On Fri, Apr 30, 2010 at 09:22:35PM +0200, Andrea Arcangeli wrote:
> I'm building a mergeable THP+memory compaction tree ready for mainline
> merging based on new anon-vma code, so I'm integrating your patch1 and
> this below should be the port of my alternate fix to your patch2 to
> fix the longstanding crash in migrate (not a bug in new anon-vma code
> but longstanding). patch1 is instead about the bugs introduced by the
> new anon-vma code that might crash migrate (even without memory
> compaction and/or THP) the same way as the bug fixed by the below.
I noticed I didn't qrefresh to sync the patch to the working dir and I
had some change in working dir not picked up in the patch. A
INIT_LIST_HEAD was missing and this was the patch I actually tested.
===
Subject: fix race between shift_arg_pages and rmap_walk
From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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 +518,46 @@ 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);
+ if (unlikely(anon_vma_clone(tmp_vma, vma))) {
+ kmem_cache_free(vm_area_cachep, tmp_vma);
+ return -ENOMEM;
+ }
+
+ /*
* cover the whole range: [new_start, old_end)
+ *
+ * The vma is attached only to vma->anon_vma so one lock is
+ * enough. Even this lock might be removed and we could run it
+ * out of order but it's nicer to make atomic updates to
+ * vm_start and so we won't need a smb_wmb() before calling
+ * move_page_tables.
*/
- if (vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL))
- return -ENOMEM;
+ spin_lock(&vma->anon_vma->lock);
+ vma->vm_start = new_start;
+ 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 */
+ unlink_anon_vmas(tmp_vma);
+ kmem_cache_free(vm_area_cachep, tmp_vma);
+
+ if (length != moved_length)
return -ENOMEM;
lru_add_drain();
@@ -551,7 +583,7 @@ 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;
}
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