Re: [RFC V3] mm: change mm_advise_free to clear page dirty

[Minchan Kim <minchan@kernel.org>]

On Tue, Mar 03, 2015 at 11:59:17AM +0800, Wang, Yalin wrote:
> > -----Original Message-----
> > From: Minchan Kim [mailto:minchan.kim@gmail.com] On Behalf Of Minchan Kim
> > Sent: Tuesday, March 03, 2015 11:26 AM
> > 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'; Hugh Dickins; Cyrill Gorcunov
> > Subject: Re: [RFC V3] mm: change mm_advise_free to clear page dirty
> > 
> > Could you separte this patch in this patchset thread?
> > It's tackling differnt problem.
> > 
> > As well, I had a question to previous thread about why shared page
> > has a problem now but you didn't answer and send a new patchset.
> > It makes reviewers/maintainer time waste/confuse. Please, don't
> > hurry to send a code. Before that, resolve reviewers's comments.
> > 
> > On Tue, Mar 03, 2015 at 10:06:40AM +0800, Wang, Yalin wrote:
> > > This patch add ClearPageDirty() to clear AnonPage dirty flag,
> > > if not clear page dirty for this anon page, the page will never be
> > > treated as freeable. We also make sure the shared AnonPage is not
> > > freeable, we implement it by dirty all copyed AnonPage pte,
> > > so that make sure the Anonpage will not become freeable, unless
> > > all process which shared this page call madvise_free syscall.
> > 
> > Please, spend more time to make description clear. I really doubt
> > who understand this description without code inspection. :(
> > Of course, I'm not a person to write description clear like native
> > , either but just I'm sure I spend a more time to write description
> > rather than coding, at least. :)
> > 
> I see, I will send another mail for file private map pages.
> Sorry for my English expressions.
> I think your solution is ok,
> Your patch will make sure the anonpage pte will always be dirty.
> I add some comments for your patch:
> 
> > ---
> >  mm/madvise.c | 1 -
> >  mm/memory.c  | 9 +++++++--
> >  mm/rmap.c    | 2 +-
> >  mm/vmscan.c  | 3 +--
> >  4 files changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 6d0fcb8..d64200e 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -309,7 +309,6 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned
> > long addr,
> >  				continue;
> >  			}
> > 
> > -			ClearPageDirty(page);
> >  			unlock_page(page);
> >  		}
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 8ae52c9..2f45e77 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2460,9 +2460,14 @@ static int do_swap_page(struct mm_struct *mm, struct
> > vm_area_struct *vma,
> > 
> >  	inc_mm_counter_fast(mm, MM_ANONPAGES);
> >  	dec_mm_counter_fast(mm, MM_SWAPENTS);
> > -	pte = mk_pte(page, vma->vm_page_prot);
> > +
> > +	/*
> > +	 * Every page swapped-out was pte_dirty so we makes pte dirty again.
> > +	 * MADV_FREE relys on it.
> > +	 */
> > +	pte = mk_pte(pte_mkdirty(page), vma->vm_page_prot);
> pte_mkdirty() usage seems wrong here.

Argh, it reveals I didn't test even build. My shame.
But RFC tag might mitigate my shame. :)
I will fix it if I send a formal version.
Thanks for the review.

> 
> >  	if ((flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
> > -		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> > +		pte = maybe_mkwrite(pte, vma);
> >  		flags &= ~FAULT_FLAG_WRITE;
> >  		ret |= VM_FAULT_WRITE;
> >  		exclusive = 1;
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 47b3ba8..34c1d66 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1268,7 +1268,7 @@ static int try_to_unmap_one(struct page *page, struct
> > vm_area_struct *vma,
> > 
> >  		if (flags & TTU_FREE) {
> >  			VM_BUG_ON_PAGE(PageSwapCache(page), page);
> > -			if (!dirty && !PageDirty(page)) {
> > +			if (!dirty) {
> >  				/* It's a freeable page by MADV_FREE */
> >  				dec_mm_counter(mm, MM_ANONPAGES);
> >  				goto discard;
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 671e47e..7f520c9 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -805,8 +805,7 @@ static enum page_references
> > page_check_references(struct page *page,
> >  		return PAGEREF_KEEP;
> >  	}
> > 
> > -	if (PageAnon(page) && !pte_dirty && !PageSwapCache(page) &&
> > -			!PageDirty(page))
> > +	if (PageAnon(page) && !pte_dirty && !PageSwapCache(page))
> >  		*freeable = true;
> > 
> >  	/* Reclaim if clean, defer dirty pages to writeback */
> > --
> > 1.9.3
> Could we remove SetPageDirty(page); in try_to_free_swap() function based on this patch?
> Because your patch will make sure the pte is always dirty,
> We don't need setpagedirty(),
> The try_to_unmap() path will re-dirty the page during reclaim path,
> Isn't it?

I dont't know what side-effect we will have if we removes SetPageDirty.
It might regress on tmpfs which would page without pte.
I don't want to have such risk in this patch.
If you want it, you could suggest it separately if this patch lands.

-- 
Kind regards,
Minchan Kim

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