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

["Wang, Yalin" <Yalin.Wang@sonymobile.com>]

> -----Original Message-----
> From: Michal Hocko [mailto:mstsxfx@gmail.com] On Behalf Of Michal Hocko
> Sent: Saturday, February 28, 2015 5:03 AM
> To: Wang, Yalin
> Cc: 'Minchan Kim'; 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 27-02-15 11:37:18, 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.
> 
> Very well spotted! I haven't noticed that during the review.
> 
> > 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;
> 
> PageAnon check seems to be redundant because we are not allowing
> MADV_FREE on any !anon private mappings AFAIR.
I only see this check:
/* MADV_FREE works for only anon vma at the moment */
	if (vma->vm_file)
		return -EINVAL;

but for file private map, there are also AnonPage sometimes, do we need change
to like this:
	if (vma->vm_flags & VM_SHARED)
		return -EINVAL;
> >
> >  		if (PageSwapCache(page)) {
> > -			if (!trylock_page(page))
> > +			if (!try_to_free_swap(page))
> >  				continue;
> 
> You need to unlock the page here.
Good spot.

> > -
> > -			if (!try_to_free_swap(page)) {
> > -				unlock_page(page);
> > -				continue;
> > -			}
> > -
> > -			ClearPageDirty(page);
> > -			unlock_page(page);
> >  		}
> >
> > +		if (page_mapcount(page) == 1)
> > +			ClearPageDirty(page);
> 
> Please add a comment about why we need to ClearPageDirty even
> !PageSwapCache. Anon pages are usually not marked dirty AFAIR. The
> reason seem to be racing try_to_free_swap which sets the page that way
> (although I do not seem to remember why are we doing that in the first
> place...)
> 
Use page_mapcount to judge if a page can be clear dirty flag seems
Not a very good solution, that is because we don't know how many
ptes are share this page, I am thinking if there is some good solution
For shared AnonPage.