Re: [PATCH 2/2] ext4: Fix deadlock in journal_unmap_buffer()

[Theodore Ts'o <tytso@mit.edu>]

On Wed, Dec 12, 2012 at 09:50:16PM +0100, Jan Kara wrote:
> We cannot wait for transaction commit in journal_unmap_buffer() because we hold
> page lock which ranks below transaction start. We solve the issue by bailing
> out of journal_unmap_buffer() and jbd2_journal_invalidatepage() with -EBUSY.

ocfs2 also calls jbd2_journal_invalidatepage().  I would think we
would need to apply a similar fix up to ocfs2, lest they end up having
jbd2_journal_invalidatepage() silently failing for them.

I'm going to hold off on this patch until we're sure it's not going to
cause problems for ocfs2.  A quick check indicates that they also
support sub-page block sizes, which would tend to indicate that they
could get hit by the same dead lock, yes?

						- Ted

> Caller is then responsible for waiting for transaction commit to finish and try
> invalidation again. Since the issue can happen only for page stradding i_size,
> it is simple enough to manually call jbd2_journal_invalidatepage() for such
> page from ext4_setattr(), check the return value and wait if necessary.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/inode.c             |   82 +++++++++++++++++++++++++++++++++++++------
>  fs/jbd2/transaction.c       |   27 +++++++-------
>  include/linux/jbd2.h        |    2 +-
>  include/trace/events/ext4.h |    4 +-
>  4 files changed, 88 insertions(+), 27 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 66abac7..cc0abeb 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2813,8 +2813,8 @@ static void ext4_invalidatepage(struct page *page, unsigned long offset)
>  	block_invalidatepage(page, offset);
>  }
>  
> -static void ext4_journalled_invalidatepage(struct page *page,
> -					   unsigned long offset)
> +static int __ext4_journalled_invalidatepage(struct page *page,
> +					    unsigned long offset)
>  {
>  	journal_t *journal = EXT4_JOURNAL(page->mapping->host);
>  
> @@ -2826,7 +2826,14 @@ static void ext4_journalled_invalidatepage(struct page *page,
>  	if (offset == 0)
>  		ClearPageChecked(page);
>  
> -	jbd2_journal_invalidatepage(journal, page, offset);
> +	return jbd2_journal_invalidatepage(journal, page, offset);
> +}
> +
> +/* Wrapper for aops... */
> +static void ext4_journalled_invalidatepage(struct page *page,
> +					   unsigned long offset)
> +{
> +	WARN_ON(__ext4_journalled_invalidatepage(page, offset) < 0);
>  }
>  
>  static int ext4_releasepage(struct page *page, gfp_t wait)
> @@ -4230,6 +4237,47 @@ int ext4_write_inode(struct inode *inode, struct writeback_control *wbc)
>  }
>  
>  /*
> + * In data=journal mode ext4_journalled_invalidatepage() may fail to invalidate
> + * buffers that are attached to a page stradding i_size and are undergoing
> + * commit. In that case we have to wait for commit to finish and try again.
> + */
> +static void ext4_wait_for_tail_page_commit(struct inode *inode)
> +{
> +	struct page *page;
> +	unsigned offset;
> +	journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
> +	tid_t commit_tid = 0;
> +	int ret;
> +
> +	offset = inode->i_size & (PAGE_CACHE_SIZE - 1);
> +	/*
> +	 * All buffers in the last page remain valid? Then there's nothing to
> +	 * do. We do the check mainly to optimize the common PAGE_CACHE_SIZE ==
> +	 * blocksize case
> +	 */
> +	if (offset > PAGE_CACHE_SIZE - (1 << inode->i_blkbits))
> +		return;
> +	while (1) {
> +		page = find_lock_page(inode->i_mapping,
> +				      inode->i_size >> PAGE_CACHE_SHIFT);
> +		if (!page)
> +			return;
> +		ret = __ext4_journalled_invalidatepage(page, offset);
> +		unlock_page(page);
> +		page_cache_release(page);
> +		if (ret != -EBUSY)
> +			return;
> +		commit_tid = 0;
> +		read_lock(&journal->j_state_lock);
> +		if (journal->j_committing_transaction)
> +			commit_tid = journal->j_committing_transaction->t_tid;
> +		read_unlock(&journal->j_state_lock);
> +		if (commit_tid)
> +			jbd2_log_wait_commit(journal, commit_tid);
> +	}
> +}
> +
> +/*
>   * ext4_setattr()
>   *
>   * Called from notify_change.
> @@ -4342,16 +4390,28 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
>  	}
>  
>  	if (attr->ia_valid & ATTR_SIZE) {
> -		if (attr->ia_size != i_size_read(inode)) {
> -			truncate_setsize(inode, attr->ia_size);
> -			/* Inode size will be reduced, wait for dio in flight.
> -			 * Temporarily disable dioread_nolock to prevent
> -			 * livelock. */
> +		if (attr->ia_size != inode->i_size) {
> +			loff_t oldsize = inode->i_size;
> +
> +			i_size_write(inode, attr->ia_size);
> +			/*
> +			 * Blocks are going to be removed from the inode. Wait
> +			 * for dio in flight.  Temporarily disable
