Re: [linux-lvm] LVM2 modifies the buffer_head struct?

[Andrew Morton]

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. That breaks more than just device mapper,
> that will break any stacked driver (such as loop, for instance).

It requires that b_private be stable across the lifetime of the buffer.

hmm.

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