Re: vm_ops.page_mkwrite() fails with vmalloc on 2.6.23

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2007-10-29 at 11:11 +0100, Peter Zijlstra wrote:
> On Mon, 2007-10-29 at 01:17 -0700, Jaya Kumar wrote:
> > On 10/29/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> > > On Mon, 22 Oct 2007 16:40:57 +0200 Stefani Seibold <stefani@seibold.net> wrote:
> > > >
> > > > The problem original occurs with the fb_defio driver (driver/video/fb_defio.c).
> > > > This driver use the vm_ops.page_mkwrite() handler for tracking the modified pages,
> > > > which will be in an extra thread handled, to perform the IO and clean and
> > > > write protect all pages with page_clean().
> > > >
> > 
> > Hi,
> > 
> > An aside, I just tested that deferred IO works fine on 2.6.22.10/pxa255.
> > 
> > I understood from the thread that PeterZ is looking into page_mkclean
> > changes which I guess went into 2.6.23. I'm also happy to help in any
> > way if the way we're doing fb_defio needs to change.
> 
> Yeah, its the truncate race stuff introduced by Nick in
>   d0217ac04ca6591841e5665f518e38064f4e65bd
> 
> I'm a bit at a loss on how to go around fixing this. One ugly idea I had
> was to check page->mapping before going into page_mkwrite() and when
> that is null, don't bother with the truncate check.

Something like this

---
 mm/memory.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux-2.6/mm/memory.c
===================================================================
--- linux-2.6.orig/mm/memory.c
+++ linux-2.6/mm/memory.c
@@ -2300,6 +2300,8 @@ static int __do_fault(struct mm_struct *
 			 * to become writable
 			 */
 			if (vma->vm_ops->page_mkwrite) {
+				struct address_space *mapping = page->mapping;
+
 				unlock_page(page);
 				if (vma->vm_ops->page_mkwrite(vma, page) < 0) {
 					ret = VM_FAULT_SIGBUS;
@@ -2314,7 +2316,7 @@ static int __do_fault(struct mm_struct *
 				 * reworking page_mkwrite locking API, which
 				 * is better done later.
 				 */
-				if (!page->mapping) {
+				if (mapping != page->mapping) {
 					ret = 0;
 					anon = 1; /* no anon but release vmf.page */
 					goto out;