Subject: Re: linux-next: manual merge of the block tree with the ext4 tree

From: Jeremiah Mahler <[email-masked]>
Date: Sat, 29 Nov 2014 14:08:43 -0800

Hi,

On Sat, Nov 29, 2014 at 11:08:33AM +0100, Sabrina Dubroca wrote:
> Hello,
> 
> [adding Jeremiah Mahler to CC]
> 
> 2014-11-27, 14:53:47 +1100, Stephen Rothwell wrote:
> > Hi Jens,
> > 
> > Today's linux-next merge of the block tree got a conflict in
> > fs/fs-writeback.c between commit ef7fdf5e8c87 ("vfs: add support for a
> > lazytime mount option") from the ext4 tree and commit 9c6ac78eb352
> > ("writeback: fix a subtle race condition in I_DIRTY clearing") from the
> > block tree.
> > 
> > I fixed it up (I took a guess, plese check - see below) and can carry
> > the fix as necessary (no action is required).
> > 
> > -- 
> > Cheers,
> > Stephen Rothwell                    [email-masked]
> > 
> > diff --cc fs/fs-writeback.c
> > index 3d87174408ae,2d609a5fbfea..000000000000
> > --- a/fs/fs-writeback.c
> > +++ b/fs/fs-writeback.c
> > @@@ -482,14 -479,30 +482,30 @@@ __writeback_single_inode(struct inode *
> >   	 * write_inode()
> >   	 */
> >   	spin_lock(&inode->i_lock);
> > - 	/* Clear I_DIRTY_PAGES if we've written out all dirty pages */
> > - 	if (!mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> > - 		inode->i_state &= ~I_DIRTY_PAGES;
> > + 
> >  -	dirty = inode->i_state & I_DIRTY;
> >  -	inode->i_state &= ~I_DIRTY;
> >  +	dirty = inode->i_state & I_DIRTY_INODE;
> >  +	inode->i_state &= ~I_DIRTY_INODE;
> > + 
> > + 	/*
> > + 	 * Paired with smp_mb() in __mark_inode_dirty().  This allows
> > + 	 * __mark_inode_dirty() to test i_state without grabbing i_lock -
> > + 	 * either they see the I_DIRTY bits cleared or we see the dirtied
> > + 	 * inode.
> > + 	 *
> > + 	 * I_DIRTY_PAGES is always cleared together above even if @mapping
> > + 	 * still has dirty pages.  The flag is reinstated after smp_mb() if
> > + 	 * necessary.  This guarantees that either __mark_inode_dirty()
> > + 	 * sees clear I_DIRTY_PAGES or we see PAGECACHE_TAG_DIRTY.
> > + 	 */
> > + 	smp_mb();
> > + 
> > + 	if (mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> > + 		inode->i_state |= I_DIRTY_PAGES;
> > + 
> >   	spin_unlock(&inode->i_lock);
> > + 
> >   	/* Don't write the inode if only I_DIRTY_PAGES was set */
> >  -	if (dirty & (I_DIRTY_SYNC | I_DIRTY_DATASYNC)) {
> >  +	if (dirty) {
> >   		int err = write_inode(inode, wbc);
> >   		if (ret == 0)
> >   			ret = err;
> 
> 
> I think there's a problem in your fix, Stephen.
> 
> I'm getting hangs at boot (strangely -- in QEMU -- only when booting
> via grub, not when using -kernel) and during shutdown.  Jeremiah seems
> to have the same problem and his bisection led to the merge commit:
> https://lkml.org/lkml/2014/11/29/17
> 
> The following solves both issues for me.  I think it makes sense given
> the #defines from ef7fdf5e8c87, since Tejun intended to clear
> I_DIRTY_PAGES.
> 
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index b70e45f45afa..6b2510d97a0a 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -484,7 +484,7 @@ __writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
>  	spin_lock(&inode->i_lock);
>  
>  	dirty = inode->i_state & I_DIRTY_INODE;
> -	inode->i_state &= ~I_DIRTY_INODE;
> +	inode->i_state &= ~I_DIRTY;
>  
>  	/*
>  	 * Paired with smp_mb() in __mark_inode_dirty().  This allows
> 
> 
> -- 
> Thanks,
> Sabrina

That change fixes the problem.

Thanks Sabrina and Stephen!

-- 
- Jeremiah Mahler