Re: Access content of file via inodes

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

Hi,

On Mon, 30 May 2005, Martin Jambor wrote:
> On 5/29/05, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> > > 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.
> 
> Do they? Documentation/filesystems/Locking only says they want their
> page locked... but thanks for telling me, I will check that.

Well I was being simplistic.  For most (I believe) fs, i_sem is use to 
protect against changes in i_size.  So both file write and truncate are 
done under i_sem.  They are the only ops that can change i_size.  (Asid: 
As a consequence of this the lseek op of those fs is also done under 
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. 
> 
> Some folks on irc sent me a link to an article that explains this
> coherency stuff:
> http://www.informit.com/articles/article.asp?p=29961&seqNum=6&rl=1
> Basically, you need to call this only if the page might afterwards be
> read from the userspace. Directories are not, so ext2 doesn't have to.

Ah, that's a great article!  Thanks for the pointer.  I will be able to 
remove quite a few dcache flushes from ntfs now.  (-:

Thanks,

	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/