[RFC PATCH V4 11/13] Btrfs: Clean pte corresponding to page straddling i_size

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

When extending a file by either "truncate up" or by writing beyond i_size, the
page which had i_size needs to be marked "read only" so that future writes to
the page via mmap interface causes btrfs_page_mkwrite() to be invoked. If not,
a write performed after extending the file via the mmap interface will find
the page to be writaeable and continue writing to the page without invoking
btrfs_page_mkwrite() i.e. we end up writing to a file without reserving disk
space.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
---
 fs/btrfs/file.c  | 12 ++++++++++--
 fs/btrfs/inode.c |  2 +-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 360d56d..5715e29 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1757,6 +1757,8 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	ssize_t err;
 	loff_t pos;
 	size_t count;
+	loff_t oldsize;
+	int clean_page = 0;
 
 	mutex_lock(&inode->i_mutex);
 	err = generic_write_checks(iocb, from);
@@ -1795,14 +1797,17 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	pos = iocb->ki_pos;
 	count = iov_iter_count(from);
 	start_pos = round_down(pos, root->sectorsize);
-	if (start_pos > i_size_read(inode)) {
+	oldsize = i_size_read(inode);
+	if (start_pos > oldsize) {
 		/* Expand hole size to cover write data, preventing empty gap */
 		end_pos = round_up(pos + count, root->sectorsize);
-		err = btrfs_cont_expand(inode, i_size_read(inode), end_pos);
+		err = btrfs_cont_expand(inode, oldsize, end_pos);
 		if (err) {
 			mutex_unlock(&inode->i_mutex);
 			goto out;
 		}
+		if (start_pos > round_up(oldsize, root->sectorsize))
+			clean_page = 1;
 	}
 
 	if (sync)
@@ -1814,6 +1819,9 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 		num_written = __btrfs_buffered_write(file, from, pos);
 		if (num_written > 0)
 			iocb->ki_pos = pos + num_written;
+		if (clean_page)
+			pagecache_isize_extended(inode, oldsize,
+						i_size_read(inode));
 	}
 
 	mutex_unlock(&inode->i_mutex);
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index c937357..f31da87 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4853,7 +4853,6 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 	}
 
 	if (newsize > oldsize) {
-		truncate_pagecache(inode, newsize);
 		/*
 		 * Don't do an expanding truncate while snapshoting is ongoing.
 		 * This is to ensure the snapshot captures a fully consistent
@@ -4876,6 +4875,7 @@ static int btrfs_setsize(struct inode *inode, struct iattr *attr)
 
 		i_size_write(inode, newsize);
 		btrfs_ordered_update_i_size(inode, i_size_read(inode), NULL);
+		pagecache_isize_extended(inode, oldsize, newsize);
 		ret = btrfs_update_inode(trans, root, inode);
 		btrfs_end_write_no_snapshoting(root);
 		btrfs_end_transaction(trans, root);
-- 
2.1.0