sunrpc: Allocate up to RPCSVC_MAXPAGES per svc_rqst

svcrdma needs 259 pages allocated to receive 1MB NFSv4.0 WRITE requests:

 - 1 page for the transport header and head iovec
 - 256 pages for the data payload
 - 1 page for the trailing GETATTR request (since NFSD XDR decoding
   does not look for a tail iovec, the GETATTR is stuck at the end
   of the rqstp->rq_arg.pages list)
 - 1 page for building the reply xdr_buf

But RPCSVC_MAXPAGES is already 259 (on x86_64). The problem is that
svc_alloc_arg never allocates that many pages. To address this:

1. The final element of rq_pages always points to NULL. To
   accommodate up to 259 pages in rq_pages, add an extra element
   to rq_pages for the array termination sentinel.

2. Adjust the calculation of "pages" to match how RPCSVC_MAXPAGES
   is calculated, so it can go up to 259. Bruce noted that the
   calculation assumes sv_max_mesg is a multiple of PAGE_SIZE,
   which might not always be true. I didn't change this assumption.

3. Change the loop boundaries to allow 259 pages to be allocated.

Additional clean-up: WARN_ON_ONCE adds an extra conditional branch,
which is basically never taken. And there's no need to dump the
stack here because svc_alloc_arg has only one caller.

Keeping that NULL "array termination sentinel"; there doesn't appear to
be any code that depends on it, only code in nfsd_splice_actor() which
needs the 259th element to be initialized to *something*.  So it's
possible we could just keep the array at 259 elements and drop that
final NULL, but we're being conservative for now.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 7bfe1fb42add..d16a8b423c20 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -659,11 +659,13 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
         int i;
 
         /* now allocate needed pages.  If we get a failure, sleep briefly */
-        pages = (serv->sv_max_mesg + PAGE_SIZE) / PAGE_SIZE;
-        WARN_ON_ONCE(pages >= RPCSVC_MAXPAGES);
-        if (pages >= RPCSVC_MAXPAGES)
+        pages = (serv->sv_max_mesg + 2 * PAGE_SIZE) >> PAGE_SHIFT;
+        if (pages > RPCSVC_MAXPAGES) {
+                pr_warn_once("svc: warning: pages=%u > RPCSVC_MAXPAGES=%lu\n",
+                             pages, RPCSVC_MAXPAGES);
                 /* use as many pages as possible */
-                pages = RPCSVC_MAXPAGES - 1;
+                pages = RPCSVC_MAXPAGES;
+        }
         for (i = 0; i < pages ; i++)
                 while (rqstp->rq_pages[i] == NULL) {
                         struct page *p = alloc_page(GFP_KERNEL);