linux-kernel - Re: Testing for dirty buffers on a block device

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Message-ID: <20100224190942.GG1025@kernel.dk>
Date:	Wed, 24 Feb 2010 20:09:42 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	linux-pm <linux-pm@...ts.linux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Testing for dirty buffers on a block device

On Wed, Feb 24 2010, Alan Stern wrote:
> On Tue, 23 Feb 2010, Jens Axboe wrote:
> 
> > > That's not what I meant.  Dirty buffers on a filesystem make no 
> > > difference because they always get written out when the filesystem is 
> > > unmounted.  The device file remains open as long as the filesystem 
> > > is mounted, which would prevent the device from being powered down.
> > > 
> > > I was asking about dirty buffers on a block device that isn't holding a 
> > > filesystem -- where the raw device is being used directly for I/O.
> > 
> > OK, so just specifically the page cache of the device. Is that really
> > enough of an issue to warrant special checking? I mean, what normal
> > setup would even use buffer raw device access?
> 
> Doesn't fdisk use it?  There might be other applications too.

It does, but that sound be a very short lived issue (since the dirty
buffers will get flushed).

> > But if you wanted, I guess the only way would be to lookup
> > dirty/writeback pages on the bdev inode mapping. For that you'd need the
> > bdev, not the gendisk or the queue though.
> 
> I can get the bdev from the gendisk by calling bdget_disk() with a 
> partition number of 0, right?  What would the next step be?  Would this 
> check for dirty pages associated with any of the partitions or would it 
> only look at pages associated with the inode for the entire disk?

It would cover the entire bdev.

-- 
Jens Axboe

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