Re: Access content of file via inodes

[Anton Altaparmakov <aia21@cam.ac.uk>]

Hi,

On Sat, 28 May 2005, Martin Jambor wrote:
> On 5/28/05, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> > > What do I have to do to when I need to append something to a file?
> > > BTW, if that matters, I'll be implementing the adress_space_operations
> > > of that file as well...
> > 
> > That depends on where you want to do that from and what that file is.
> 
> The file is a file system internal file that is used to translate
> inode numbers to  addresses of inodes on the disk. It cannot be
> written to from userspace. When I need one more inode than I currently
> have, I also need to create a new mapping info.

Aha.  Just like NTFS then.  (-:

> I implemented it usng the same approach which ext2 uses to modify its
> directories and believe that is the corect way (by calling aops
> prepare and commit methods). The only thing that puzzles me is that

Yes that isw fine.  Just note that prepare/commit write need to run under 
i_sem protection.  But if you are already serializing access to the file 
some other way then you can ignore i_sem.

> ext2 does not call flush_dcache_page that you suggested. Since it
> seems to be an architecture specific function, I have no clue what so
> ever whether I need to call it or not.

Well, as far as I understand it, this function causes changes to the page 
contents to become visible on all CPUs and from all processes and needs to 
be run before you unlock a page/mark it up to date otherwise someone who 
then locks it and/or reads it will possibly read old data from the page 
that is no longer correct.  Thus I always call it after modifying a page.  
Perhaps it is sometimes or even never actually needed for NTFS but I do 
not want to take that risk so I prefer to always call it.  It certainly 
does not do any harm to call it except perhaps making the code a bit 
slower.

> Thanks for your email and for any clarification,

No problem.

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/