linux-kernel - Re: get_user

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Message-ID: <20091130115425.GA21639@wotan.suse.de>
Date:	Mon, 30 Nov 2009 12:54:25 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mark Veltzer <mark.veltzer@...il.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: get_user_pages question

On Sat, Nov 28, 2009 at 07:50:52PM +0100, Andrea Arcangeli wrote:
> All other patches floating around spread an mm-wide semaphore over
> fork fast path, and across O_DIRECT, nfs, and aio, and they most
> certainly didn't fix the two races for all gup users, and they weren't
> stable because of having to identify the closure of the I/O across all
> possible put_page. That approach kind of opens a can of worms and it
> looks the wrong way to go to me, and I think they scale worse too for
> the fast path (no O_DIRECT or no fork). Identifying the gup closure
> points and replacing the raw put_page with gup_put_page would not be
> an useless effort though and I felt if the gup API was just a little
> bit more sophisticated I could simplify a bit the put_compound_page to
> serialize the race against split_huge_page_refcount, but this is an
> orthogonal issue with the mm-wide semaphore release addition which I
> personally dislike.

IIRC, the last time this came up, it kind of became stalled on this
point. Linus hated our "preemptive cow" approaches, and thought the
above approach was better.

I don't think we need to bother arguing details between our former
approaches until we get past this sticking point.

FWIW, I need to change get_user_pages semantics somewhat because we
have filesystems that cannot tolerate a set_page_dirty() to dirty a
clean page (it must only be dirtied with page_mkwrite).

This should probably require converting callers to use put_user_pages
and disallowing lock_page, mmap_sem, user-copy etc. within these
sections.

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