NFSv4.1: Fix an off-by-one error in pnfs_generic_pg_test

And document what is going on there...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 16 insertions, 5 deletions

diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
index 0f42e02436e..29c0ca7fc34 100644
--- a/fs/nfs/pnfs.c
+++ b/fs/nfs/pnfs.c
@@ -1078,11 +1078,22 @@ pnfs_generic_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
                         return true;
         }
 
-        if (req_offset(req) > end_offset(pgio->pg_lseg->pls_range.offset,
-                                         pgio->pg_lseg->pls_range.length))
-                return false;
-
-        return true;
+        /*
+         * Test if a nfs_page is fully contained in the pnfs_layout_range.
+         * Note that this test makes several assumptions:
+         * - that the previous nfs_page in the struct nfs_pageio_descriptor
+         *   is known to lie within the range.
+         *   - that the nfs_page being tested is known to be contiguous with the
+         *   previous nfs_page.
+         *   - Layout ranges are page aligned, so we only have to test the
+         *   start offset of the request.
+         *
+         * Please also note that 'end_offset' is actually the offset of the
+         * first byte that lies outside the pnfs_layout_range. FIXME?
+         *
+         */
+        return req_offset(req) < end_offset(pgio->pg_lseg->pls_range.offset,
+                                         pgio->pg_lseg->pls_range.length);
 }
 EXPORT_SYMBOL_GPL(pnfs_generic_pg_test);