> +			 * dioread_nolock to prevent livelock.
> +			 */
>  			if (orphan) {
> -				ext4_inode_block_unlocked_dio(inode);
> -				inode_dio_wait(inode);
> -				ext4_inode_resume_unlocked_dio(inode);
> +				if (!ext4_should_journal_data(inode)) {
> +					ext4_inode_block_unlocked_dio(inode);
> +					inode_dio_wait(inode);
> +					ext4_inode_resume_unlocked_dio(inode);
> +				} else
> +					ext4_wait_for_tail_page_commit(inode);
>  			}
> +			/*
> +			 * Truncate pagecache after we've waited for commit
> +			 * in data=journal mode to make pages freeable.
> +			 */
> +			truncate_pagecache(inode, oldsize, inode->i_size);
>  		}
>  		ext4_truncate(inode);
>  	}
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index a74ba46..e1475b4 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -1850,7 +1850,6 @@ static int journal_unmap_buffer(journal_t *journal, struct buffer_head *bh,
>  
>  	BUFFER_TRACE(bh, "entry");
>  
> -retry:
>  	/*
>  	 * It is safe to proceed here without the j_list_lock because the
>  	 * buffers cannot be stolen by try_to_free_buffers as long as we are
> @@ -1945,14 +1944,11 @@ retry:
>  		 * for commit and try again.
>  		 */
>  		if (partial_page) {
> -			tid_t tid = journal->j_committing_transaction->t_tid;
> -
>  			jbd2_journal_put_journal_head(jh);
>  			spin_unlock(&journal->j_list_lock);
>  			jbd_unlock_bh_state(bh);
>  			write_unlock(&journal->j_state_lock);
> -			jbd2_log_wait_commit(journal, tid);
> -			goto retry;
> +			return -EBUSY;
>  		}
>  		/*
>  		 * OK, buffer won't be reachable after truncate. We just set
> @@ -2013,21 +2009,23 @@ zap_buffer_unlocked:
>   * @page:    page to flush
>   * @offset:  length of page to invalidate.
>   *
> - * Reap page buffers containing data after offset in page.
> - *
> + * Reap page buffers containing data after offset in page. Can return -EBUSY
> + * if buffers are part of the committing transaction and the page is straddling
> + * i_size. Caller then has to wait for current commit and try again.
>   */
> -void jbd2_journal_invalidatepage(journal_t *journal,
> -		      struct page *page,
> -		      unsigned long offset)
> +int jbd2_journal_invalidatepage(journal_t *journal,
> +				struct page *page,
> +				unsigned long offset)
>  {
>  	struct buffer_head *head, *bh, *next;
>  	unsigned int curr_off = 0;
>  	int may_free = 1;
> +	int ret = 0;
>  
>  	if (!PageLocked(page))
>  		BUG();
>  	if (!page_has_buffers(page))
> -		return;
> +		return 0;
>  
>  	/* We will potentially be playing with lists other than just the
>  	 * data lists (especially for journaled data mode), so be
> @@ -2041,9 +2039,11 @@ void jbd2_journal_invalidatepage(journal_t *journal,
>  		if (offset <= curr_off) {
>  			/* This block is wholly outside the truncation point */
>  			lock_buffer(bh);
> -			may_free &= journal_unmap_buffer(journal, bh,
> -							 offset > 0);
> +			ret = journal_unmap_buffer(journal, bh, offset > 0);
>  			unlock_buffer(bh);
> +			if (ret < 0)
> +				return ret;
> +			may_free &= ret;
>  		}
>  		curr_off = next_off;
>  		bh = next;
> @@ -2054,6 +2054,7 @@ void jbd2_journal_invalidatepage(journal_t *journal,
>  		if (may_free && try_to_free_buffers(page))
>  			J_ASSERT(!page_has_buffers(page));
>  	}
> +	return 0;
>  }
>  
>  /*
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 3efc43f..cb7d6af 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -1099,7 +1099,7 @@ extern int	 jbd2_journal_dirty_metadata (handle_t *, struct buffer_head *);
>  extern void	 jbd2_journal_release_buffer (handle_t *, struct buffer_head *);
>  extern int	 jbd2_journal_forget (handle_t *, struct buffer_head *);
>  extern void	 journal_sync_buffer (struct buffer_head *);
> -extern void	 jbd2_journal_invalidatepage(journal_t *,
> +extern int	 jbd2_journal_invalidatepage(journal_t *,
>  				struct page *, unsigned long);
>  extern int	 jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t);
>  extern int	 jbd2_journal_stop(handle_t *);
> diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
> index df972d9..3ef522e 100644
> --- a/include/trace/events/ext4.h
> +++ b/include/trace/events/ext4.h
> @@ -476,13 +476,13 @@ DECLARE_EVENT_CLASS(ext4_invalidatepage_op,
>  		  (unsigned long) __entry->index, __entry->offset)
>  );
>  
> -DEFINE_EVENT(ext4_invalidatepage_op, ext4_invalidatepage
> +DEFINE_EVENT(ext4_invalidatepage_op, ext4_invalidatepage,
>  	TP_PROTO(struct page *page, unsigned long offset),
>  
>  	TP_ARGS(page, offset)
>  );
>  
> -DEFINE_EVENT(ext4_invalidatepage_op, ext4_journalled_invalidatepage
> +DEFINE_EVENT(ext4_invalidatepage_op, ext4_journalled_invalidatepage,
>  	TP_PROTO(struct page *page, unsigned long offset),
>  
>  	TP_ARGS(page, offset)
> -- 
> 1.7.1
>