linux-kernel - Re: [PATCH, v3] shmat: introduce flag SHM_MAP_NOT

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Date:	Wed, 8 Oct 2008 09:33:25 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Hugh Dickins <hugh@...itas.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH, v3] shmat: introduce flag SHM_MAP_NOT_FIXED

> Perhaps I'm confused but my /dev/shm doesn't have any such files,
> but I see a variety of shm segments in ipcs.
> 
> What would the path passed to shm_open look like?

Well right now they don't get exposed as they are created unlinked but
that is a trivial tweak to mm/shmem.c.

> > and nobody is wanting to map those at fixed addresses.
> 
> You're saying it should always use the address as a search hint?

Nothing of the sort. I'm pointing out that mmap and shm_open already
provide all the needed interfaces for this for real world applications
today.
> 
> Just changing the semantics unconditionally would seem risky to me. After 
> all as you point out they are primarily for compatibility and for that keeping
> old semantics would seem better to me.

We don't need to change any semantics, there is a perfectly good
alternative standards based interface. At most you might want to make the
sys3 shared memory segments appear in /dev/shm/ somewhere.

Alan
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