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 _______________________________________________ linux-lvm mailing list linux-lvm@sistina.com http://lists.sistina.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://www.sistina.com/lvm/Pages/howto.html
