9p: Fix DIO read through netfs

If a program is watching a file on a 9p mount, it won't see any change in
size if the file being exported by the server is changed directly in the
source filesystem, presumably because 9p doesn't have change notifications,
and because netfs skips the reads if the file is empty.

Fix this by attempting to read the full size specified when a DIO read is
requested (such as when 9p is operating in unbuffered mode) and dealing
with a short read if the EOF was less than the expected read.

To make this work, filesystems using netfslib must not set
NETFS_SREQ_CLEAR_TAIL if performing a DIO read where that read hit the EOF.
I don't want to mandatorily clear this flag in netfslib for DIO because,
say, ceph might make a read from an object that is not completely filled,
but does not reside at the end of file - and so we need to clear the
excess.

This can be tested by watching an empty file over 9p within a VM (such as
in the ktest framework):

        while true; do read content; if [ -n "$content" ]; then echo $content; break; fi; done < /host/tmp/foo

then writing something into the empty file.  The watcher should immediately
display the file content and break out of the loop.  Without this fix, it
remains in the loop indefinitely.

Fixes: 80105ed2fd ("9p: Use netfslib read/write_iter")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218916
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/1229195.1723211769@warthog.procyon.org.uk
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Trond Myklebust <trond.myklebust@hammerspace.com>
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/9p/vfs_addr.c

@@ -75,7 +75,8 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq)
 
 	/* if we just extended the file size, any portion not in
 	 * cache won't be on server and is zeroes */
-	__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
+	if (subreq->rreq->origin != NETFS_DIO_READ)
+		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 
 	netfs_subreq_terminated(subreq, err ?: total, false);
 }

fs/afs/file.c

@@ -242,7 +242,8 @@ static void afs_fetch_data_notify(struct afs_operation *op)
 
 	req->error = error;
 	if (subreq) {
-		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
+		if (subreq->rreq->origin != NETFS_DIO_READ)
+			__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 		netfs_subreq_terminated(subreq, error ?: req->actual_len, false);
 		req->subreq = NULL;
 	} else if (req->done) {

fs/ceph/addr.c

@@ -246,7 +246,8 @@ static void finish_netfs_read(struct ceph_osd_request *req)
 	if (err >= 0) {
 		if (sparse && err > 0)
 			err = ceph_sparse_ext_map_end(op);
-		if (err < subreq->len)
+		if (err < subreq->len &&
+		    subreq->rreq->origin != NETFS_DIO_READ)
 			__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 		if (IS_ENCRYPTED(inode) && err > 0) {
 			err = ceph_fscrypt_decrypt_extents(inode,
@@ -282,7 +283,8 @@ static bool ceph_netfs_issue_op_inline(struct netfs_io_subrequest *subreq)
 	size_t len;
 	int mode;
 
-	__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
+	if (rreq->origin != NETFS_DIO_READ)
+		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 	__clear_bit(NETFS_SREQ_COPY_TO_CACHE, &subreq->flags);
 
 	if (subreq->start >= inode->i_size)

fs/netfs/io.c

@@ -530,7 +530,8 @@ incomplete:
 
 	if (transferred_or_error == 0) {
 		if (__test_and_set_bit(NETFS_SREQ_NO_PROGRESS, &subreq->flags)) {
-			subreq->error = -ENODATA;
+			if (rreq->origin != NETFS_DIO_READ)
+				subreq->error = -ENODATA;
 			goto failed;
 		}
 	} else {
@@ -601,9 +602,14 @@ netfs_rreq_prepare_read(struct netfs_io_request *rreq,
 			}
 			if (subreq->len > ictx->zero_point - subreq->start)
 				subreq->len = ictx->zero_point - subreq->start;
+
+			/* We limit buffered reads to the EOF, but let the
+			 * server deal with larger-than-EOF DIO/unbuffered
+			 * reads.
+			 */
+			if (subreq->len > rreq->i_size - subreq->start)
+				subreq->len = rreq->i_size - subreq->start;
 		}
-		if (subreq->len > rreq->i_size - subreq->start)
-			subreq->len = rreq->i_size - subreq->start;
 		if (rreq->rsize && subreq->len > rreq->rsize)
 			subreq->len = rreq->rsize;
 
@@ -739,11 +745,10 @@ int netfs_begin_read(struct netfs_io_request *rreq, bool sync)
 	do {
 		_debug("submit %llx + %llx >= %llx",
 		       rreq->start, rreq->submitted, rreq->i_size);
-		if (rreq->origin == NETFS_DIO_READ &&
-		    rreq->start + rreq->submitted >= rreq->i_size)
-			break;
 		if (!netfs_rreq_submit_slice(rreq, &io_iter))
 			break;
+		if (test_bit(NETFS_SREQ_NO_PROGRESS, &rreq->flags))
+			break;
 		if (test_bit(NETFS_RREQ_BLOCKED, &rreq->flags) &&
 		    test_bit(NETFS_RREQ_NONBLOCK, &rreq->flags))
 			break;

fs/nfs/fscache.c

@@ -363,7 +363,8 @@ void nfs_netfs_read_completion(struct nfs_pgio_header *hdr)
 		return;
 
 	sreq = netfs->sreq;
-	if (test_bit(NFS_IOHDR_EOF, &hdr->flags))
+	if (test_bit(NFS_IOHDR_EOF, &hdr->flags) &&
+	    sreq->rreq->origin != NETFS_DIO_READ)
 		__set_bit(NETFS_SREQ_CLEAR_TAIL, &sreq->flags);
 
 	if (hdr->error)

fs/smb/client/file.c

@@ -217,7 +217,8 @@ static void cifs_req_issue_read(struct netfs_io_subrequest *subreq)
 			goto out;
 	}
 
-	__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
+	if (subreq->rreq->origin != NETFS_DIO_READ)
+		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 
 	rc = rdata->server->ops->async_readv(rdata);
 out: