linux-kernel - Re: [GIT PULL] please pull ummunotify

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Message-ID: <4AA9EAF7.5010401@inria.fr>
Date:	Fri, 11 Sep 2009 08:15:19 +0200
From:	Brice Goglin <Brice.Goglin@...ia.fr>
To:	Roland Dreier <rdreier@...co.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	jsquyres@...co.com, linux-rdma@...r.kernel.org,
	general@...ts.openfabrics.org, linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] please pull ummunotify

Roland Dreier wrote:
>  > Can I this version already solved fork() + COW issue? if so, could you
>  > please explain what happen at fork. Obviously RDMA point to either parent
>  > or child page, not both. but Corrent COW rule is, first touch process
>  > get copyed page and other process still own original page. I think it's 
>  > unpecected behavior form RDMA.
>
> No, ummunotify doesn't really help that much with fork() + COW.  If a
> parent forks and then touches pages that are actively in use for RDMA,
> then of course they get COWed and RDMA goes to the wrong memory (from
> the point of view of the parent).
>   

My understanding of the code is that fork will end-up calling
copy_page_range() on all VMA, and copy_page_range() calls
mmu_notifier_invalidate_range_start() if  is_cow_mapping() is true,
which should be the case here. So you should get some invalidate events
on fork.

Brice

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