Re: [PATCH] lazy freeing of memory through MADV_FREE

Nick Piggin
2007-04-24T02:16:06Z

Rik van Riel wrote:
> This should fix the MADV_FREE code for PPC's hashed tlb.
>
> Signed-off-by: Rik van Riel <riel@redhat.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);

--
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