Re: [RFC] mm: change mm_advise_free to clear page dirty

[Minchan Kim <minchan@kernel.org>]

On Fri, Feb 27, 2015 at 10:37:14PM +0900, Minchan Kim wrote:
> On Fri, Feb 27, 2015 at 03:50:29PM +0800, Wang, Yalin wrote:
> > > -----Original Message-----
> > > From: Minchan Kim [mailto:minchan.kim@gmail.com] On Behalf Of Minchan Kim
> > > Sent: Friday, February 27, 2015 2:44 PM
> > > To: Wang, Yalin
> > > Cc: Michal Hocko; Andrew Morton; linux-kernel@vger.kernel.org; linux-
> > > mm@kvack.org; Rik van Riel; Johannes Weiner; Mel Gorman; Shaohua Li
> > > Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty
> > > 
> > > On Fri, Feb 27, 2015 at 01:48:48PM +0800, Wang, Yalin wrote:
> > > > > -----Original Message-----
> > > > > From: Minchan Kim [mailto:minchan.kim@gmail.com] On Behalf Of Minchan
> > > Kim
> > > > > Sent: Friday, February 27, 2015 1:28 PM
> > > > > To: Wang, Yalin
> > > > > Cc: Michal Hocko; Andrew Morton; linux-kernel@vger.kernel.org; linux-
> > > > > mm@kvack.org; Rik van Riel; Johannes Weiner; Mel Gorman; Shaohua Li
> > > > > Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty
> > > > >
> > > > > Hello,
> > > > >
> > > > > On Fri, Feb 27, 2015 at 11:37:18AM +0800, Wang, Yalin wrote:
> > > > > > This patch add ClearPageDirty() to clear AnonPage dirty flag,
> > > > > > the Anonpage mapcount must be 1, so that this page is only used by
> > > > > > the current process, not shared by other process like fork().
> > > > > > if not clear page dirty for this anon page, the page will never be
> > > > > > treated as freeable.
> > > > >
> > > > > In case of anonymous page, it has PG_dirty when VM adds it to
> > > > > swap cache and clear it in clear_page_dirty_for_io. That's why
> > > > > I added ClearPageDirty if we found it in swapcache.
> > > > > What case am I missing? It would be better to understand if you
> > > > > describe specific scenario.
> > > > >
> > > > > Thanks.
> > > > >
> > > > > >
> > > > > > Signed-off-by: Yalin Wang <yalin.wang@sonymobile.com>
> > > > > > ---
> > > > > >  mm/madvise.c | 15 +++++----------
> > > > > >  1 file changed, 5 insertions(+), 10 deletions(-)
> > > > > >
> > > > > > diff --git a/mm/madvise.c b/mm/madvise.c
> > > > > > index 6d0fcb8..257925a 100644
> > > > > > --- a/mm/madvise.c
> > > > > > +++ b/mm/madvise.c
> > > > > > @@ -297,22 +297,17 @@ static int madvise_free_pte_range(pmd_t *pmd,
> > > > > unsigned long addr,
> > > > > >  			continue;
> > > > > >
> > > > > >  		page = vm_normal_page(vma, addr, ptent);
> > > > > > -		if (!page)
> > > > > > +		if (!page || !PageAnon(page) || !trylock_page(page))
> > > > > >  			continue;
> > > > > >
> > > > > >  		if (PageSwapCache(page)) {
> > > > > > -			if (!trylock_page(page))
> > > > > > +			if (!try_to_free_swap(page))
> > > > > >  				continue;
> > > > > > -
> > > > > > -			if (!try_to_free_swap(page)) {
> > > > > > -				unlock_page(page);
> > > > > > -				continue;
> > > > > > -			}
> > > > > > -
> > > > > > -			ClearPageDirty(page);
> > > > > > -			unlock_page(page);
> > > > > >  		}
> > > > > >
> > > > > > +		if (page_mapcount(page) == 1)
> > > > > > +			ClearPageDirty(page);
> > > > > > +		unlock_page(page);
> > > > > >  		/*
> > > > > >  		 * Some of architecture(ex, PPC) don't update TLB
> > > > > >  		 * with set_pte_at and tlb_remove_tlb_entry so for
> > > > > > --
> > > > Yes, for page which is in SwapCache, it is correct,
> > > > But for anon page which is not in SwapCache, it is always
> > > > PageDirty(), so we should also clear dirty bit to make it freeable,
> > > 
> > > No. Every anon page starts from !PageDirty and it has PG_dirty
> > > only when it's addeded into swap cache. If vm_swap_full turns on,
> > > a page in swap cache could have PG_dirty via try_to_free_swap again.
> > 
> > mmm..
> > sometimes you can see an anon page PageDirty(), but it is not in swapcache,
> > for example, handle_pte_fault()-->do_swap_page()-->try_to_free_swap(),
> > at this time, the page is deleted from swapcache and is marked PageDirty(),
> 
> That's what I missed. It's clear and would be simple patch so
> could you send a patch to fix this issue with detailed description
> like above?
> 
> > 
> > 
> > > So, Do you have concern about swapped-out pages when MADV_FREE is
> > > called? If so, please look at my patch.
> > > 
> > > https://lkml.org/lkml/2015/2/25/43
> > > 
> > > It will zap the swapped out page. So, this is not a issue any more?
> > > 
> > > >
> > > > Another problem  is that if an anon page is shared by more than one
> > > process,
> > > > This happened when fork(), the anon page will be copy on write,
> > > > In this case, we should not clear page dirty,
> > > > This is not correct for other process which don't call MADV_FREE syscall.
> > > 
> > > You mean we shouldn't inherit MADV_FREE attribute?
> > > Why?
> > 
> > Is it correct behavior if code like this:
> > 
> > Parent:
> > ptr1 = malloc(len);
> > memset(ptr1, 'a', len);
> > fork();
> > if (I am parent)
> > 	madvise_free(ptr1, len);
> > 
> > child:
> > sleep(10);
> > parse_data(ptr1, len);  // child may see zero, not 'a',
> > 			// is it the right behavior that the programer want?
> > 
> > Because child don't call madvise_free(), so it should see 'a', not zero page.
> > Isn't it ?
> 
> You're absolutely right. Thanks.
> But I doubt your fix is best. Most of fork will do exec soonish so
> it's not a good idea to make MADV_FREE void even though hinted pages
> are shared when the syscall was called.
> How about checking the page is shared or not in reclaim path?
> If it is still shared, we shouldn't discard it.

I got confused. With looking at copy_one_pte, it copys from src_pte
and not clear dirty bit if it's not a shared mapping.
If so, in your example, child pte has pte dirty bit on while parent
has clean bit by madvise_free so that VM shouldn't discard the page.
No?

> 
> Thanks.
> 
> > Thanks
> > 
> > 
> > 
> > 
> > 
> > 
> 
> -- 
> Kind regards,
> Minchan Kim

-- 
Kind regards,
Minchan Kim