linux-kernel - Re: [patch 0/4] [RFC] MMU Notifiers V1

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Message-ID: <479DFE7F.9030305@qumranet.com>
Date:	Mon, 28 Jan 2008 18:10:39 +0200
From:	Izik Eidus <izike@...ranet.com>
To:	Andrea Arcangeli <andrea@...ranet.com>
CC:	Christoph Lameter <clameter@....com>, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, Nick Piggin <npiggin@...e.de>,
	kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 0/4] [RFC] MMU Notifiers V1

Andrea Arcangeli wrote:
> On Thu, Jan 24, 2008 at 09:56:06PM -0800, Christoph Lameter wrote:
>   
>> Andrea's mmu_notifier #4 -> RFC V1
>>
>> - Merge subsystem rmap based with Linux rmap based approach
>> - Move Linux rmap based notifiers out of macro
>> - Try to account for what locks are held while the notifiers are
>>   called.
>> - Develop a patch sequence that separates out the different types of
>>   hooks so that it is easier to review their use.
>> - Avoid adding #include to linux/mm_types.h
>> - Integrate RCU logic suggested by Peter.
>>     
>
> I'm glad you're converging on something a bit saner and much much
> closer to my code, plus perfectly usable by KVM optimal rmap design
> too. It would have preferred if you would have sent me patches like
> Peter did for review and merging etc... that would have made review
> especially easier. Anyway I'm used to that on lkml so it's ok, I just
> need this patch to be included in mainline, everything else is
> irrelevant to me.
>
> On a technical merit this still partially makes me sick and I think
> it's the last issue to debate.
>
> @@ -971,6 +974,9 @@ int try_to_unmap(struct page *page, int 
>         else
>                 ret = try_to_unmap_file(page, migration);
>
> +       if (unlikely(PageExternalRmap(page)))
> +               mmu_rmap_notifier(invalidate_page, page);
> +
>         if (!page_mapped(page))
>                 ret = SWAP_SUCCESS;
>         return ret;
>
> I find the above hard to accept, because the moment you work with
> physical pages and not "mm+address" I think you couldn't possibly care
> if page_mapped is true or false, and I think the above notifier should
> be called _outside_ try_to_unmap. Infact I'd call
> mmu_rmap_notifier(invalidate_page, page); only if page_unmapped is
> false and the linux pte is gone already (practically just before the
> page_count == 2 check and after try_to_unmap).
>   

i dont understand how is that better than notification on tlb flush?
i mean cpus have very smiler problem as we do,
so why not deal with the notification at the same place as they do (tlb 
flush) ?

moreover notification on tlb flush allow unmodified applications to work 
with us
for example the memory merging driver that i wrote was able to work with kvm
without any change to its source code.
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