linux-kernel - Re: [PATCH] Fix queueing work if !bdi_cap_writeback

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Date:	Fri, 14 Sep 2012 19:14:29 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	viro@...iv.linux.org.uk, jack@...e.cz, hch@....de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix queueing work if !bdi_cap_writeback_dirty()

> >> @@ -120,6 +120,9 @@ __bdi_start_writeback(struct backing_dev
> >>  {
> >>  	struct wb_writeback_work *work;
> >>  
> >> +	if (!bdi_cap_writeback_dirty(bdi))
> >> +		return;
> >
> > Will someone in the current kernel actually call
> > __bdi_start_writeback() on a BDI_CAP_NO_WRITEBACK bdi?
> >
> > If the answer is no, VM_BUG_ON(!bdi_cap_writeback_dirty(bdi)) looks better.
> 
> I guess nobody call it in current kernel though. Hmm.., but we also have
> check in __mark_inode_dirty(), nobody should be using it, right?
> 
> If we defined it as the bug, I can't see what BDI_CAP_NO_WRITEBACK wants
> to do actually.  We are not going to allow to disable the writeback task?

> I was going to use this to disable writeback task on my developing FS...

That sounds like an interesting use case. Can you elaborate a bit more?
Note that even if you disable __bdi_start_writeback() here, the kernel
may also start writeback in the page reclaim path, the fsync() path,
and perhaps more.

Thanks,
Fengguang
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