fs: Protect write paths by sb_start_write - sb_end_write

There are several entry points which dirty pages in a filesystem.  mmap
(handled by block_page_mkwrite()), buffered write (handled by
__generic_file_aio_write()), splice write (generic_file_splice_write),
truncate, and fallocate (these can dirty last partial page - handled inside
each filesystem separately). Protect these places with sb_start_write() and
sb_end_write().

->page_mkwrite() calls are particularly complex since they are called with
mmap_sem held and thus we cannot use standard sb_start_write() due to lock
ordering constraints. We solve the problem by using a special freeze protection
sb_start_pagefault() which ranks below mmap_sem.

BugLink: https://bugs.launchpad.net/bugs/897421
Tested-by: Kamal Mostafa <kamal@canonical.com>
Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com>
Tested-by: Dann Frazier <dann.frazier@canonical.com>
Tested-by: Massimo Morana <massimo.morana@canonical.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 13 insertions, 19 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index d5ec360e332d..9f6d2e41281d 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2306,8 +2306,8 @@ EXPORT_SYMBOL(block_commit_write);
  * beyond EOF, then the page is guaranteed safe against truncation until we
  * unlock the page.
  *
- * Direct callers of this function should call vfs_check_frozen() so that page
- * fault does not busyloop until the fs is thawed.
+ * Direct callers of this function should protect against filesystem freezing
+ * using sb_start_write() - sb_end_write() functions.
  */
 int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                          get_block_t get_block)
@@ -2345,18 +2345,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 
         if (unlikely(ret < 0))
                 goto out_unlock;
-        /*
-         * Freezing in progress? We check after the page is marked dirty and
-         * with page lock held so if the test here fails, we are sure freezing
-         * code will wait during syncing until the page fault is done - at that
-         * point page will be dirty and unlocked so freezing code will write it
-         * and writeprotect it again.
-         */
         set_page_dirty(page);
-        if (inode->i_sb->s_frozen != SB_UNFROZEN) {
-                ret = -EAGAIN;
-                goto out_unlock;
-        }
         wait_on_page_writeback(page);
         return 0;
 out_unlock:
@@ -2371,12 +2360,9 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
         int ret;
         struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;
 
-        /*
-         * This check is racy but catches the common case. The check in
-         * __block_page_mkwrite() is reliable.
-         */
-        vfs_check_frozen(sb, SB_FREEZE_WRITE);
+        sb_start_pagefault(sb);
         ret = __block_page_mkwrite(vma, vmf, get_block);
+        sb_end_pagefault(sb);
         return block_page_mkwrite_return(ret);
 }
 EXPORT_SYMBOL(block_page_mkwrite);
diff --git a/fs/open.c b/fs/open.c
index 9ddc18565503..f3d96e7e7b19 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -164,11 +164,13 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
         if (IS_APPEND(inode))
                 goto out_putf;
 
+        sb_start_write(inode->i_sb);
         error = locks_verify_truncate(inode, file, length);
         if (!error)
                 error = security_path_truncate(&file->f_path);
         if (!error)
                 error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, file);
+        sb_end_write(inode->i_sb);
 out_putf:
         fput(file);
 out:
@@ -266,7 +268,10 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
         if (!file->f_op->fallocate)
                 return -EOPNOTSUPP;
 
-        return file->f_op->fallocate(file, mode, offset, len);
+        sb_start_write(inode->i_sb);
+        ret = file->f_op->fallocate(file, mode, offset, len);
+        sb_end_write(inode->i_sb);
+        return ret;
 }
 
 SYSCALL_DEFINE(fallocate)(int fd, int mode, loff_t offset, loff_t len)
diff --git a/fs/splice.c b/fs/splice.c
index 7bf08fa22ec9..41514dd89462 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -996,6 +996,8 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
         };
         ssize_t ret;
 
+        sb_start_write(inode->i_sb);
+
         pipe_lock(pipe);
 
         splice_from_pipe_begin(&sd);
@@ -1034,6 +1036,7 @@ generic_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
                         *ppos += ret;
                 balance_dirty_pages_ratelimited_nr(mapping, nr_pages);
         }
+        sb_end_write(inode->i_sb);
 
         return ret;
 }