMACTXT_F_LAST_CTXT so that when this WR completes,
the CQ handler code can enqueue the RPC for processing.

The code in rdma_read_xdr was setting this bit on the last two
contexts on this list when the last read-list chunk spanned multiple
pages. This caused the svc_rdma_recvfrom logic to incorrectly build
the RPC and caused the kernel to crash because the second-to-last
context doesn't contain the iovec page list.

Modified the condition that sets this bit so that it correctly detects
the last context for the RPC.

Signed-off-by: Tom Tucker <[email protected]>
Tested-by: Roland Dreier <[email protected]>

Bruce, this should be queued for 2.6.25 if possible.

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 21 +++++++++++----------
1 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index 301f619..85788e7 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -326,15 +326,6 @@ next_sge:
ctxt->direction = DMA_FROM_DEVICE;
clear_bit(RDMACTXT_F_READ_DONE, &ctxt->flags);
clear_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
- if ((ch+1)->rc_discrim == 0) {
- /*
- * Checked in sq_cq_reap to see if we need to
- * be enqueued
- */
- set_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
- ctxt->next = hdr_ctxt;
- hdr_ctxt->next = head;
- }

/* Prepare READ WR */
memset(&read_wr, 0, sizeof read_wr);
@@ -352,7 +343,17 @@ next_sge:
rdma_set_ctxt_sge(ctxt, &sge[ch_sge_ary[ch_no].start],
+ if (((ch+1)->rc_discrim == 0) &&
+ (read_wr.num_sge == ch_sge_ary[ch_no].count)) {
+ /*
+ * Mark the last RDMA_READ with a bit to
+ * indicate all RPC data has been fetched from
+ * the client and the RPC needs to be enqueued.
+ */
+ set_bit(RDMACTXT_F_LAST_CTXT, &ctxt->flags);
+ ctxt->next = hdr_ctxt;
+ hdr_ctxt->next = head;
+ }
/* Post the read */
err = svc_rdma_send(xprt, &read_wr);
if (err) {