linux-kernel - Re: [PATCH 6/7] hugetlb: hugepage migration core

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Date:	Wed, 7 Jul 2010 15:05:13 +0900
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Wu Fengguang <fengguang.wu@...el.com>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] hugetlb: hugepage migration core

On Tue, Jul 06, 2010 at 09:13:37AM +0200, Andi Kleen wrote:
> On Tue, Jul 06, 2010 at 12:33:42PM +0900, Naoya Horiguchi wrote:
> > > There's more code that handles LRU in this file. Do they all handle huge pages
> > > correctly?
> > > 
> > > I also noticed we do not always lock all sub pages in the huge page. Now if
> > > IO happens it will lock on subpages, not the head page. But this code
> > > handles all subpages as a unit. Could this cause locking problems?
> > > Perhaps it would be safer to lock all sub pages always? Or would 
> > > need  to audit other page users to make sure they always lock on the head
> > > and do the same here.
> > > 
> > > Hmm page reference counts may have the same issue?
> > 
> > If we try to implement paging out of hugepage in the future, we need to
> > solve all these problems straightforwardly. But at least for now we can
> > skirt them by not touching LRU code for hugepage extension.
> 
> We need the page lock to avoid migrating pages that are currently
> under IO. This can happen even without swapping when the process 
> manually starts IO. 

I see.  I understood we should work on locking problem in now.
I digged and learned hugepage IO can happen in direct IO from/to
hugepage or coredump of hugepage user.

We can resolve race between memory failure and IO by checking
page lock and writeback flag, right?

BTW I surveyed direct IO code, but page lock seems not to be taken.
Am I missing something?
(Before determining whether we lock all subpages or only headpage,
I want to clarify how current code for non-hugepage resolves this problem.)

Thanks,
Naoya Horiguchi
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