[PATCH 10/9] netfs: Fix interaction of streaming writes with zero-point tracker

[David Howells]

When a folio that is marked for streaming write (dirty, but not uptodate,
with partial content specified in the private data) is written back, the
folio is effectively switched to the blank state upon completion of the
write.  This means that if we want to read it in future, we need to reread
the whole folio.

However, if the folio is above the zero_point position, when it is read
back, it will just be cleared and the read skipped, leading to apparent
local corruption.

Fix this by increasing the zero_point to the end of the dirty data in the
folio when clearing the folio state after writeback.  This is analogous to
the folio having ->release_folio() called upon it.

This was causing the config.log generated by configuring a cpython tree on
a cifs share to get corrupted because the scripts involved were appending
text to the file in small pieces.

Fixes: 288ace2f57c9 ("netfs: New writeback implementation")
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Steve French <sfrench at samba.org>
cc: Paulo Alcantara <pc at manguebit.com>
cc: Jeff Layton <jlayton at kernel.org>
cc: linux-cifs at vger.kernel.org
cc: netfs at lists.linux.dev
cc: linux-fsdevel at vger.kernel.org
---
 fs/netfs/write_collect.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/netfs/write_collect.c b/fs/netfs/write_collect.c
index 426cf87aaf2e..ae7a2043f670 100644
--- a/fs/netfs/write_collect.c
+++ b/fs/netfs/write_collect.c
@@ -33,6 +33,7 @@
 int netfs_folio_written_back(struct folio *folio)
 {
 	enum netfs_folio_trace why = netfs_folio_trace_clear;
+	struct netfs_inode *ictx = netfs_inode(folio->mapping->host);
 	struct netfs_folio *finfo;
 	struct netfs_group *group = NULL;
 	int gcount = 0;
@@ -41,6 +42,12 @@ int netfs_folio_written_back(struct folio *folio)
 		/* Streaming writes cannot be redirtied whilst under writeback,
 		 * so discard the streaming record.
 		 */
+		unsigned long long fend;
+
+		fend = folio_pos(folio) + finfo->dirty_offset + finfo->dirty_len;
+		if (fend > ictx->zero_point)
+			ictx->zero_point = fend;
+
 		folio_detach_private(folio);
 		group = finfo->netfs_group;
 		gcount++;