linux-kernel - [PATCH] 9p: Don't revert the I/O iterator after reading

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Message-ID: <2299159.1729543103@warthog.procyon.org.uk>
Date: Mon, 21 Oct 2024 21:38:23 +0100
From: David Howells <dhowells@...hat.com>
To: Antony Antony <antony@...nome.org>
Cc: dhowells@...hat.com, Sedat Dilek <sedat.dilek@...il.com>,
    Maximilian Bosch <maximilian@...sch.me>,
    Linux regressions mailing list <regressions@...ts.linux.dev>,
    LKML <linux-kernel@...r.kernel.org>, linux-fsdevel@...r.kernel.org,
    Christian Brauner <brauner@...nel.org>
Subject: [PATCH] 9p: Don't revert the I/O iterator after reading

Hi Antony,

I think I may have a fix already lurking on my netfs-writeback branch for the
next merge window.  Can you try the attached?

David
---
Don't revert the I/O iterator before returning from p9_client_read_once().
netfslib doesn't require the reversion and nor doed 9P directory reading.

Make p9_client_read() use a temporary iterator to call down into
p9_client_read_once(), and advance that by the amount read.

Reported-by: Antony Antony <antony@...nome.org>
Signed-off-by: David Howells <dhowells@...hat.com>
cc: Eric Van Hensbergen <ericvh@...nel.org>
cc: Latchesar Ionkov <lucho@...kov.net>
cc: Dominique Martinet <asmadeus@...ewreck.org>
cc: Christian Schoenebeck <linux_oss@...debyte.com>
cc: v9fs@...ts.linux.dev
cc: netfs@...ts.linux.dev
cc: linux-fsdevel@...r.kernel.org
---
 net/9p/client.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index 5cd94721d974..be59b0a94eaf 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -1519,13 +1519,15 @@ p9_client_read(struct p9_fid *fid, u64 offset, struct iov_iter *to, int *err)
 	*err = 0;
 
 	while (iov_iter_count(to)) {
+		struct iov_iter tmp = *to;
 		int count;
 
-		count = p9_client_read_once(fid, offset, to, err);
+		count = p9_client_read_once(fid, offset, &tmp, err);
 		if (!count || *err)
 			break;
 		offset += count;
 		total += count;
+		iov_iter_advance(to, count);
 	}
 	return total;
 }
@@ -1567,16 +1569,12 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 	}
 	if (IS_ERR(req)) {
 		*err = PTR_ERR(req);
-		if (!non_zc)
-			iov_iter_revert(to, count - iov_iter_count(to));
 		return 0;
 	}
 
 	*err = p9pdu_readf(&req->rc, clnt->proto_version,
 			   "D", &received, &dataptr);
 	if (*err) {
-		if (!non_zc)
-			iov_iter_revert(to, count - iov_iter_count(to));
 		trace_9p_protocol_dump(clnt, &req->rc);
 		p9_req_put(clnt, req);
 		return 0;
@@ -1596,8 +1594,6 @@ p9_client_read_once(struct p9_fid *fid, u64 offset, struct iov_iter *to,
 			p9_req_put(clnt, req);
 			return n;
 		}
-	} else {
-		iov_iter_revert(to, count - received - iov_iter_count(to));
 	}
 	p9_req_put(clnt, req);
 	return received;