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

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Message-ID: <20081008004030.7a0e9915@lxorguk.ukuu.org.uk>
Date:	Wed, 8 Oct 2008 00:40:30 +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

> > Can use shm_open and mmap to get POSIX standard shm behaviour via a sane
> 
> I don't think shm_open can attach to SYSV shared segments. Or are you
> proposing to add "sysvshmfs" to make that possible? 

Actually you can do so. As it stands today the SYS3 SHM interface code
does the following

	create a char array in the form SYS%08ld, key
	open it on shmfs

> - There are legacy interfaces that cannot be really changed who use sysv shm
> (e.g. X shm and others -- just do a ipcs on your system) 

They can be changed and nobody is wanting to map those at fixed addresses.

> - An system call emulation as in qemu obviously has to implement the
> existing system call semantics.

Which it can do perfectly well using shm_open to create its SYS3
SHM objects. In fact theoertically we could bin the whole of SYS3 shm and
push it into glibc emulation if we wanted.

Alan
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