On Wed, Jul 03 2002, Joe Thornber wrote:
> On Wed, Jul 03, 2002 at 12:08:38PM +0200, Jens Axboe wrote:
> > On Tue, Jul 02 2002, Joe Thornber wrote:
> > > Tom,
> > >
> > > On Tue, Jul 02, 2002 at 09:40:56AM -0400, Tom Walcott wrote:
> > > > Hello,
> > > >
> > > > Browsing the patch submitted for 2.4 inclusion, I noticed that LVM2
> > > > modifies the buffer_head struct. Why does LVM2 require the addition of it's
> > > > own private field in the buffer_head? It seems that it should be able to
> > > > use the existing b_private field.
> > >
> > > This is a horrible hack to get around the fact that ext3 uses the
> > > b_private field for its own purposes after the buffer_head has been
> > > handed to the block layer (it doesn't just use b_private when in the
> > > b_end_io function). Is this acceptable behaviour ? Other filesystems
> > > do not have similar problems as far as I know.
> > >
> > > device-mapper uses the b_private field to 'hook' the buffer_heads so
> > > it can keep track of in flight ios (essential for implementing
> > > suspend/resume correctly). See dm.c:dec_pending()
> >
> > Your driver is required to properly stack b_private uses, however if
> > ext3 (well jbd really) over writes b_private after bh i/o submission I
> > would say that it is broken.
>
> AFAIK ext3 doesn't overwrite b_private after submission, but does
> expect the value not to change (ie. no stacking to be taking place).
Now we are in a grey area. The 'usual' stacked drivers work like this:
some fs path
submit_bh(bh_orig);
...
stacked driver make_request_fn:
bh_new = alloc_bh
bh_new->b_private = bh_orig;
...
submit_bh(bh_new);
if you are just modifying b_private, how exactly is your stacking
working? ie what about lvm2 on lvm2?
-- Jens Axboe- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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