On Fri, 4 Feb 2005, Anton Altaparmakov wrote:
> On Thu, 2005-02-03 at 11:23 -0800, Bryan Henderson wrote:
> >
> > I think the point is that we can't have a "handler for writes," because
> > the writes are being done by simple CPU Store instructions in a user
> > program. The handler we're talking about is just for page faults. Other
>
> That was my understanding.
>
> > operating systems approach this by actually _having_ a handler for a CPU
> > store instruction, in the form of a page protection fault handler -- the
> > nopage routine adds the page to the user's address space, but write
> > protects it. The first time the user tries to store into it, the
> > filesystem driver gets a chance to do what's necessary to support a dirty
> > cache page -- allocate a block, add additional dirty pages to the cache,
> > etc. It would be wonderful to have that in Linux.
>
> It would. This would certainly solve this problem.
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.
Didn't look further, think you should take a look at page_mkwrite,
but don't expect the implementation to be correct yet.
Hugh
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