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SubjectRe: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks
On Tue, May 04, 2010 at 11:32:13AM +0100, Mel Gorman wrote:
> Unfortunately, the same bug triggers after about 18 minutes. The objective of
> your fix is very simple - have a VMA covering the new range so that rmap can
> find it. However, no lock is held during move_page_tables() because of the
> need to call the page allocator. Due to the lack of locking, is it possible
> that something like the following is happening?
>
> Exec Process Migration Process
> begin move_page_tables
> begin rmap walk
> take anon_vma locks
> find new location of pte (do nothing)
> copy migration pte to new location
> #### Bad PTE now in place
> find old location of pte
> remove old migration pte
> release anon_vma locks
> remove temporary VMA
> some time later, bug on migration pte
>
> Even with the care taken, a migration PTE got copied and then left behind. What
> I haven't confirmed at this point is if the ordering of the walk in "migration
> process" is correct in the above scenario. The order is important for
> the race as described to happen.

Ok so this seems the ordering dependency on the anon_vma list that
strikes again, I didn't realize the ordering would matter here, but it
does as shown above, great catch! The destination vma of the
move_page_tables has to be at the tail of the anon_vma list like the
child vma have to be at the end to avoid the equivalent race in
fork. This has to be a requirement for mremap too. We just want to
enforce the same invariants that mremap already enforces, to avoid
adding new special cases to the VM.

== for new anon-vma code ==
Subject: fix race between shift_arg_pages and rmap_walk

From: Andrea Arcangeli <aarcange@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@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>
@@ -503,7 +504,9 @@ 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);

@@ -515,17 +518,43 @@ 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;
+ INIT_LIST_HEAD(&tmp_vma->anon_vma_chain);
+ /*
* cover the whole range: [new_start, old_end)
*/
- if (vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL))
+ tmp_vma->vm_start = new_start;
+ /*
+ * The tmp_vma destination of the copy (with the new vm_start)
+ * has to be at the end of the anon_vma list for the rmap_walk
+ * to find the moved pages at all times.
+ */
+ if (unlikely(anon_vma_clone(tmp_vma, vma))) {
+ kmem_cache_free(vm_area_cachep, tmp_vma);
return -ENOMEM;
+ }

/*
* 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);
+
+ vma->vm_start = new_start;
+ /* rmap walk will already find all pages using the new_start */
+ unlink_anon_vmas(tmp_vma);
+ kmem_cache_free(vm_area_cachep, tmp_vma);
+
+ if (length != moved_length)
return -ENOMEM;

lru_add_drain();
@@ -551,7 +580,7 @@ static int shift_arg_pages(struct vm_are
/*
* Shrink the vma to just the new range. Always succeeds.
*/
- vma_adjust(vma, new_start, new_end, vma->vm_pgoff, NULL);
+ vma->vm_end = new_end;

return 0;
}

== for old anon-vma code ==
Subject: fix race between shift_arg_pages and rmap_walk

From: Andrea Arcangeli <aarcange@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@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 +517,41 @@ 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;
+
+ /*
* cover the whole range: [new_start, old_end)
*/
- vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL);
+ tmp_vma->vm_start = new_start;
+
+ /*
+ * The tmp_vma destination of the copy (with the new vm_start)
+ * has to be at the end of the anon_vma list for the rmap_walk
+ * to find the moved pages at all times.
+ */
+ anon_vma_link(tmp_vma);

/*
* 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);
+
+ vma->vm_start = new_start;
+ /* 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 +577,7 @@ 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;
}

I'll release new THP-23 and THP-23-anon_vma_chain (prerelease is
already in aa.git but it misses the above bit) soon enough...


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