Re: [PATCH 1 of 2] Implement generic block_page_mkwrite() functionality

David Chinner
2007-02-08T22:31:24Z

On Thu, Feb 08, 2007 at 08:11:00AM -0500, Chris Mason wrote:
> On Thu, Feb 08, 2007 at 09:50:13AM +1100, David Chinner wrote:
> > > You don't need to lock out all truncation, but you do need to lock
> > > out truncation of the page in question. Instead of your i_size
> > > checks, check page->mapping isn't NULL after the lock_page?
> >
> > Yes, that can be done, but we still need to know if part of
> > the page is beyond EOF for when we call block_commit_write()
> > and mark buffers dirty. Hence we need to check the inode size.
> >
> > I guess if we block the truncate with the page lock, then the
> > inode size is not going to change until we unlock the page.
> > If the inode size has already been changed but the page not yet
> > removed from the mapping we'll be beyond EOF.
> >
> > So it seems to me that we can get away with not using the i_mutex
> > in the generic code here.
>
> vmtruncate changes the inode size before waiting on any pages. So,
> i_size could change any time during page_mkwrite.

Which would put us beyond EOF. Ok.

> It would be a good idea to read i_size once and put it in a local var
> instead.

Will do - I'll snap it once the page is locked....

Thanks Chris.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group