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

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Date:	Mon, 23 Apr 2007 21:58:20 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

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...

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.

View attachment "linux-2.6-madv-ppcfix.patch" of type "text/x-patch" (454 bytes)