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

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Date:	Fri, 14 Sep 2012 15:19:52 +0200
From:	Jan Kara <jack@...e.cz>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	Fengguang Wu <fengguang.wu@...el.com>, 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()

On Fri 14-09-12 21:12:02, OGAWA Hirofumi wrote:
> Fengguang Wu <fengguang.wu@...el.com> writes:
> >> >> @@ -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.
> 
> page reclaim and fsync path have FS handler. So, FS can control those.
> 
> The modern FS have to control to flush carefully. Many FSes are already
> ignoring if wbc->sync_mode != WB_SYNC_ALL (e.g. ext3_write_inode,
> nilfs_writepages), and have own FS task to flush.
  Out of curiosity, what exactly do you need to control in your filesystem
that makes flusher thread unusable for you? You still have a lot of
flexibility with ->write_inode() and ->writepages() callbacks...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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