Re: [PATCH] hugetlb: call mmu notifiers on hugepage cow

[Doug Doan]

On 06/01/2010 11:16 PM, Andrew Morton wrote:
> On Thu, 27 May 2010 13:43:00 -0700 Doug Doan<dougd@xxxxxxxx>  wrote:
>
> > When a copy-on-write occurs, we take one of two paths in handle_mm_fault:
> > through handle_pte_fault for normal pages, or through hugetlb_fault for huge pages.
> >
> > In the normal page case, we eventually get to do_wp_page and call mmu notifiers
> > via ptep_clear_flush_notify. There is no callout to the mmmu notifiers in the
> > huge page case. This patch fixes that.
> >
> > Signed-off-by: Doug Doan<dougd@xxxxxxxx>
> > ---
> >
> > [patch  text/plain (802B)]
> > --- mm/hugetlb.c.orig	2010-05-27 13:07:58.569546314 -0700
> > +++ mm/hugetlb.c	2010-05-26 14:41:06.449296524 -0700
>
> (In patch -p1 form, please.  So a/mm/hugetlb.c)
>
> > @@ -2345,11 +2345,17 @@ retry_avoidcopy:
> >   	ptep = huge_pte_offset(mm, address&  huge_page_mask(h));
> >   	if (likely(pte_same(huge_ptep_get(ptep), pte))) {
> >   		/* Break COW */
> > +		mmu_notifier_invalidate_range_start(mm,
> > +			address&  huge_page_mask(h),
> > +			(address&  huge_page_mask(h)) + huge_page_size(h));
> >   		huge_ptep_clear_flush(vma, address, ptep);
> >   		set_huge_pte_at(mm, address, ptep,
> >   				make_huge_pte(vma, new_page, 1));
> >   		/* Make the old page be freed below */
> >   		new_page = old_page;
> > +		mmu_notifier_invalidate_range_end(mm,
> > +			address&  huge_page_mask(h),
> > +			(address&  huge_page_mask(h)) + huge_page_size(h));
> >   	}
> >   	page_cache_release(new_page);
> >   	page_cache_release(old_page);
>
> This causes mmu_notifier_invalidate_range_start() to be called under
> page_table_lock.  The immediately preceding code seems to take some
> care to avoid doing that.  I took a quick look at other callsites and
> cannot immediately see other cases where
> mmu_notifier_invalidate_range_start/end() are called under that lock.
>
> This may not introduce bugs with current notifier implementations (I
> didn't check), but it does lessen flexibility?

In the normal page case, handle_pte_fault calls do_wp_page inside a spinlock on 
ptl = pte_lockptr(mm, pmd), which uses mm->page_table_lock if USE_SPLIT_PTLOCKS 
is not defined.

I don't understand what you mean by lessen flexibilty.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/