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SubjectRe: [prepatch] address_space-based writeback
Anton Altaparmakov writes:
> At 16:53 27/04/02, Jan Harkes wrote:
> >On Fri, Apr 12, 2002 at 08:57:17AM +0100, Anton Altaparmakov wrote:
> > > Yet, this really begs the question of defining the concept of a file. I am
> > > quite happy with files being the io entity in ntfs. It is just that each
> > > file in ntfs can contain loads of different data holding attributes which
> > > are all worth placing in address spaces. Granted, a dummy inode could be
> > > setup for each of those which just means a lot of wasted ram but ntfs is
> > > not that important so I have to take the penalty there. But if I also need
> > > unique inode numbers in those dummy inodes then the overhead is becoming
> > > very, very high...
> >
> >You could have all additional IO streams use the same inode number and
> >use iget4. Several inodes can have the same i_ino and the additional
> >argument would be a stream identifier that selects the correct 'IO
> >identity'.
> Great idea! I quickly looked into the implementation details and using
> iget4/read_inode2 perfectly reconciles my ideas of using an address space
> mapping for each ntfs attribute with the kernels requirement of using
> inodes as the i/o entity by allowing a clean and unique mapping between
> multiple inodes with the same inode numbers and their attributes and
> address spaces.

Please note that ->read_inode2() is reiserfs-specific hack. Adding more
users for it would make it permanent. The only reason for ->read_inode2
existence was that iget() was called by code external to the
file-system, knfsd used to do this, now it can call ->fh_to_dentry() in
stead. As iget() is never called outside of file-ssytem, you can set
ntfs->read_inode to no-op and write your own function ntfs_iget(...) to
be called from ntfs_lookup() and ntfs_fill_super().

ntfs_iget() calls iget() (->read_inode is no-op, hence iget doesn't
access disk) and, if new inode were allocated, reads data from the disk
and initializes inode, etc.

I guess coda_iget() is example of this.

> I need to work out exactly how to do it but I will definitely go that way.
> That will make everything nice and clean and get rid of the existing
> kludges of passing around other types of objects instead of struct file *
> to my various readpage functions. Also I will be able to have fewer
> readpage functions... (-:
> Thanks for the suggestion!
> Best regards,
> Anton

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