Re: [PATCH] Add notification of page becoming writable to VMA ops

[David Howells <dhowells@redhat.com>]

Hugh Dickins <hugh@veritas.com> wrote:

> > +			if (vma->vm_ops->page_mkwrite &&
> > +			    vma->vm_ops->page_mkwrite(vma, new_page) < 0)
> > +				return VM_FAULT_SIGBUS;
> > +		}
> >  	}
> 
> This isn't necessarily wrong, and may be exactly how it was before,
> I don't remember.  But it implies that when page_mkwrite fails,
> it page_cache_releases the page.  Is that desirable?  Or should
> that be left to the caller?

You're right. I've added a release. That may explain a memory leak I was
seeing that I couldn't find.

> > @@ -1945,7 +1998,7 @@ static int do_file_page(struct mm_struct
> 
> Drop all those changes to do_file_page (which I added), they're no
> longer necessary.  A case appeared which made it clear that we cannot
> rely on resolving this issue for get_user_pages in a single call to
> handle_mm_fault, and that's why the VM_FAULT_WRITE stuff got added. 

I take it then that:

 (1) the write_access parameter to do_file_page() is there purely so that
     handle_pte_fault() can jump to it rather than calling it since they have
     the same parameter set and return value;

 (2) and that do_file_page() always installs a read-only PTE so that
     do_wp_page() will be called subsequently on a write attempt.

David