Re: [PATCH 2/8] vfs: Protect write paths by sb_start_write - sb_end_write

Eric Sandeen
2012-02-05T06:13:40Z

On 1/20/12 2:34 PM, Jan Kara wrote:
> There are three entry points which dirty pages in a filesystem. mmap (handled
> by block_page_mkwrite()), buffered write (handled by
> __generic_file_aio_write()), and truncate (it can dirty last partial page -
> handled inside each filesystem separately). Protect these places with
> sb_start_write() and sb_end_write().

The protection for truncate got lost since the first patchset, was that
on purpose?

-Eric

> Acked-by: "Theodore Ts'o" <tytso@mit.edu>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/buffer.c | 22 ++++------------------
> mm/filemap.c | 3 ++-
> 2 files changed, 6 insertions(+), 19 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 19d8eb7..550714d 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2338,8 +2338,8 @@ 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)
> @@ -2371,18 +2371,7 @@ 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:
> @@ -2397,12 +2386,9 @@ 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_write(sb, SB_FREEZE_WRITE);
> ret = __block_page_mkwrite(vma, vmf, get_block);
> + sb_end_write(sb, SB_FREEZE_WRITE);
> return block_page_mkwrite_return(ret);
> }
> EXPORT_SYMBOL(block_page_mkwrite);
> diff --git a/mm/filemap.c b/mm/filemap.c
> index c0018f2..471b9ae 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2529,7 +2529,7 @@ 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);
> + sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
>
> /* We can write back this queue in page reclaim */
> current->backing_dev_info = mapping->backing_dev_info;
> @@ -2601,6 +2601,7 @@ ssize_t __generic_file_aio_write(struct kiocb *iocb, const struct iovec *iov,
> pos, ppos, count, written);
> }
> out:
> + sb_end_write(inode->i_sb, SB_FREEZE_WRITE);
> current->backing_dev_info = NULL;
> return written ? written : err;
> }