Re: [PATCH] lazy freeing of memory through MADV_FREE

[Nick Piggin <nickpiggin@yahoo.com.au>]

Rik van Riel wrote:
> Use TLB batching for MADV_FREE.  Adds another 10-15% extra performance
> to the MySQL sysbench results on my quad core system.
> 
> Signed-off-by: Rik van Riel <riel@redhat.com>
> ---
> 
> 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.


>>> 4) because we flush the tlb before releasing the page
>>>    table lock, other CPUs cannot remove this page from
>>>    the address space - they will block on the page
>>>    table lock before looking at this pte
>>
>>
>> We don't when the ptl is split.
> 
> 
> 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.

-- 
SUSE Labs, Novell Inc.