linux-kernel - Re: [PATCH] lazy freeing of memory through MADV

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Date:	Tue, 24 Apr 2007 12:16:06 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Rik van Riel <riel@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, shak <dshaks@...hat.com>,
	jakub@...hat.com, drepper@...hat.com
Subject: Re: [PATCH] lazy freeing of memory through MADV_FREE

Rik van Riel wrote:
> This should fix the MADV_FREE code for PPC's hashed tlb.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
> 
> Nick Piggin wrote:
> 
>>> Nick Piggin wrote:
>>>
>>>>> 3) because of this, we can treat any such accesses as
>>>>>    happening simultaneously with the MADV_FREE and
>>>>>    as illegal, aka undefined behaviour territory and
>>>>>    we do not need to worry about them
>>>>
>>>>
>>>>
>>>> Yes, but I'm wondering if it is legal in all architectures.
>>>
>>>
>>>
>>> It's similar to trying to access memory during an munmap.
>>>
>>> You may be able to for a short time, but it'll come back to
>>> haunt you.
>>
>>
>> The question is whether the architecture specific tlb
>> flushing code will break or not.
> 
> 
> I guess we'll need to call tlb_remove_tlb_entry() inside the
> MADV_FREE code to keep powerpc happy.
> 
> Thanks for pointing this one out.
> 
>>> Even then we do.  Each invocation of zap_pte_range() only touches
>>> one page table page, and it flushes the TLB before releasing the
>>> page table lock.
>>
>>
>> What kernel are you looking at? -rc7 and rc6-mm1 don't, AFAIKS.
> 
> 
> Oh dear.  I see it now...
> 
> The tlb end things inside zap_pte_range() are actually
> noops and the actual tlb flush only happens inside
> zap_page_range().
> 
> I guess the fact that munmap gets the mmap_sem for
> writing should save us, though...

What about an unmap_mapping_range, or another MADV_FREE or
MADV_DONTNEED?

> 
> 
> ------------------------------------------------------------------------
> 
> --- linux-2.6.20.x86_64/mm/memory.c.noppc	2007-04-23 21:50:09.000000000 -0400
> +++ linux-2.6.20.x86_64/mm/memory.c	2007-04-23 21:48:59.000000000 -0400
> @@ -679,6 +679,7 @@ static unsigned long zap_pte_range(struc
>  					}
>  					ptep_test_and_clear_dirty(vma, addr, pte);
>  					ptep_test_and_clear_young(vma, addr, pte);
> +					tlb_remove_tlb_entry(tlb, pte, addr);
>  					SetPageLazyFree(page);
>  					if (PageActive(page))
>  						deactivate_tail_page(page);


-- 
SUSE Labs, Novell Inc.
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