[PATCH 06/14] fuse: Trust kernel i_size only - v3

["Maxim V. Patlasov" <MPatlasov@parallels.com>]

Make fuse think that when writeback is on the inode's i_size is always
up-to-date and not update it with the value received from the userspace.
This is done because the page cache code may update i_size without letting
the FS know.

This assumption implies fixing the previously introduced short-read helper --
when a short read occurs the 'hole' is filled with zeroes.

fuse_file_fallocate() is also fixed because now we should keep i_size up to
date, so it must be updated if FUSE_FALLOCATE request succeeded.

Changed in v2:
 - improved comment in fuse_short_read()
 - fixed fuse_file_fallocate() for KEEP_SIZE mode

Changed in v3:
 - fixed fuse_fillattr() not to use local i_size if writeback-cache is off
 - added a comment explaining why we cannot trust attr.size from server

Original patch by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Maxim V. Patlasov <MPatlasov@parallels.com>
---
 fs/fuse/dir.c   |   13 +++++++++++--
 fs/fuse/file.c  |   43 +++++++++++++++++++++++++++++++++++++++++--
 fs/fuse/inode.c |   11 +++++++++--
 3 files changed, 61 insertions(+), 6 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 8506522..8672ee4 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -845,6 +845,11 @@ static void fuse_fillattr(struct inode *inode, struct fuse_attr *attr,
 			  struct kstat *stat)
 {
 	unsigned int blkbits;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+
+	/* see the comment in fuse_change_attributes() */
+	if (fc->writeback_cache && S_ISREG(inode->i_mode))
+		attr->size = i_size_read(inode);
 
 	stat->dev = inode->i_sb->s_dev;
 	stat->ino = attr->ino;
@@ -1571,6 +1576,7 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
 	struct fuse_setattr_in inarg;
 	struct fuse_attr_out outarg;
 	bool is_truncate = false;
+	bool is_wb = fc->writeback_cache;
 	loff_t oldsize;
 	int err;
 
@@ -1643,7 +1649,9 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
 	fuse_change_attributes_common(inode, &outarg.attr,
 				      attr_timeout(&outarg));
 	oldsize = inode->i_size;
-	i_size_write(inode, outarg.attr.size);
+	/* see the comment in fuse_change_attributes() */
+	if (!is_wb || is_truncate || !S_ISREG(inode->i_mode))
+		i_size_write(inode, outarg.attr.size);
 
 	if (is_truncate) {
 		/* NOTE: this may release/reacquire fc->lock */
@@ -1655,7 +1663,8 @@ static int fuse_do_setattr(struct dentry *entry, struct iattr *attr,
 	 * Only call invalidate_inode_pages2() after removing
 	 * FUSE_NOWRITE, otherwise fuse_launder_page() would deadlock.
 	 */
-	if (S_ISREG(inode->i_mode) && oldsize != outarg.attr.size) {
+	if ((is_truncate || !is_wb) &&
+			S_ISREG(inode->i_mode) && oldsize != outarg.attr.size) {
 		truncate_pagecache(inode, oldsize, outarg.attr.size);
 		invalidate_inode_pages2(inode->i_mapping);
 	}
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index ee44b24..af58bbf 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -15,6 +15,7 @@
 #include <linux/module.h>
 #include <linux/compat.h>
 #include <linux/swap.h>
+#include <linux/falloc.h>
 
 static const struct file_operations fuse_direct_io_file_operations;
 
@@ -543,9 +544,31 @@ static void fuse_short_read(struct fuse_req *req, struct inode *inode,
 			    u64 attr_ver)
 {
 	size_t num_read = req->out.args[0].size;
+	struct fuse_conn *fc = get_fuse_conn(inode);
+
+	if (fc->writeback_cache) {
+		/*
+		 * A hole in a file. Some data after the hole are in page cache,
+		 * but have not reached the client fs yet. So, the hole is not
+		 * present there.
+		 */
+		int i;
+		int start_idx = num_read >> PAGE_CACHE_SHIFT;
+		size_t off = num_read & (PAGE_CACHE_SIZE - 1);
 
-	loff_t pos = page_offset(req->pages[0]) + num_read;
-	fuse_read_update_size(inode, pos, attr_ver);
+		for (i = start_idx; i < req->num_pages; i++) {
+			struct page *page = req->pages[i];
+			void *mapaddr = kmap_atomic(page);
+
+			memset(mapaddr + off, 0, PAGE_CACHE_SIZE - off);
+
+			kunmap_atomic(mapaddr);
+			off = 0;
+		}
+	} else {
+		loff_t pos = page_offset(req->pages[0]) + num_read;
+		fuse_read_update_size(inode, pos, attr_ver);
+	}
 }
 
 static int fuse_readpage(struct file *file, struct page *page)
@@ -2286,6 +2309,8 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		.mode = mode
 	};
 	int err;
+	bool change_i_size = fc->writeback_cache &&
+		!(mode & FALLOC_FL_KEEP_SIZE);
 
 	if (fc->no_fallocate)
 		return -EOPNOTSUPP;
@@ -2294,6 +2319,11 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	if (IS_ERR(req))
 		return PTR_ERR(req);
 
+	if (change_i_size) {
+		struct inode *inode = file->f_mapping->host;
+		mutex_lock(&inode->i_mutex);
+	}
+
 	req->in.h.opcode = FUSE_FALLOCATE;
 	req->in.h.nodeid = ff->nodeid;
 	req->in.numargs = 1;
@@ -2307,6 +2337,15 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	}
 	fuse_put_request(fc, req);
 
+	if (change_i_size) {
+		struct inode *inode = file->f_mapping->host;
+
+		if (!err)
+			fuse_write_update_size(inode, offset + length);
+
+		mutex_unlock(&inode->i_mutex);
+	}
+
 	return err;
 }
 
diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index 01353ed..94319e6 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -197,6 +197,7 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
 {
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	struct fuse_inode *fi = get_fuse_inode(inode);
+	bool is_wb = fc->writeback_cache;
 	loff_t oldsize;
 	struct timespec old_mtime;
 
@@ -210,10 +211,16 @@ void fuse_change_attributes(struct inode *inode, struct fuse_attr *attr,
 	fuse_change_attributes_common(inode, attr, attr_valid);
 
 	oldsize = inode->i_size;
-	i_size_write(inode, attr->size);
+        /*
+	 * In case of writeback_cache enabled, the cached writes beyond EOF
+	 * extend local i_size without keeping userspace server in sync. So,
+	 * attr->size coming from server can be stale. We cannot trust it.
+	 */
+	if (!is_wb || !S_ISREG(inode->i_mode))
+		i_size_write(inode, attr->size);
 	spin_unlock(&fc->lock);
 
-	if (S_ISREG(inode->i_mode)) {
+	if (!is_wb && S_ISREG(inode->i_mode)) {
 		bool inval = false;
 
 		if (oldsize != attr->size) {