[Ocfs2-devel] [-mm PATCH] ocfs2: Shared writeable mmap
Peter Zijlstra
a.p.zijlstra at chello.nl
Tue Jun 20 02:07:47 CDT 2006
On Mon, 2006-06-19 at 17:07 -0700, Andrew Morton wrote:
> Mark Fasheh <mark.fasheh at oracle.com> wrote:
> >
> > I finally got some time to sit down and implement an OCFS2 patch to make use
> > of the ->page_mkwrite() callback added by David Howells' patch (named
> > 'add-page_mkwrite-vm_operations-method.patch' in -mm). The patches, and an
> > MPI program to test this can be found at:
> >
> > http://kernel.org/pub/linux/kernel/people/mfasheh/ocfs2/mmap/
> >
> > There's one bug however, which will cause the test program on one of the
> > reading nodes to see stale data if it is run several times in a row against
> > the same file. I have verified that the same thing works fine on a local
> > file system (ext3). I'm not sure where the issue is, but I have a feeling
> > I'm doing something bad in ocfs2_data_convert_worker(). Another possibility
> > is that we missed a place to put the ->page_mkwrite callback.
> >
> > Unfortunately, I have to step away from this patch for a bit as I have some
> > higher priority issues to deal with :/ Luckily, it seems to be in a state
> > which I think warrants it being pushed out to the public for general review,
> > testing, etc. If anyone is interested, I'd also appreciate any advice or
> > help regarding the bug -- my VM-foo is very weak :)
>
> Peter Zijlstra told me yesterday:
>
> There is a problem with the page-mkwrite last posted to lkml. /me
> checks your tree... Yeah, that version has a problem:
> http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.17-rc6/2.6.17-rc6-mm2/broken-out/add-page_mkwrite-vm_operations-method.patch
>
> The thing is that get_user_pages(.write=1, .force=1) can generate COW
> hits on read-only shared mappings, this patch traps those as mkpage_write
> candidates and fails to handle them the old way.
The -v9 version of the dirty page tracking I send out fixes this problem
by affiliation; the following patch should also be enough:
---
mm/memory.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
Index: 2.6-mm/mm/memory.c
===================================================================
--- 2.6-mm.orig/mm/memory.c 2006-06-20 09:02:58.000000000 +0200
+++ 2.6-mm/mm/memory.c 2006-06-20 09:06:01.000000000 +0200
@@ -1464,7 +1464,8 @@ static int do_wp_page(struct mm_struct *
if (!old_page)
goto gotten;
- if (unlikely(vma->vm_flags & VM_SHARED)) {
+ if (unlikely(vma->vm_flags & (VM_SHARED|VM_WRITE) ==
+ VM_SHARED|VM_WRITE) {
if (vma->vm_ops && vma->vm_ops->page_mkwrite) {
/*
* Notify the address space that the page is about to
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