Re: [PATCH] mm: fix NR_ISOLATED_[ANON|FILE] mismatch

[Johannes Weiner]

On Wed, Sep 19, 2012 at 01:04:56PM -0400, KOSAKI Motohiro wrote:
> On Wed, Sep 19, 2012 at 3:45 AM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > When I looked at zone stat mismatch problem, I found
> > migrate_to_node doesn't decrease NR_ISOLATED_[ANON|FILE]
> > if check_range fails.

This is a bit misleading.  It's not that the stats would be
inaccurate, it's that the pages would be leaked from the LRU, no?

> > It can make system hang out.

Did you spot this by code review only or did you actually run into
this?  Because...

> > Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Christoph Lameter <cl@xxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > ---
> >  mm/mempolicy.c |   16 ++++++++--------
> >  1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 3d64b36..6bf0860 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -953,16 +953,16 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
> >
> >         vma = check_range(mm, mm->mmap->vm_start, mm->task_size, &nmask,
> >                         flags | MPOL_MF_DISCONTIG_OK, &pagelist);
> > -       if (IS_ERR(vma))
> > -               return PTR_ERR(vma);
> > -
> > -       if (!list_empty(&pagelist)) {
> > +       if (IS_ERR(vma)) {
> > +               err = PTR_ERR(vma);
> > +               goto out;
> > +       }
> > +       if (!list_empty(&pagelist))
> >                 err = migrate_pages(&pagelist, new_node_page, dest,
> >                                                         false, MIGRATE_SYNC);
> > -               if (err)
> > -                       putback_lru_pages(&pagelist);
> > -       }
> > -
> > +out:
> > +       if (err)
> > +               putback_lru_pages(&pagelist);
> 
> Good catch!
> This is a regression since following commit. So, I doubt we need
> all or nothing semantics. Can we revert it instead? (and probably
> we need more kind comment for preventing an accident)

I think it makes sense to revert.  Not because of the semantics, but I
just don't see how check_range() could even fail for this callsite:

1. we pass mm->mmap->vm_start in there, so we should not fail due to
   find_vma()

2. we pass MPOL_MF_DISCONTIG_OK, so the discontig checks do not apply
   and so can not fail

3. we pass MPOL_MF_MOVE | MPOL_MF_MOVE_ALL, the page table loops will
   continue until addr == end, so we never fail with -EIO

> commit 0def08e3acc2c9c934e4671487029aed52202d42
> Author: Vasiliy Kulikov <segooon@xxxxxxxxx>
> Date:   Tue Oct 26 14:21:32 2010 -0700
> 
>     mm/mempolicy.c: check return code of check_range

We don't use this code to "check" the range, we use it to collect
migrate pages.  There is no failure case.
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