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>

diff --git a/fs/buffer.c b/fs/buffer.c
index d5ec360..9f6d2e4 100644 (file)
--- 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 9ddc185..f3d96e7 100644 (file)
--- 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 7bf08fa..41514dd 100644 (file)
--- 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;
 }

diff --git a/mm/filemap.c b/mm/filemap.c
index 51efee6..fa5ca30 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1718,6 +1718,7 @@ int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
        struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
        int ret = VM_FAULT_LOCKED;
 
+       sb_start_pagefault(inode->i_sb);
        file_update_time(vma->vm_file);
        lock_page(page);
        if (page->mapping != inode->i_mapping) {
@@ -1725,7 +1726,14 @@ int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
                ret = VM_FAULT_NOPAGE;
                goto out;
        }
+       /*
+        * We mark the page dirty already here so that when freeze is in
+        * progress, we are guaranteed that writeback during freezing will
+        * see the dirty page and writeprotect it again.
+        */
+       set_page_dirty(page);
 out:
+       sb_end_pagefault(inode->i_sb);
        return ret;
 }
 EXPORT_SYMBOL(filemap_page_mkwrite);
@@ -2426,8 +2434,6 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
        count = ocount;
        pos = *ppos;
 
-       vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
-
        /* We can write back this queue in page reclaim */
        current->backing_dev_info = mapping->backing_dev_info;
        written = 0;
@@ -2526,6 +2532,7 @@ ssize_t generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
 
        BUG_ON(iocb->ki_pos != pos);
 
+       sb_start_write(inode->i_sb);
        mutex_lock(&inode->i_mutex);
        blk_start_plug(&plug);
        ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
@@ -2539,6 +2546,7 @@ ssize_t generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
                        ret = err;
        }
        blk_finish_plug(&plug);
+       sb_end_write(inode->i_sb);
        return ret;
 }
 EXPORT_SYMBOL(generic_file_aio_write);

diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index 80b34ef..13e013b 100644 (file)
--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -402,6 +402,8 @@ xip_file_write(struct file *filp, const char __user *buf, size_t len,
        loff_t pos;
        ssize_t ret;
 
+       sb_start_write(inode->i_sb);
+
        mutex_lock(&inode->i_mutex);
 
        if (!access_ok(VERIFY_READ, buf, len)) {
@@ -412,8 +414,6 @@ xip_file_write(struct file *filp, const char __user *buf, size_t len,
        pos = *ppos;
        count = len;
 
-       vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
-
        /* We can write back this queue in page reclaim */
        current->backing_dev_info = mapping->backing_dev_info;
 
@@ -437,6 +437,7 @@ xip_file_write(struct file *filp, const char __user *buf, size_t len,
        current->backing_dev_info = NULL;
  out_up:
        mutex_unlock(&inode->i_mutex);
+       sb_end_write(inode->i_sb);
        return ret;
 }
 EXPORT_SYMBOL_GPL(xip_file_write);