Date: Tue, 13 Dec 2005 23:07:58 +0200
From: "Michael S. Tsirkin" <mst@mellanox.co.il>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Hugh Dickins <hugh@veritas.com>, Gleb Natapov <gleb@minantech.com>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Petr Vandrovec <vandrove@vc.cvut.cz>,
       Badari Pulavarty <pbadari@us.ibm.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: set_page_dirty vs set_page_dirty_lock
Message-ID: <20051213210757.GC6715@mellanox.co.il>
Reply-To: "Michael S. Tsirkin" <mst@mellanox.co.il>
References: <439D417E.903@yahoo.com.au>
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Quoting  Nick Piggin <nickpiggin@yahoo.com.au>:
> > 
> >>or try to do something tricky with page_count to determine
> >>if we need to do a copy in fork() rather than a COW.
> > 
> > 
> > I'm actually reasonably happy with the trick that I'm using:
> > performing a second get_user_pages after DMA and comparing
> > the page lists.
> > However, doing this every time on the off chance that a
> > page was made COW forces me into task context, every time.
> > 
> 
> I think it might be possible to solve it with the early-copy in
> fork(). I'll tinker with it.

A possible way to do this would be adding an option to get_user_pages,
and if selected, setting a flag on all pages in range.
This flag should be cleared in put_page when the reference count drops to 1.

The only thing left would be to do the actual copy if the flag is set.

HTH,

-- 
MST
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