Re: PROBLEM: [REGRESSION 7.1-rc4 -> 7.1-rc5] 9p: silent NUL corruption of file data on cache modes with CACHE_WRITEBACK

[David Howells <dhowells@redhat.com>]

Does the attached work for you?  With it I see:

  cache=none       read   0/20  write   0/20  [cache=0x0]
  cache=readahead  read   0/20  write   0/20  [cache=0x1]
  cache=mmap       read   0/20  write   0/20  [cache=0x5]
  cache=loose      read   0/20  write   0/20  [cache=0xf]
  cache=fscache    read   0/20  write   0/20  [cache=0x8f]

David
---
commit a7f0af2e23c3ab191c08c1041dc67afd971a8c49
Author: David Howells <dhowells@redhat.com>
Date:   Fri Sep 11 10:15:43 2026 +0100

    9p: Fix v9fs_issue_write() to update i_size and remote_i_size
    
    Fix v9fs_issue_write() to update i_size and remote_i_size to the new size
    of the server file if we made it larger, using the start fpos and the count
    returned by p9_client_write() to calculate the new minimum file size.
    
    This assumes that if the 9P server makes a short write (say it hits
    ENOSPC), a reduced count is returned.
    
    Fixes: 5fb70e7275a6 ("netfs, 9p: Implement helpers for new write code")
    Reported-by: Michael Mulqueen <mike@method-b.uk>
    Closes: https://lore.kernel.org/r/fbb9e395-1e07-4212-8f70-23f3cd498074@method-b.uk/
    Signed-off-by: David Howells <dhowells@redhat.com>
    cc: Eric Van Hensbergen <ericvh@kernel.org>
    cc: Latchesar Ionkov <lucho@ionkov.net>
    cc: Dominique Martinet <asmadeus@codewreck.org>
    cc: Christian Schoenebeck <linux_oss@crudebyte.com>
    cc: Paulo Alcantara <pc@manguebit.org>
    cc: v9fs@lists.linux.dev
    cc: netfs@lists.linux.dev
    cc: linux-fsdevel@vger.kernel.org

diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
index 1ac0b3dcc077..13cf87a5f90c 100644
--- a/fs/9p/vfs_addr.c
+++ b/fs/9p/vfs_addr.c
@@ -54,11 +54,37 @@ static void v9fs_begin_writeback(struct netfs_io_request *wreq)
 static void v9fs_issue_write(struct netfs_io_subrequest *subreq)
 {
 	struct p9_fid *fid = subreq->rreq->netfs_priv;
+	struct inode *inode = subreq->rreq->inode;
+	struct netfs_inode *ictx = netfs_inode(inode);
 	int err, len;
 
 	len = p9_client_write(fid, subreq->start, &subreq->io_iter, &err);
-	if (len > 0)
+	if (len > 0) {
+		uoff_t end = subreq->start + len, i_size, remote, zp;
+		bool set = false;
+
+		spin_lock(&inode->i_lock);
+
+		/* We can read the sizes directly as we hold i_lock. */
+		i_size = inode->i_size;
+		remote = ictx->_remote_i_size;
+		zp = ictx->_zero_point;
+
+		if (end > i_size) {
+			i_size = end;
+			set = true;
+		}
+		if (end > remote) {
+			remote = end;
+			set = true;
+		}
+
+		if (set)
+			netfs_write_sizes(inode, i_size, remote, zp);
+		spin_unlock(&inode->i_lock);
+
 		__set_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
+	}
 	netfs_write_subrequest_terminated(subreq, len ?: err);
 }