f2fs: do not preallocate blocks which has wrong buffer

Sheng Yong reports needless preallocation if write(small_buffer, large_size)
is called.

In that case, f2fs preallocates large_size, but vfs returns early due to
small_buffer size. Let's detect it before preallocation phase in f2fs.
Reported-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -749,6 +749,9 @@ int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
 	struct f2fs_map_blocks map;
 	int err = 0;
 
+	if (is_inode_flag_set(inode, FI_NO_PREALLOC))
+		return 0;
+
 	map.m_lblk = F2FS_BLK_ALIGN(iocb->ki_pos);
 	map.m_len = F2FS_BYTES_TO_BLK(iocb->ki_pos + iov_iter_count(from));
 	if (map.m_len > map.m_lblk)
@@ -1653,7 +1656,8 @@ static int prepare_write_begin(struct f2fs_sb_info *sbi,
 	 * we already allocated all the blocks, so we don't need to get
 	 * the block addresses when there is no need to fill the page.
 	 */
-	if (!f2fs_has_inline_data(inode) && len == PAGE_SIZE)
+	if (!f2fs_has_inline_data(inode) && len == PAGE_SIZE &&
+			!is_inode_flag_set(inode, FI_NO_PREALLOC))
 		return 0;
 
 	if (f2fs_has_inline_data(inode) ||

fs/f2fs/f2fs.h

@@ -1665,6 +1665,7 @@ enum {
 	FI_INLINE_DOTS,		/* indicate inline dot dentries */
 	FI_DO_DEFRAG,		/* indicate defragment is running */
 	FI_DIRTY_FILE,		/* indicate regular/symlink has dirty pages */
+	FI_NO_PREALLOC,		/* indicate skipped preallocated blocks */
 };
 
 static inline void __mark_inode_dirty_flag(struct inode *inode,

fs/f2fs/file.c

@@ -20,6 +20,7 @@
 #include <linux/uaccess.h>
 #include <linux/mount.h>
 #include <linux/pagevec.h>
+#include <linux/uio.h>
 #include <linux/uuid.h>
 #include <linux/file.h>
 
@@ -2258,8 +2259,12 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	inode_lock(inode);
 	ret = generic_write_checks(iocb, from);
 	if (ret > 0) {
-		int err = f2fs_preallocate_blocks(iocb, from);
+		int err;
 
+		if (iov_iter_fault_in_readable(from, iov_iter_count(from)))
+			set_inode_flag(inode, FI_NO_PREALLOC);
+
+		err = f2fs_preallocate_blocks(iocb, from);
 		if (err) {
 			inode_unlock(inode);
 			return err;
@@ -2267,6 +2272,7 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		blk_start_plug(&plug);
 		ret = __generic_file_write_iter(iocb, from);
 		blk_finish_plug(&plug);
+		clear_inode_flag(inode, FI_NO_PREALLOC);
 	}
 	inode_unlock(inode);