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SubjectRe: [PATCH 2/2] mm,migration: Fix race between shift_arg_pages and rmap_walk by guaranteeing rmap_walk finds PTEs created within the temporary stack
On Thu, 6 May 2010 18:40:39 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

>
>
> On Fri, 7 May 2010, Mel Gorman wrote:
> >
> > Page migration requires rmap to be able to find all migration ptes
> > created by migration. If the second rmap_walk clearing migration PTEs
> > misses an entry, it is left dangling causing a BUG_ON to trigger during
> > fault. For example;
>
> So I still absolutely detest this patch.
>
> Why didn't the other - much simpler - patch work? The one Rik pointed to:
>
> http://lkml.org/lkml/2010/4/30/198
>
> and didn't do that _disgusting_ temporary anon_vma?
>
I vote for simple one rather than temporal anon_vma. IIUC, this patch is selected
for not to leak exec's problem out to mm/ by magical check.


> Alternatively, why don't we just take the anon_vma lock over this region,
> so that rmap can't _walk_ the damn thing?
>

IIUC, move_page_tables() may call "page table allocation" and it cannot be
done under spinlock.

Thanks,
-Kame



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