Message-ID: <20090331044544.GB5979@skywalker>
Date:	Tue, 31 Mar 2009 10:15:44 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ext3: Avoid false EIO errors

On Mon, Mar 30, 2009 at 06:05:17PM +0200, Jan Kara wrote:
> On Mon 30-03-09 16:28:21, Aneesh Kumar K.V wrote:
> > On Mon, Mar 30, 2009 at 12:32:48PM +0200, Jan Kara wrote:
> > > > > > > -				struct address_space *mapping,
> > > > > > > -				loff_t pos, unsigned len, unsigned copied,
> > > > > > > -				struct page *page, void *fsdata)
> > > > > > > +static void update_file_sizes(struct inode *inode, loff_t pos, unsigned len,
> > > > > > > +			      unsigned copied)
> > > > > > >  {
> > > > > > > -	struct inode *inode = file->f_mapping->host;
> > > > > > > -
> > > > > > > -	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
> > > > > > > +	int mark_dirty = 0;
> > > > > > > 
> > > > > > > -	if (pos+copied > inode->i_size) {
> > > > > > > -		i_size_write(inode, pos+copied);
> > > > > > > -		mark_inode_dirty(inode);
> > > > > > > +	if (pos + len > EXT3_I(inode)->i_disksize) {
> > > > > > > +		mark_dirty = 1;
> > > > > > > +		EXT3_I(inode)->i_disksize = pos + len;
> > > > > > >  	}
> > > > > > 
> > > > > > Won't this result in file having wrong contents if we failed to copy
> > > > > > the contents from user space? Now if we successfully allocated
> > > > > > blocks and we failed to copy the contents from user space, the above
> > > > > > would result in update of i_disksize and a mark_inode_dirty. Doesn't
> > > > > > that imply we have wrong contents in those block for which we failed to
> > > > > > copy the contents from user space ?
> > > > >   block_write_end() zeros all new buffers. So yes, if we crash after
> > > > > this transaction commits but before we manage to redo the write, then user
> > > > > could see zeros at the end of file (previously inode could have blocks
> > > > > allocated beyond EOF).
> > > > >   I was also thinking about truncating the newly created buffers but it's a
> > > > > bit tricky. We need to do it in the same transaction (otherwise the race
> > > > > would be still there) but standard truncate path would like to add inode
> > > > > to the orphan list, lock pages etc and we have no credits for that and also
> > > > > lock ordering might be troublesome. So I've chosen the simple path.
> > > > > 
> > > > We do a vmtruncate if we failed to allocate blocks in
> > > > ext3_write_begin. That is done after the closing the current
> > > > transaction. If we crash in between (ie, after committing the
> > > > transaction allocating blocks and before committing the transaction that
> > > > is doing truncate) we would only have  some data blocks leaking. But
> > > > that would be better than user seeing zero's in the file ?. Also if we
> > > > happen to add the inode to the orphan list and crash, the recovery would
> > > > truncate it properly. So by doing a vmtruncate I guess the window would be
> > > > small and we are already doing that in ext3_write_begin.
> > >   Hmm, are you sure some assertion would not fire if we find allocated
> > > blocks beyond i_size (which could happen with the old code)? Frankly, I
> > > prefer user seeing zeros at the end of file (so that he can come and yell
> > > at me ;) rather than silently leaking blocks, getting inode into an
> > > unexpected state and then debug some mysterious problem. But hopefully this
> > > problem has a solution which can make both of us happy ;): We can reserve
> > > enough credits (actually just one block more) and when we see we need to
> > > do truncate because of failed write, we first add inode to the orphan list
> > > before stopping the current handle (so that if we crash it gets properly
> > > truncated) and then truncate the blocks in a separate transaction. Does it
> > > sound good to you?
> > 
> > Yes. We also should unlock the page before the truncate
>   OK, below is improved patch that adds inode to orphan list before
> stopping the current handle.
> 
> 									Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
> 
> From 8f02ffb17a23c52ec980800fdccf0fa11d96f2a7 Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@...e.cz>
> Date: Wed, 25 Mar 2009 18:51:52 +0100
> Subject: [PATCH] ext3: Avoid false EIO errors
> 
> Sometimes block_write_begin() can map buffers in a page but later we fail to
> copy data into those buffers (because the source page has been paged out in the
> mean time). We then end up with !uptodate mapped buffers. To add a bit more to
> the confusion, block_write_end() does not commit any data (and thus does not
> any mark buffers as uptodate) if we didn't succeed with copying all the data.
