linux-kernel - Re: [PATCH] sys_remap_file_pages: fix ->vm

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Message-ID: <20080211101530.GA162@tv-sign.ru>
Date:	Mon, 11 Feb 2008 13:15:30 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Miklos Szeredi <mszeredi@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	William Lee Irwin III <wli@...omorphy.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Matt Helsley <matthltc@...ibm.com>
Subject: Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting

Sorry for delay,

On 02/06, Hugh Dickins wrote:
> 
> On Sun, 3 Feb 2008, Oleg Nesterov wrote:
> > Off-topic question to all. sys_remap_file_pages() doesn't work with
> > shared readonly mappings, why?
> 
> Slight correction: it works with shared readonly mappings, doesn't
> it, so long as the mmap'ed file was opened for reading and writing?

Yes sure. I meant, if the file was opened without FMODE_WRITE, then
mmap(PROT_READ, MAP_SHARED) doesn't actually set VM_SHARED, it only
sets VM_MAYSHARE.

(this looks understandable, but means that !VM_SHARED doesn't necessary
 imply the possible cow).

> > IOW, why it checks VM_SHARED but not VM_MAYSHARE?
> 
> My guess has always been that it was just a misunderstanding of how
> those VM_ flags end up working: assume so unless Ingo corrects me.
> 
> By the time I realized that oddity, we'd been driven into several
> tiresome corners by the very existence of sys_remap_file_pages.
> So whereas my usual instinct would have been to relax the restriction
> and generalize, in its case I wanted to hold on to every restriction
> we had.

OK, thanks a lot. I was afraid I missed some "obvious" reason why we
can't do this.

Oleg.

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