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

[Mingming Cao]

On Mon, 2008-05-19 at 00:37 +0200, Jan Kara wrote:
>   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>
> 

This is less intrusive way to fix this problem. The gfp_mask was marked
as unused in try_to_free_page(). I looked at filesystems in the kernel,
there is only a few defined releasepage() callback, and only xfs checks
the flag(but not used). btrfs is actually using it though. I thought
about the way you have suggested, i.e.clean up this gfp_mask and and
replace with a flag.  I am not entirely sure if it we need to change the
address_space_operations and fix all the filesystems for this matter.

Andrew, what do you think? Is this approach acceptable? 


Thanks and regards,
Mingming
> > 
> > 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

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