Re: [PATCH] lazy freeing of memory through MADV_FREE

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

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);


--
SUSE Labs, Novell Inc.
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