linux-kernel - Re: [PATCH] migration_entry

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20080618155233.7dd79312.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 18 Jun 2008 15:52:33 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	"hugh@...itas.com" <hugh@...itas.com>
Subject: Re: [PATCH] migration_entry_wait fix.

On Wed, 18 Jun 2008 16:42:37 +1000
Nick Piggin <nickpiggin@...oo.com.au> wrote:

> > (This fix is not related to lock_page() problem.)
> >
> > If I read your advice correctly, we shouldn't use lock_page() here.
> >
> > Before speculative page cache, page_table_entry of a page under migration
> > has a pte entry which encodes pfn as special pte entry. and wait for the
> > end of page migration by lock_page().
> 
> What I don't think I understand, is how we can have a page in the
> page tables (and with the ptl held) but with a zero refcount... Oh,
> it's not actually a page but a migration entry! I'm not quite so
> familiar with that code.
> 
> Hmm, so we might possibly see a page there that has a zero refcount
> due to page_freeze_refs? In which case, I think the direction of you
> fix is good. Sorry for my misunderstanding the problem, and thank
> you for fixing up my code!
> 
> I would ask you to use get_page_unless_zero rather than
> page_cache_get_speculative(), because it's not exactly a speculative
> reference -- a speculative reference is one where we elevate _count
> and then must recheck that the page we have is correct.
> 
ok.

> Also, please add a comment. It would really be nicer to hide this
> transiently-frozen state away from migration_entry_wait, but I can't
> see any lock that would easily solve it.
> 
ok, will adds comments.

Thanks,
-Kame


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/