Re: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 10 May 2010 21:05:59 +0200
Andrea Arcangeli <aarcange@redhat.com> wrote:

> On Mon, May 10, 2010 at 12:41:07PM -0500, Christoph Lameter wrote:
> > A simple way to disallow migration of pages is to increment the refcount
> > of a page.
> 
> Ok for migrate but it won't prevent to crash in split_huge_page rmap
> walk, nor the PG_lock. Why for a rmap bug have a migrate specific fix?
> The fix that makes execve the only special place to handle in every
> rmap walk, is at least more maintainable than a fix that makes one of
> the rmap walk users special and won't fix the others, as there will be
> more than just 1 user that requires this. My fix didn't make execve
> special and it didn't require execve knowledge into the every rmap
> walk like migrate (split_huge_page etc...) but as long as the kernel
> doesn't crash I'm fine ;).
> 

At first, I like step-by-step approach even if it makes our cost double
because it's easy to understand and makes chasing change-log easy.

Ok, your split_huge_page() has some problems with current rmap+migration.
But I don't like a patch for never-happen-now bug in change-log.

I believe it can be fixed by the same approach for execs.
Renaming 
#define VM_STACK_INCOMPLETE_SETUP
to be
#define VM_TEMPORARY_INCONSITENT_RMAP
in _your_ patch series and add some check in rmap_walk() seems enough.

Of course, I may misunderstand your problem. Could you show your patch
which meets the problem with rmap+migration ?

Thanks,
-Kame




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