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

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Date:	Thu, 13 Sep 2012 14:03:51 +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()

On Thu, Sep 13, 2012 at 02:41:31PM +0900, OGAWA Hirofumi wrote:
> Fengguang Wu <fengguang.wu@...el.com> writes:
> 
> >> If used custom bdi with BDI_CAP_NO_WRITEBACK, wait_for_completion()
> >> (e.g. sync_inodes_sb()) will be blocked forever.
> >
> > The sync(2) block cannot be fixed by this patch?
> 
> This patch fixes block problem too.

Ah OK, my sight was somehow limited by the screen size and only saw
the first chunk...

> >> I tested by custom bdi with BDI_CAP_NO_WRITEBACK - sync(2) blocked
> >> forever by this reason.
> >
> > What's your test script? How do you create/use that custom bdi?
> 
> Ah, I wrote my kernel module to test. I guess there is no users in
> current kernel for now, because it doesn't work.

OK. Got it. So I'd suggest to change the first chunk to a VM_BUG_ON(),
which should be enough to catch new bad callers. The other two chunks
look fine.

Thanks,
Fengguang
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