page_mkwrite seems broken

[Hugh Dickins <hugh@veritas.com>]

On Fri, 4 Feb 2005, Hugh Dickins wrote in another thread:
> 
> Isn't this exactly what David Howells' page_mkwrite stuff in -mm's
> add-page-becoming-writable-notification.patch is designed for?
> 
> Though it looks a little broken to me as it stands (beyond the two
> fixup patches already there).  I've not found time to double-check
> or test, apologies in advance if I'm libelling, but...
> 
> (a) I thought the prot bits do_nopage gives a pte in a shared writable
>     mapping include write permission, even when it's a read fault:
>     that can't be allowed if there's a page_mkwrite.
> 
> (b) I don't understand how do_wp_page's "reuse" logic for whether it
>     can just go ahead and use the existing anonymous page, would have
>     any relevance to calling page_mkwrite on a shared writable page,
>     which must be used and not COWed however many references there are.

I have now looked further, and both points still seem valid to me:
the page_mkwrite calling code looks doubly broken.  (Tested?)

Nor has there been any movement on the points raised by Christoph,
that aops->page_mkwrite is redundant, and do_wp_page_mk_pte_writable
separation unhelpful.

I could probably put page_mkwrite to use in tmpfs (to eliminate its
unsatisfactory but never over-troubling shmem_recalc_inode), but not
as it currently stands.

Are you planning any movement on this, David?
Or should I have a go sometime?

Hugh