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Subject: [PATCH v1 19/42] SUNRPC: Fix xdr_get_next_encode_buffer() page
 boundary handling
From:   Chuck Lever <chuck.lever@oracle.com>
To:     linux-nfs@vger.kernel.org
Date:   Mon, 01 Mar 2021 10:17:13 -0500
Message-ID: <161461183307.8508.17196295994390119297.stgit@klimt.1015granger.net>
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The description of commit 2825a7f90753 ("nfsd4: allow encoding
across page boundaries") states:

> Also we can't handle a new operation starting close to the end of
> a page.

But does not detail why this is the case.

Subtracting the scratch buffer's "shift" value from the remaining
stream space seems to make reserving space close to the end of the
buf->pages array reliable.

This change is needed to make entry encoding with struct xdr_stream,
introduced in a subsequent patch, work correctly when it approaches
the end of the dirlist buffer.

Fixes: 2825a7f90753 ("nfsd4: allow encoding across page boundaries")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xdr.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
index 3964ff74ee51..043b67229792 100644
--- a/net/sunrpc/xdr.c
+++ b/net/sunrpc/xdr.c
@@ -978,7 +978,7 @@ static __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
 	 * shifted this one back:
 	 */
 	xdr->p = (void *)p + frag2bytes;
-	space_left = xdr->buf->buflen - xdr->buf->len;
+	space_left = xdr->buf->buflen - xdr->buf->len - frag1bytes;
 	xdr->end = (void *)p + min_t(int, space_left, PAGE_SIZE);
 	xdr->buf->page_len += frag2bytes;
 	xdr->buf->len += nbytes;