Re: [PATCH] lazy freeing of memory through MADV_FREE

Rik van Riel
2007-04-24T01:58:42Z

[-- Attachment #1: Type: text/plain, Size: 1458 bytes --]

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...

--
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.

[-- Attachment #2: linux-2.6-madv-ppcfix.patch --]
[-- Type: text/x-patch, Size: 453 bytes --]

--- 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);