linux-kernel - Re: [RFC][PATCH v3 4/6] aio: Don't inherit aio ring memory at fork

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Message-Id: <20090415115858.AC31.A69D9226@jp.fujitsu.com>
Date:	Wed, 15 Apr 2009 12:00:16 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>, linux-api@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH v3 4/6] aio: Don't inherit aio ring memory at fork

Hi

> > drawback worst aio scenario here
> > -----------------------------------------------------------------------
> > io_setup() and gup			inc page_count
> >
> > fork					inc mapcount
> > 					and make write-protect to pte
> >
> > write ring from userland(*)		page fault and
> > 					COW break.
> > 					parent process get copyed page and
> > 					child get original page owner-ship.
> >
> > kmap and memcpy from kernel		change child page. (it mean data lost)
> >
> > (*) Is this happend?
> 
> I guess it's possible, but I don't know of any programs that do this.

Yup, I also think this isn't happen in real world.

> 
> > MADV_DONTFORK or down_read(mmap_sem) or down_read(mm_pinned_sem) 
> > or copy-at-fork mecanism(=Nick/Andrea patch) solve it.
> 
> OK, thanks for the explanation.
> 
> +	/*
> +	 * aio context doesn't inherit while fork. (see mm_init())
> +	 * Then, aio ring also mark DONTFORK.
> +	 */
> 
> Would you mind if I did some word-smithing on that comment?  Something
> like:
> 	/*
> 	 * The io_context is not inherited by the child after fork()
>          * (see mm_init).  Therefore, it makes little sense for the
>          * completion ring to be inherited.
>          */
> 
> +	ret = sys_madvise(info->mmap_base, info->mmap_size, MADV_DONTFORK);
> +	BUG_ON(ret);
> +
> 
> It appears there's no other way to set the VM_DONTCOPY flag, so I guess
> calling sys_madvise is fine.  I'm not sure I agree with the BUG_ON(ret),
> however, as EAGAIN may be feasible.
> 
> So, fix that up and you can add my reviewed-by.  I think you should push
> this patch independent of the other patches in this series.

Done :)




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