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From: Andrea Arcangeli on 28 Apr 2010 16:50
On Wed, Apr 28, 2010 at 05:45:08PM +0200, Andrea Arcangeli wrote:
> On Wed, Apr 28, 2010 at 04:23:54PM +0100, Mel Gorman wrote:
> > Is it possible to delay the linkage like that? As arguments get copied into
> > the temporary stack before it gets moved, I'd have expected the normal fault
> > path to prepare and attach the anon_vma. We could special case it but
> > that isn't very palatable either.
>
> I'm not sure what is more palatable, but I feel it should be fixed in
> execve.

Ok the best idea so far I had is to add a fake temporary fake vma to
the anon_vma list with the old vm_start and same vm_pgoff before
shifting down vma->vm_start and calling move_page_tables. Then after
the move is complete we remove the fake vma. So all the fast paths
will remain unmodified and no magic is required. I'll try to fix this
for the old stable anon-vma code and test in aa.git first as the code
will differ. If it works ok anybody can port it to new anon-vma code.
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From: Andrea Arcangeli on 28 Apr 2010 17:10
On Wed, Apr 28, 2010 at 10:40:43PM +0200, Andrea Arcangeli wrote:
> Ok the best idea so far I had is to add a fake temporary fake vma to
> the anon_vma list with the old vm_start and same vm_pgoff before
> shifting down vma->vm_start and calling move_page_tables. Then after
> the move is complete we remove the fake vma. So all the fast paths
> will remain unmodified and no magic is required. I'll try to fix this
> for the old stable anon-vma code and test in aa.git first as the code
> will differ. If it works ok anybody can port it to new anon-vma code.

here the new try for aa.git or 2.6.33 anon-vma code.

----
Subject: fix race between shift_arg_pages and rmap_walk

From: Andrea Arcangeli <aarcange(a)redhat.com>

migrate.c requires rmap to be able to find all ptes mapping a page at
all times, otherwise the migration entry can be instantiated, but it
can't be removed if the second rmap_walk fails to find the page.

And split_huge_page() will have the same requirements as migrate.c
already has.

Signed-off-by: Andrea Arcangeli <aarcange(a)redhat.com>
---

diff --git a/fs/exec.c b/fs/exec.c
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -55,6 +55,7 @@
#include <linux/fsnotify.h>
#include <linux/fs_struct.h>
#include <linux/pipe_fs_i.h>
+#include <linux/rmap.h>

#include <asm/uaccess.h>
#include <asm/mmu_context.h>
@@ -502,7 +503,9 @@ static int shift_arg_pages(struct vm_are
unsigned long length = old_end - old_start;
unsigned long new_start = old_start - shift;
unsigned long new_end = old_end - shift;
+ unsigned long moved_length;
struct mmu_gather *tlb;
+ struct vm_area_struct *tmp_vma;

BUG_ON(new_start > new_end);

@@ -514,16 +517,36 @@ static int shift_arg_pages(struct vm_are
return -EFAULT;

/*
+ * We need to create a fake temporary vma and index it in the
+ * anon_vma list in order to allow the pages to be reachable
+ * at all times by the rmap walk for migrate, while
+ * move_page_tables() is running.
+ */
+ tmp_vma = kmem_cache_alloc(vm_area_cachep, GFP_KERNEL);
+ if (!tmp_vma)
+ return -ENOMEM;
+ *tmp_vma = *vma;
+
+ spin_lock(&vma->anon_vma->lock);
+ /*
* cover the whole range: [new_start, old_end)
*/
- vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL);
+ vma->vm_start = new_start;
+ __anon_vma_link(tmp_vma);
+ spin_unlock(&vma->anon_vma->lock);

/*
* move the page tables downwards, on failure we rely on
* process cleanup to remove whatever mess we made.
*/
- if (length != move_page_tables(vma, old_start,
- vma, new_start, length))
+ moved_length = move_page_tables(vma, old_start,
+ vma, new_start, length);
+
+ /* rmap walk will already find all pages using the new_start */
+ anon_vma_unlink(tmp_vma);
+ kmem_cache_free(vm_area_cachep, tmp_vma);
+
+ if (length != moved_length)
return -ENOMEM;

lru_add_drain();
@@ -549,7 +572,7 @@ static int shift_arg_pages(struct vm_are
/*
* shrink the vma to just the new range.
*/
- vma_adjust(vma, new_start, new_end, vma->vm_pgoff, NULL);
+ vma->vm_end = new_end;

return 0;
}
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