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SubjectRe: 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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