Re: [PATCH 3/3] mm,migration: Remove straggling migration PTEswhen page tables are being moved after the VMA has already moved

[Andrea Arcangeli]

On Tue, Apr 27, 2010 at 10:30:52PM +0100, Mel Gorman wrote:
> During exec(), a temporary stack is setup and moved later to its final
> location. There is a race between migration and exec whereby a migration
> PTE can be placed in the temporary stack. When this VMA is moved under the
> lock, migration no longer knows where the PTE is, fails to remove the PTE
> and the migration PTE gets copied to the new location.  This later causes
> a bug when the migration PTE is discovered but the page is not locked.

This is the real bug, the patch 1 should be rejected and the
expanation-trace has the ordering wrong. The ordering is subtle but
fundamental to prevent that race, split_huge_page also requires the
same anon-vma list_add_tail to avoid the same race between fork and
rmap_walk. It should work fine already with old and new anon-vma code
as they both add new vmas always to the tail of the list.

So the bug in very short, is that "move_page_tables runs out of sync
with vma_adjust in shift_arg_pages"?

> This patch handles the situation by removing the migration PTE when page
> tables are being moved in case migration fails to find them. The alternative
> would require significant modification to vma_adjust() and the locks taken
> to ensure a VMA move and page table copy is atomic with respect to migration.

I'll now evaluate the fix and see if I can find any other
way to handle this.

Great, I'm quite sure with patch 3 we'll move the needle and fix the
bug, it perfectly explains why we only get the oops inside execve in
the stack page.

Patch 2 I didn't check it yet but it's only relevant for the new
anon-vma code, I suggest to handle it separately from the rest.
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