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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 5/5] x86,mm: make pagefault killable
Cc: kosaki.motohiro@jp.fujitsu.com, Oleg Nesterov <oleg@redhat.com>,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>, Rik van Riel <riel@redhat.com>,
        linux-mm <linux-mm@kvack.org>, Andrey Vagin <avagin@openvz.org>,
        Hugh Dickins <hughd@google.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
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Date: Mon, 28 Mar 2011 16:00:49 +0900 (JST)
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> On Thu, Mar 24, 2011 at 10:13 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > I am wondering, can't we set FAULT_FLAG_KILLABLE unconditionally
> > but check PF_USER when we get VM_FAULT_RETRY? I mean,
> >
> >        if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
> >                if (!(error_code & PF_USER))
> >                        no_context(...);
> >                return;
> >        }
> 
> I agree, we should do this.
> 
> > Probably not... but I can't find any example of in-kernel fault which
> > can be broken by -EFAULT if current was killed.
> 
> There's no way that can validly break anything, since any such
> codepath has to be able to handle -EFAULT for other reasons anyway.
> 
> The only issue is whether we're ok with a regular write() system call
> (for example) not being atomic in the presence of a fatal signal. So
> it does change semantics, but I think it changes it in a good way
> (technically POSIX requires atomicity, but on the other hand,
> technically POSIX also doesn't talk about the process being killed,
> and writes would still be atomic for the case where they actually
> return. Not to mention NFS etc where writes have never been atomic
> anyway, so a program that relies on strict "all or nothing" write
> behavior is fundamentally broken to begin with).

Ok, I didn't have enough brave. Will do.