> 
> Commit f4fc66a894546bdc88a775d0e83ad20a65210bcb (ext3: convert to new aops)
> missed these cases and thus we were inserting non-uptodate buffers to
> transaction's list which confuses JBD code and it reports IO errors, aborts
> a transaction and generally makes users afraid about their data ;-P.
> 
> This patch fixes the problem by reorganizing ext3_..._write_end() code to
> first call block_write_end() to mark buffers with valid data uptodate and
> after that we file only uptodate buffers to transaction's lists.
> 
> We also fix a problem where we could leave blocks allocated beyond i_size
> (i_disksize in fact) because of failed write. We now add inode to orphan
> list when write fails (to be safe in case we crash) and then truncate blocks
> beyond i_size in a separate transaction.
> 
> Signed-off-by: Jan Kara <jack@...e.cz>

ext4 would need the orphan_add and truncate changes. 

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>

> ---
>  fs/ext3/inode.c |  123 +++++++++++++++++++++++++++----------------------------
>  1 files changed, 61 insertions(+), 62 deletions(-)
> 
> diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> index 4a09ff1..40eb569 100644
> --- a/fs/ext3/inode.c
> +++ b/fs/ext3/inode.c
> @@ -1149,12 +1149,15 @@ static int ext3_write_begin(struct file *file, struct address_space *mapping,
>  				struct page **pagep, void **fsdata)
>  {
>  	struct inode *inode = mapping->host;
> -	int ret, needed_blocks = ext3_writepage_trans_blocks(inode);
> +	int ret;
>  	handle_t *handle;
>  	int retries = 0;
>  	struct page *page;
>  	pgoff_t index;
>  	unsigned from, to;
> +	/* Reserve one block more for addition to orphan list in case
> +	 * we allocate blocks but write fails for some reason */
> +	int needed_blocks = ext3_writepage_trans_blocks(inode) + 1;
> 
>  	index = pos >> PAGE_CACHE_SHIFT;
>  	from = pos & (PAGE_CACHE_SIZE - 1);
> @@ -1184,15 +1187,20 @@ retry:
>  	}
>  write_begin_failed:
>  	if (ret) {
> -		ext3_journal_stop(handle);
> -		unlock_page(page);
> -		page_cache_release(page);
>  		/*
>  		 * block_write_begin may have instantiated a few blocks
>  		 * outside i_size.  Trim these off again. Don't need
>  		 * i_size_read because we hold i_mutex.
> +		 *
> +		 * Add inode to orphan list in case we crash before truncate
> +		 * finishes.
>  		 */
>  		if (pos + len > inode->i_size)
> +			ext3_orphan_add(handle, inode);
> +		ext3_journal_stop(handle);
> +		unlock_page(page);
> +		page_cache_release(page);
> +		if (pos + len > inode->i_size)
>  			vmtruncate(inode, inode->i_size);
>  	}
>  	if (ret == -ENOSPC && ext3_should_retry_alloc(inode->i_sb, &retries))
> @@ -1211,6 +1219,18 @@ int ext3_journal_dirty_data(handle_t *handle, struct buffer_head *bh)
>  	return err;
>  }
> 
> +/* For ordered writepage and write_end functions */
> +static int journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
> +{
> +	/*
> +	 * Write could have mapped the buffer but it didn't copy the data in
> +	 * yet. So avoid filing such buffer into a transaction.
> +	 */
> +	if (buffer_mapped(bh) && buffer_uptodate(bh))
> +		return ext3_journal_dirty_data(handle, bh);
> +	return 0;
> +}
> +
>  /* For write_end() in data=journal mode */
>  static int write_end_fn(handle_t *handle, struct buffer_head *bh)
>  {
> @@ -1221,26 +1241,20 @@ static int write_end_fn(handle_t *handle, struct buffer_head *bh)
>  }
> 
>  /*
> - * Generic write_end handler for ordered and writeback ext3 journal modes.
> - * We can't use generic_write_end, because that unlocks the page and we need to
> - * unlock the page after ext3_journal_stop, but ext3_journal_stop must run
> - * after block_write_end.
> + * This is nasty and subtle: ext3_write_begin() could have allocated blocks
> + * for the whole page but later we failed to copy the data in. Update inode
> + * size according to what we managed to copy. The rest is going to be
> + * truncated in write_end function.
