Anton Altaparmakov <[EMAIL PROTECTED]> wrote:
>
> On Wed, 2005-02-02 at 14:34 -0800, Andrew Morton wrote:
> > Anton Altaparmakov <[EMAIL PROTECTED]> wrote:
> > >
> > > Below is a patch which adds a function
> > > mm/filemap.c::find_or_create_pages(), locks a range of pages. Please
> > > see
> > > the function description in the patch for details.
> >
> > This isn't very nice, is it, really? Kind of a square peg in a round hole.
>
> Only followed your advice. (-; But yes, it is not very nice at all.
>
> > If you took the approach of defining a custom file_operations.write() then
> > I'd imagine that the write() side of things would fall out fairly neatly:
> > no need for s_umount and i_sem needs to be taken anyway. No trylocking.
>
> But the write() side of things don't need s_umount or trylocking with
> the proposed find_or_create_pages(), either...
i_sem nests outside lock_page, normally. I guess that can be avoided though.
> Unfortunately it is not possible to do this since removing
> ->{prepare,commit}_write() from NTFS would mean that we cannot use loop
> devices on NTFS any more and this is a really important feature for
> several Linux distributions (e.g. TopologiLinux) which install Linux on
> a loopback mounted NTFS file which they then use to place an ext3 (or
> whatever) fs on and use that as the root fs...
>
> So we definitely need full blown prepare/commit write. (Unless we
> modify the loop device driver not to use ->{prepare,commit}_write
> first.)
>
> Any ideas how to solve that one?
I did a patch which switched loop to use the file_operations.read/write
about a year ago. Forget what happened to it. It always seemed the right
thing to do..
> > And for the vmscan->writepage() side of things I wonder if it would be
> > possible to overload the mapping's ->nopage handler. If the target page
> > lies in a hole, go off and allocate all the necessary pagecache pages, zero
> > them, mark them dirty?
>
> I guess it would be possible but ->nopage is used for the read case and
> why would we want to then cause writes/allocations?
yup, we'd need to create a new handler for writes, or pass `write_access'
into ->nopage. I think others (dwdm2?) have seen a need for that.
> At the moment I cannot see a way to solve my problem without the
> proposed find_or_create_pages(). )-:
Unpleasant, isn't it.
I guess the path of least resistance is to do it within ntfs for now.
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