Re: [PATCH 5/7] hugetlb: pin oldpage in page migration

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

Hi,

Thank you for your reviewing.

On Tue, Jul 06, 2010 at 10:54:38AM -0500, Christoph Lameter wrote:
> On Fri, 2 Jul 2010, Naoya Horiguchi wrote:
> 
> > This patch introduces pinning the old page during page migration
> > to avoid freeing it before we complete copying.
> 
> The old page is already pinned due to the reference count that is taken
> when the page is put onto the list of pages to be migrated. See
> do_move_pages() f.e.

OK.

> Huge pages use a different scheme?

Different scheme is in soft offline, where the target page is not pinned
before migration.  So I should have pinned in soft offline side.
I'll fix it.

> > This race condition can happen for privately mapped or anonymous hugepage.
> 
> It cannot happen unless you come up with your own scheme of managing pages
> to be migrated and bypass migrate_pages(). There you should take the
> refcount.

Yes.

> >  	/*
> > +	 * It's reasonable to pin the old page until unmapping and copying
> > +	 * complete, because when the original page is an anonymous hugepage,
> > +	 * it will be freed in try_to_unmap() due to the fact that
> > +	 * all references of anonymous hugepage come from mapcount.
> > +	 * Although in the other cases no problem comes out without pinning,
> > +	 * it looks logically correct to do it.
> > +	 */
> > +	get_page(page);
> > +
> > +	/*
> 
> Its already pinned. Dont do this. migrate_pages() relies on the caller
> having pinned the page already.

I agree.

Thanks,
Naoya Horiguchi

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