Re: [PATCH 0/3] Fix migration races in rmap_walk() V2

[Andrea Arcangeli]

On Tue, Apr 27, 2010 at 05:27:36PM -0500, Christoph Lameter wrote:
> Can we simply wait like in the fault path?

There is no bug there, no need to wait either. I already audited it
before, and I didn't see any bug. Unless you can show a bug with CPU A
running the rmap_walk on process1 before process2, there is no bug to
fix there.

> 
> > Patch 3 notes that while a VMA is moved under the anon_vma lock, the page
> > 	tables are not similarly protected. Where migration PTEs are
> > 	encountered, they are cleaned up.
> 
> This means they are copied / moved etc and "cleaned" up in a state when
> the page was unlocked. Migration entries are not supposed to exist when
> a page is not locked.

patch 3 is real, and the first thought I had was to lock down the page
before running vma_adjust and unlock after move_page_tables. But these
are virtual addresses. Maybe there's a simpler way to keep migration
away while we run those two operations.
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