Re: [PATCH] JBD: Fix DIO EIO error caused by race between free buffer and commit trasanction

[Jan Kara]

  Hi,

> This patch fixed a few races between direct IO and kjournald commit
> transaction.  An unexpected EIO error gets returned to direct IO
> caller when it failed to free those data buffers. This could be
> reproduced easily with parallel direct write and buffered write to the
> same file
> 
> More specific, those races could cause journal_try_to_free_buffers()
> fail to free the data buffers, when jbd is committing the transaction
> that has those data buffers on its t_syncdata_list or t_locked_list.
> journal_commit_transaction() still holds the reference to those
> buffers before data reach to disk and buffers are removed from the
> t_syncdata_list of t_locked_list. This prevent the concurrent
> journal_try_to_free_buffers() to free those buffers at the same time,
> but cause EIO error returns back to direct IO.
> 
> With this patch, in case of direct IO and when try_to_free_buffers() failed,
> let's waiting for journal_commit_transaction() to finish
> flushing the current committing transaction's data buffers to disk, 
> then try to free those buffers again.
  If Andrew or Christoph wouldn't beat you for "inventive use" of
gfp_mask, I'm fine with the patch as well ;). You can add
  Acked-by: Jan Kara <jack@xxxxxxx>

> 
> Signed-off-by: Mingming Cao <cmm@xxxxxxxxxx>
> Reviewed-by: Badari Pulavarty <pbadari@xxxxxxxxxx> 
> ---
>  fs/jbd/transaction.c |   57 +++++++++++++++++++++++++++++++++++++++++++++++++--
>  mm/truncate.c        |    3 +-
>  2 files changed, 57 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.26-rc2/fs/jbd/transaction.c
> ===================================================================
> --- linux-2.6.26-rc2.orig/fs/jbd/transaction.c	2008-05-16 11:51:02.000000000 -0700
> +++ linux-2.6.26-rc2/fs/jbd/transaction.c	2008-05-16 13:43:02.000000000 -0700
> @@ -1648,12 +1648,39 @@ out:
>  	return;
>  }
>  
> +/*
> + * journal_try_to_free_buffers() could race with journal_commit_transaction()
> + * The later might still hold the reference count to the buffers when inspecting
> + * them on t_syncdata_list or t_locked_list.
> + *
> + * Journal_try_to_free_buffers() will call this function to
> + * wait for the current transaction to finish syncing data buffers, before
> + * try to free that buffer.
> + *
> + * Called with journal->j_state_lock hold.
> + */
> +static void journal_wait_for_transaction_sync_data(journal_t *journal)
> +{
> +	transaction_t *transaction = NULL;
> +	tid_t tid;
> +
> +	transaction = journal->j_committing_transaction;
> +
> +	if (!transaction)
> +		return;
> +
> +	tid = transaction->t_tid;
> +	spin_unlock(&journal->j_state_lock);
> +	log_wait_commit(journal, tid);
> +	spin_lock(&journal->j_state_lock);
> +}
>  
>  /**
>   * int journal_try_to_free_buffers() - try to free page buffers.
>   * @journal: journal for operation
>   * @page: to try and free
> - * @unused_gfp_mask: unused
> + * @gfp_mask: unused for allocation purpose. Here is used
> + * 	      as a flag to tell if direct IO is attemping to free buffers.
>   *
>   *
>   * For all the buffers on this page,
> @@ -1682,9 +1709,11 @@ out:
>   * journal_try_to_free_buffer() is changing its state.  But that
>   * cannot happen because we never reallocate freed data as metadata
>   * while the data is part of a transaction.  Yes?
> + *
> + * Return 0 on failure, 1 on success
>   */
>  int journal_try_to_free_buffers(journal_t *journal,
> -				struct page *page, gfp_t unused_gfp_mask)
> +				struct page *page, gfp_t gfp_mask)
>  {
>  	struct buffer_head *head;
>  	struct buffer_head *bh;
> @@ -1713,7 +1742,31 @@ int journal_try_to_free_buffers(journal_
>  		if (buffer_jbd(bh))
>  			goto busy;
>  	} while ((bh = bh->b_this_page) != head);
> +
>  	ret = try_to_free_buffers(page);
> +
> + 	/*
> +	 * In the case of concurrent direct IO and buffered IO,
> +	 * There are a number of places where we
> +	 * could race with journal_commit_transaction(), the later still
> +	 * holds the reference to the buffers to free while processing them.
> +	 * try_to_free_buffers() failed to free those buffers,
> +	 * resulting in an unexpected EIO error
> +	 * returns back to the generic_file_direct_IO()
> +	 *
> +	 * So let's wait for the current transaction to finish flush of
> +	 * dirty data buffers before we try to free those buffers
> +	 * again. This wait is needed by direct IO code path only,
> +	 * gfp_mask __GFP_REPEAT is passed from the direct IO code
> +	 * path to flag if we need to wait and retry free buffers.
> +	 */
> +	if (ret == 0 && gfp_mask & __GFP_REPEAT) {
> +        	spin_lock(&journal->j_state_lock);
> +		journal_wait_for_transaction_sync_data(journal);
> +		ret = try_to_free_buffers(page);
> +		spin_unlock(&journal->j_state_lock);
> +	}
> +
>  busy:
>  	return ret;
>  }
> Index: linux-2.6.26-rc2/mm/truncate.c
> ===================================================================
> --- linux-2.6.26-rc2.orig/mm/truncate.c	2008-05-16 11:51:02.000000000 -0700
> +++ linux-2.6.26-rc2/mm/truncate.c	2008-05-16 13:42:18.000000000 -0700
> @@ -346,7 +346,8 @@ invalidate_complete_page2(struct address
>  	if (page->mapping != mapping)
>  		return 0;
>  
> -	if (PagePrivate(page) && !try_to_release_page(page, GFP_KERNEL))
> +	if (PagePrivate(page) &&
> +			!try_to_release_page(page,GFP_KERNEL|__GFP_REPEAT))
>  		return 0;
>  
>  	write_lock_irq(&mapping->tree_lock);
> 
> 
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								Honza
-- 
Jan Kara <jack@xxxxxxx>
SuSE CR Labs
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