>   */
> -static int ext3_generic_write_end(struct file *file,
> -				struct address_space *mapping,
> -				loff_t pos, unsigned len, unsigned copied,
> -				struct page *page, void *fsdata)
> +static void update_file_sizes(struct inode *inode, loff_t pos, unsigned copied)
>  {
> -	struct inode *inode = file->f_mapping->host;
> -
> -	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
> -
> -	if (pos+copied > inode->i_size) {
> -		i_size_write(inode, pos+copied);
> +	/* What matters to us is i_disksize. We don't write i_size anywhere */
> +	if (pos + copied > inode->i_size)
> +		i_size_write(inode, pos + copied);
> +	if (pos + copied > EXT3_I(inode)->i_disksize) {
> +		EXT3_I(inode)->i_disksize = pos + copied;
>  		mark_inode_dirty(inode);
>  	}
> -
> -	return copied;
>  }
> 
>  /*
> @@ -1260,35 +1274,29 @@ static int ext3_ordered_write_end(struct file *file,
>  	unsigned from, to;
>  	int ret = 0, ret2;
> 
> -	from = pos & (PAGE_CACHE_SIZE - 1);
> -	to = from + len;
> +	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
> 
> +	from = pos & (PAGE_CACHE_SIZE - 1);
> +	to = from + copied;
>  	ret = walk_page_buffers(handle, page_buffers(page),
> -		from, to, NULL, ext3_journal_dirty_data);
> +		from, to, NULL, journal_dirty_data_fn);
> 
> -	if (ret == 0) {
> -		/*
> -		 * generic_write_end() will run mark_inode_dirty() if i_size
> -		 * changes.  So let's piggyback the i_disksize mark_inode_dirty
> -		 * into that.
> -		 */
> -		loff_t new_i_size;
> -
> -		new_i_size = pos + copied;
> -		if (new_i_size > EXT3_I(inode)->i_disksize)
> -			EXT3_I(inode)->i_disksize = new_i_size;
> -		ret2 = ext3_generic_write_end(file, mapping, pos, len, copied,
> -							page, fsdata);
> -		copied = ret2;
> -		if (ret2 < 0)
> -			ret = ret2;
> -	}
> +	if (ret == 0)
> +		update_file_sizes(inode, pos, copied);
> +	/*
> +	 * There may be allocated blocks outside of i_size because
> +	 * we failed to copy some data. Prepare for truncate.
> +	 */
> +	if (pos + len > inode->i_size)
> +		ext3_orphan_add(handle, inode);
>  	ret2 = ext3_journal_stop(handle);
>  	if (!ret)
>  		ret = ret2;
>  	unlock_page(page);
>  	page_cache_release(page);
> 
> +	if (pos + len > inode->i_size)
> +		vmtruncate(inode, inode->i_size);
>  	return ret ? ret : copied;
>  }
> 
> @@ -1299,25 +1307,22 @@ static int ext3_writeback_write_end(struct file *file,
>  {
>  	handle_t *handle = ext3_journal_current_handle();
>  	struct inode *inode = file->f_mapping->host;
> -	int ret = 0, ret2;
> -	loff_t new_i_size;
> -
> -	new_i_size = pos + copied;
> -	if (new_i_size > EXT3_I(inode)->i_disksize)
> -		EXT3_I(inode)->i_disksize = new_i_size;
> -
> -	ret2 = ext3_generic_write_end(file, mapping, pos, len, copied,
> -							page, fsdata);
> -	copied = ret2;
> -	if (ret2 < 0)
> -		ret = ret2;
> +	int ret;
> 
> -	ret2 = ext3_journal_stop(handle);
> -	if (!ret)
> -		ret = ret2;
> +	copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);
> +	update_file_sizes(inode, pos, copied);
> +	/*
> +	 * There may be allocated blocks outside of i_size because
> +	 * we failed to copy some data. Prepare for truncate.
> +	 */
> +	if (pos + len > inode->i_size)
> +		ext3_orphan_add(handle, inode);
> +	ret = ext3_journal_stop(handle);
>  	unlock_page(page);
>  	page_cache_release(page);
> 
> +	if (pos + len > inode->i_size)
> +		vmtruncate(inode, inode->i_size);
>  	return ret ? ret : copied;
>  }
> 
> @@ -1428,17 +1433,11 @@ static int bput_one(handle_t *handle, struct buffer_head *bh)
>  	return 0;
>  }
> 
> -static int journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
> -{
> -	if (buffer_mapped(bh))
> -		return ext3_journal_dirty_data(handle, bh);
> -	return 0;
> -}
> -
>  static int buffer_unmapped(handle_t *handle, struct buffer_head *bh)
>  {
>  	return !buffer_mapped(bh);
>  }
> +
>  /*
>   * Note that we always start a transaction even if we're not journalling
>   * data.  This is to preserve ordering: any hole instantiation within
> -- 
> 1.6.0.2
> 
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Powered by blists - more mailing lists