linux-kernel - Re: [RFC][PATCH v3 2/6] mm, directio: fix fork vs direct-io race (read(2) side IOW gup(write) side)

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Message-ID: <20090414175124.GC9809@random.random>
Date:	Tue, 14 Apr 2009 19:51:24 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Hugh Dickins <hugh@...itas.com>,
	Zach Brown <zach.brown@...cle.com>,
	Andy Grover <andy.grover@...cle.com>
Subject: Re: [RFC][PATCH v3 2/6] mm, directio: fix fork vs direct-io race
	(read(2) side IOW gup(write) side)

On Tue, Apr 14, 2009 at 12:45:41PM -0400, Jeff Moyer wrote:
> So, if you're continuously submitting async read I/O, you will starve
> out the fork() call indefinitely.  I agree that you want to allow

IIRC rwsem good enough to stop the down_read when a down_write is
blocked. Otherwise page fault flood in threads would also starve any
mmap or similar call. Still with this approach fork will start to hang
indefinitely waiting for I/O, making it an I/O bound call, and not a
CPU call anymore, which may severely impact interactive-ness of
applications.

As long as fork is useful in the first place to provide memory
protection of different code with different
memory-corruption-trust-levels (otherwise nobody should use fork at
all, and vfork [or better spawn] should become the only option), then
fork from a thread pool is also reasonable. Either fork is totally
useless as a whole (which I wouldn't argue too much about), or if you
agree fork makes any sense, it can also make sense if intermixed with
clone(CLONE_VM) and hopefully it should behave CPU bound like CLONE_VM.
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