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

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Date:	Mon, 6 Feb 2012 16:33:12 +0100
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...deen.net>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	Dave Chinner <dchinner@...hat.com>,
	Surbhi Palande <csurbhi@...il.com>,
	Kamal Mostafa <kamal@...onical.com>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, xfs@....sgi.com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/8] vfs: Protect write paths by sb_start_write -
 sb_end_write

On Sun 05-02-12 00:13:38, Eric Sandeen wrote:
> 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?
  It was not lost but it got moved down into the filesystem. I forgot to
update the changelog. But after Dave's comments I think it can go back into
VFS. Just lockdep complained about deadlocks in my first naive approach -
that's why I started doing weird things with XFS locks after all.

  Anyway now I'm wiser regarding XFS locking and I also have better idea
how to achive proper lock ordering in VFS. Just we are finishing SLE11 SP2
so I didn't get to writing the patches last week... But I should get to it
maybe even today and if not then at least during this week ;)

									Honza

> > Acked-by: "Theodore Ts'o" <tytso@....edu>
> > Signed-off-by: Jan Kara <jack@...e.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;
> >  }
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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