[Devel] [PATCH 2/2] fuse: wait for writeback in fuse_file_fallocate() -v2

[Maxim Patlasov]

The patch fixes a race between mmap-ed write and fallocate(PUNCH_HOLE):

1) An user makes a page dirty via mmap-ed write.
2) The user performs fallocate(2) with mode == PUNCH_HOLE|KEEP_SIZE
   and <offset, size> covering the page.
3) Before truncate_pagecache_range call from fuse_file_fallocate,
   the page goes to write-back. The page is fully processed by fuse_writepage
   (including end_page_writeback on the page), but fuse_flush_writepages did
   nothing because fi->writectr < 0.
4) truncate_pagecache_range is called and fuse_file_fallocate is finishing
   by calling fuse_release_nowrite. The latter triggers processing queued
   write-back request which will write stale data to the hole soon.

Changed in v2 (thanks to Brian for suggestion):
 - Do not truncate page cache until FUSE_FALLOCATE succeeded. Otherwise,
   we can end up in returning -ENOTSUPP while user data is already punched
   from page cache. Use filemap_write_and_wait_range() instead.

Signed-off-by: Maxim Patlasov <mpatlasov at parallels.com>
---
 fs/fuse/file.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++++------
 1 files changed, 48 insertions(+), 6 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index d1715b3..e88ba8b 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -344,6 +344,31 @@ static bool fuse_page_is_writeback(struct inode *inode, pgoff_t index)
 	return found;
 }
 
+static bool fuse_range_is_writeback(struct inode *inode, pgoff_t idx_from,
+				    pgoff_t idx_to)
+{
+	struct fuse_conn *fc = get_fuse_conn(inode);
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	struct fuse_req *req;
+	bool found = false;
+
+	spin_lock(&fc->lock);
+	list_for_each_entry(req, &fi->writepages, writepages_entry) {
+		pgoff_t curr_index;
+
+		BUG_ON(req->inode != inode);
+		curr_index = req->misc.write.in.offset >> PAGE_CACHE_SHIFT;
+		if (!(idx_from >= curr_index + req->num_pages ||
+		      idx_to < curr_index)) {
+			found = true;
+			break;
+		}
+	}
+	spin_unlock(&fc->lock);
+
+	return found;
+}
+
 /*
  * Wait for page writeback to be completed.
  *
@@ -358,6 +383,19 @@ static int fuse_wait_on_page_writeback(struct inode *inode, pgoff_t index)
 	return 0;
 }
 
+static void fuse_wait_on_writeback(struct inode *inode, pgoff_t start,
+				   size_t bytes)
+{
+	struct fuse_inode *fi = get_fuse_inode(inode);
+	pgoff_t idx_from, idx_to;
+
+	idx_from = start >> PAGE_CACHE_SHIFT;
+	idx_to = (start + bytes - 1) >> PAGE_CACHE_SHIFT;
+
+	wait_event(fi->page_waitq,
+		   !fuse_range_is_writeback(inode, idx_from, idx_to));
+}
+
 static int fuse_flush(struct file *file, fl_owner_t id)
 {
 	struct inode *inode = file_inode(file);
@@ -2478,8 +2516,15 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 
 	if (lock_inode) {
 		mutex_lock(&inode->i_mutex);
-		if (mode & FALLOC_FL_PUNCH_HOLE)
-			fuse_set_nowrite(inode);
+		if (mode & FALLOC_FL_PUNCH_HOLE) {
+			loff_t endbyte = offset + length - 1;
+			err = filemap_write_and_wait_range(inode->i_mapping,
+							   offset, endbyte);
+			if (err)
+				goto out;
+
+			fuse_wait_on_writeback(inode, offset, length);
+		}
 	}
 
 	req = fuse_get_req_nopages(fc);
@@ -2514,11 +2559,8 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	fuse_invalidate_attr(inode);
 
 out:
-	if (lock_inode) {
-		if (mode & FALLOC_FL_PUNCH_HOLE)
-			fuse_release_nowrite(inode);
+	if (lock_inode)
 		mutex_unlock(&inode->i_mutex);
-	}
 
 	return err;
 }