linux-kernel - Re: [patch 3/6] mmu_notifier: invalidate

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Message-Id: <20080215193736.9d6e7da3.akpm@linux-foundation.org>
Date:	Fri, 15 Feb 2008 19:37:36 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Andrea Arcangeli <andrea@...ranet.com>, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [patch 3/6] mmu_notifier: invalidate_page callbacks

On Thu, 14 Feb 2008 22:49:02 -0800 Christoph Lameter <clameter@....com> wrote:

> Two callbacks to remove individual pages as done in rmap code
> 
> 	invalidate_page()
> 
> Called from the inner loop of rmap walks to invalidate pages.
> 
> 	age_page()
> 
> Called for the determination of the page referenced status.
> 
> If we do not care about page referenced status then an age_page callback
> may be be omitted. PageLock and pte lock are held when either of the
> functions is called.

The age_page mystery shallows.

It would be useful to have some rationale somewhere in the patchset for the
existence of this callback.

>  #include <asm/tlbflush.h>
>  
> @@ -287,7 +288,8 @@ static int page_referenced_one(struct pa
>  	if (vma->vm_flags & VM_LOCKED) {
>  		referenced++;
>  		*mapcount = 1;	/* break early from loop */
> -	} else if (ptep_clear_flush_young(vma, address, pte))
> +	} else if (ptep_clear_flush_young(vma, address, pte) |
> +		   mmu_notifier_age_page(mm, address))
>  		referenced++;

The "|" is obviously deliberate.  But no explanation is provided telling us
why we still call the callback if ptep_clear_flush_young() said the page
was recently referenced.  People who read your code will want to understand
this.

>  	/* Pretend the page is referenced if the task has the
> @@ -455,6 +457,7 @@ static int page_mkclean_one(struct page 
>  
>  		flush_cache_page(vma, address, pte_pfn(*pte));
>  		entry = ptep_clear_flush(vma, address, pte);
> +		mmu_notifier(invalidate_page, mm, address);

I just don't see how ths can be done if the callee has another thread in
the middle of establishing IO against this region of memory. 
->invalidate_page() _has_ to be able to block.  Confused.


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