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

[Michal Hocko <mhocko@suse.cz>]

On Sat 28-02-15 10:11:13, Wang, Yalin wrote:
> > -----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,

AFAIR MADV_FREE was intended only for private anon mappings. What would
be the use case for MADV_FREE on file backed private mappings?

[...]

-- 
Michal Hocko
SUSE Labs