Re: Why doesn't zap_pte_range() call page_mkwrite()

[Nick Piggin <npiggin@suse.de>]

On Fri, Apr 24, 2009 at 01:00:48PM -0400, Trond Myklebust wrote:
> On Fri, 2009-04-24 at 16:52 +0200, Miklos Szeredi wrote:
> > On Fri, 24 Apr 2009, Robin Holt wrote:
> > > I am not sure how you came to this conclusion.  The address_space has
> > > the vma's chained together and protected by the i_mmap_lock.  That is
> > > acquired prior to the cleaning operation.  Additionally, the cleaning
> > > operation walks the process's page tables and will remove/write-protect
> > > the page before releasing the i_mmap_lock.
> > > 
> > > Maybe I misunderstand.  I hope I have not added confusion.
> > 
> > Looking more closely, I think you're right.
> > 
> > I thought that detach_vmas_to_be_unmapped() also removed them from
> > mapping->i_mmap, but that is not the case, it only removes them from
> > the process's mm_struct.  The vma is only removed from ->i_mmap in
> > unmap_region() _after_ zapping the pte's.
> > 
> > This means that while the pte zapping is going on, any page faults
> > will fail but page_mkclean() (and all of rmap) will continue to work.
> > 
> > But then I don't see how we get a dirty pte without also first getting
> > a page fault.  Weird...
> 
> You don't, but unless you unmap the page when you write it out, you will
> not get any further page faults. The VM will just redirty the page
> without calling page_mkwrite().

Why? It should call page_mkwrite...

 
> As I said, I think I can fix the NFS problem by simply unmapping the
> page inside ->writepage() whenever we know the write request was
> originally set up by a page fault.

The biggest outstanding problem we have remaining is get_user_pages.
Callers are only required to hold a ref on the page and then they
can call set_page_dirty at any point after that.

I have a half-done patch somewhere to add a put_user_pages, and then
we could probably go from there to pinning the fs metadata (whether
by using the page lock or something else, I don't quite know).
 

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