From: Izik Eidus <ieidus@redhat.com>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Greg KH <greg@kroah.com>,
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
mtk.manpages@gmail.com, linux-man@vger.kernel.org,
linux-kernel@vger.kernel.org, Nick Piggin <npiggin@suse.de>,
Hugh Dickins <hugh@veritas.com>
Subject: Re: open(2) says O_DIRECT works on 512 byte boundries?
Date: Sat, 07 Feb 2009 15:32:03 +0200 [thread overview]
Message-ID: <498D8D53.6030007@redhat.com> (raw)
In-Reply-To: <20090206175414.GQ14011@random.random>
Andrea Arcangeli wrote:
>
> Here the latest patch. Now that I consider the 'production' trouble
> closed I'll be porting it to mainline while addressing gup-fast too
> which is an additional complication I didn't have to take care of
> yet. So expect a patch that works for you in the next few days, either
> that or complains about an unfixable gup-fast ;). But frankly I've
> been thinking it should be possible in a much simpler way that I
> ever thought before, by entirely relaying on the tlb flush.
>
> In short if I simply do the page-count vs page-mapcount check _after_
> ptep_set_wrprotect (which implies a tlb flush and after that any
> gup-fast running in other cpus should be forced through the slow path
> and block) I think I'm done. The code now does:
>
> check mapcount
> ptep_set_wrprotect
>
> I think to make the thing working with gup-fast I've only to
> ptep_set_wrprotect before the mapcount check.
>
> The reason why the normal pagetable walking of the cpu is ok with
> current code is that ptep_set_wrprotect done later will block any
> access to the page from the other cpus. Not so if it was gup-fast
> taking reference on the page. So we need to stop with
> ptep_set_wrprotect any subsequential gup-fast _before_ we check count
> vs mapcount and the fact the get_page is run inside the critical
> section with local irq disabled in gup-fast should close the race for
> gup-fast too. Will try that ASAP...
>
>
Hi Andrea,
Good approach, but I think that it isn't enough to just change the order
of check mapcount and ptep_set_wrprotect(),
the reason is that gup_fast do the get_page() AFTER it fetched the pte,
so we are opening here a tiny race:
cpu#1 do get_user_pages_fast and fetch the pte (it think the pte is
writeable)
cpu#2 do ptep_set_wrprotect()
cpu#2 check the mapcount against pagecount (it think that everything is
fine and continue)
cpu#1 only now do get_page()
Anyway this is minor issue that can be probably solved by just:
rechecking if the pte isnt read_only in gup_fast after we do the get_page()
Anyway sound like a great idea to fix this issue!
good work.
next prev parent reply other threads:[~2009-02-07 13:33 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-01-28 21:33 open(2) says O_DIRECT works on 512 byte boundries? Greg KH
2009-01-29 0:41 ` Robert Hancock
[not found] ` <20090129011758.GA26534@kroah.com>
2009-01-29 2:59 ` Michael Kerrisk
2009-01-29 3:13 ` Greg KH
2009-01-29 15:40 ` Jeff Moyer
2009-01-30 6:16 ` Greg KH
2009-01-29 5:13 ` KAMEZAWA Hiroyuki
2009-01-29 7:10 ` KOSAKI Motohiro
2009-01-30 6:17 ` Greg KH
2009-02-02 22:08 ` Andrea Arcangeli
2009-02-03 1:29 ` KAMEZAWA Hiroyuki
2009-02-03 2:31 ` Andrea Arcangeli
2009-02-03 2:55 ` KAMEZAWA Hiroyuki
2009-02-03 3:42 ` KAMEZAWA Hiroyuki
2009-02-06 17:55 ` Andrea Arcangeli
2009-02-03 3:50 ` Greg KH
2009-02-03 15:01 ` Andrea Arcangeli
2009-02-03 4:13 ` KAMEZAWA Hiroyuki
2009-02-03 4:38 ` KAMEZAWA Hiroyuki
2009-02-03 15:08 ` Andrea Arcangeli
2009-02-04 23:41 ` Greg KH
2009-02-06 17:54 ` Andrea Arcangeli
2009-02-06 18:38 ` Andrea Arcangeli
2009-02-07 13:32 ` Izik Eidus [this message]
2009-02-07 15:33 ` Andrea Arcangeli
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