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

[Chris Mason <chris.mason@oracle.com>]

On Sat, Apr 25, 2009 at 07:10:28AM +0200, Nick Piggin wrote:
> 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).

Hi everyone,

Sorry for digging up an old thread, but is there any reason we can't
just use page_mkwrite here?  I'd love to get rid of the btrfs code to
detect places that use set_page_dirty without a page_mkwrite.

-chris

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