Re: [patch 1/3] add the fsblock layer

[Nick Piggin <nickpiggin@yahoo.com.au>]

Neil Brown wrote:
> On Tuesday June 26, nickpiggin@yahoo.com.au wrote:
> 
>>Chris Mason wrote:
>>
>>>The block device pagecache isn't special, and certainly isn't that much
>>>code.  I would suggest keeping it buffer head specific and making a
>>>second variant that does only fsblocks.  This is mostly to keep the
>>>semantics of PagePrivate sane, lets not fuzz the line.
>>
>>That would require a new inode and address_space for the fsblock
>>type blockdev pagecache, wouldn't it? I just can't think of a
>>better non-intrusive way of allowing a buffer_head filesystem and
>>an fsblock filesystem to live on the same blkdev together.
> 
> 
> I don't think they would ever try to.  Both filesystems would bd_claim
> the blkdev, and only one would win.

Hmm OK, I might have confused myself thinking about partitions...

> The issue is more of a filesystem sharing a blockdev with the
> block-special device (i.e. open("/dev/sda1"), read) isn't it?
> 
> If a filesystem wants to attach information to the blockdev pagecache
> that is different to what blockdev want to attach, then I think "Yes"
> - a new inode and address space is what it needs to create.
> 
> Then you get into consistency issues between the metadata and direct
> blockdevice access.  Do we care about those?

Yeah that issue is definitely a real one. The problem is not just
consistency, but "how do the block device aops even know that the
PG_private page they have has buffer heads or fsblocks", so it is
an oopsable condition rather than just a plain consistency issue
(consistency is already not guaranteed).

-- 
SUSE Labs, Novell Inc.