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SubjectRe: [PATCH][RFC] dm: Do not open log and cow device read-write for read-only mappings
On Mon, 14 Feb 2011 16:44:30 +0100 Tejun Heo <tj@kernel.org> wrote:

> Hello,
>
> On Mon, Feb 14, 2011 at 03:23:20PM +0100, Milan Broz wrote:
> > >> I am not sure if this is complete fix... note that:
> > >> - what happens during mirror resync and read-only log?
> > >> - for COW, it there situation we need to update header in read-oly mode? (invalidated snap?)
> > >
> > > But if the underlying device is marked ro, dm shouldn't update it at
> > > all. The device should be opened ro and ro policy should be enforced.
> >
> > Sure. So we need to check these situations I described.
>
> Yeap, it seems dm folks are gonna take care of dm part.
>
> > Btw the same pattern is in MD code in lock_rdev() ...
>
> Indeed, cc'ing Neil. Hi, the whole thread can be read from the
> following URL.
>
> http://thread.gmane.org/gmane.linux.kernel/1099399/focus=1099735
>
> blkdev_get() now rejects rw open of devices which are marked
> read-only. I think the right thing to do would be opening the member
> devices ro if the array is assembled for ro access (similar to Milan's
> patch for dm). How does that sound?
>
> Thanks.
>

Sounds sensible ... though it is not all that easy to assemble an
array as 'read-only'.... it is possible though.

When the array is switched to read-write, do I have to call blkdev_get again
asking for rw access, then close the old blkdev, or can I 'upgrade'?

If a device has multiple opens: some read-only and some read-write, can I
find out when the last read-write close is gone? That would be really useful,
especially if a filesystem down-graded its open to read-only when it is
remounted read-only..

[[And if filesystems could be convinced to open the device read-only when the
fs is mounted read-only (and just do journal replay to internal data
structures) that would be really awesome!!]]

NeilBrown



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