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    SubjectRe: Testing for dirty buffers on a block device
    On Tue, Feb 23 2010, Alan Stern wrote:
    > On Tue, 23 Feb 2010, Jens Axboe wrote:
    > > > P.S.: Jens, given a pointer to a struct gendisk or to a struct
    > > > request_queue, is there a good way to tell whether there any dirty
    > > > buffers for that device waiting to be written out? This is for
    > > > purposes of runtime power management -- in the initial implementation,
    > > > I want to avoid powering-down a block device if it is open or has any
    > > > dirty buffers. In other words, only completely idle devices should be
    > > > powered down (a good example would be a card reader with no memory card
    > > > inserted).
    > >
    > > There's no fool proof way. For most file systems I think you could get
    > > away with checking the q->bdi dirty lists to see if there's anything
    > > pending. But that wont work always, if the fs uses a different backing
    > > dev info than then queue itself.
    > 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?

    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.

    Jens Axboe

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