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

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Message-ID: <Pine.LNX.4.64.0802061957110.32204@blonde.site>
Date:	Wed, 6 Feb 2008 20:13:04 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Oleg Nesterov <oleg@...sign.ru>
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

[ removed stable ]

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?

> 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.

sys_remap_file_pages does serve a useful purpose; but it subverts
vma principles, and has therefore caused us grief repeatedly.

Hugh
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