Date: Thu, 15 Sep 2005 17:47:02 +0200
From: Andrea Arcangeli <andrea@suse.de>
To: Linus Torvalds <torvalds@osdl.org>
Cc: Hugh Dickins <hugh@veritas.com>, Nick Piggin <npiggin@novell.com>,
       linux-kernel@vger.kernel.org, Andrew Morton <akpm@osdl.org>,
       Roland McGrath <roland@redhat.com>
Subject: Re: ptrace can't be transparent on readonly MAP_SHARED
Message-ID: <20050915154702.GA4122@opteron.random>
References: <20050914212405.GD4966@opteron.random> <Pine.LNX.4.61.0509151337260.16231@goblin.wat.veritas.com> <Pine.LNX.4.58.0509150805150.26803@g5.osdl.org>
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On Thu, Sep 15, 2005 at 08:12:59AM -0700, Linus Torvalds wrote:
> have a PROT_READONLY/PROT_NONE area that is visible from the debugger, but
> continues to cause SIGSEGV's if the user process itself tries to access
> it. To me, that's good.

Continue to cause sigsegv yes, but on the wrong page, when it will read
the page it can contain different data compared to what is on
disk/pagecache.

> There would have to be some real advantage to _not_ doing what we're doing 
> now. And I don't see an advantage.

The advantage is a faster fast path and less special cases to keep in
mind.

> The real complexity is not "maybe_mkwrite()", which is trivial. The real 

It is trivial yes, but for it to work without deadlocks, it requires
non-trivial changes to the page fault handler and get_user_pages.

I guess this is mostly a matter of taste, but my taste is about keeping
it simple and fast (though the difference is certainly not measurable).

Thanks.
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