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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

On Wed, Sep 19, 2012 at 7:51 PM, Minchan Kim <minchan@kernel.org> wrote:
> On Wed, Sep 19, 2012 at 02:28:10PM -0400, Johannes Weiner wrote:
>> 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@kernel.org> 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@jp.fujitsu.com>
>> > > Cc: Mel Gorman <mgorman@suse.de>
>> > > Cc: Christoph Lameter <cl@linux.com>
>> > > Signed-off-by: Minchan Kim <minchan@kernel.org>
>> > > ---
>> > >  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@gmail.com>
>> > 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.
>>
>
> Here it goes.
>
> From c2c21b551811e034eb0ede6806e0314b201d7e5b Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Thu, 20 Sep 2012 08:39:52 +0900
> Subject: [PATCH] mm: revert 0def08e3, mm/mempolicy.c: check return code of
>  check_range
>
> This patch reverts 0def08e3 because check_range can't fail in
> migrate_to_node with considering current usecases.
>
> Quote from Johannes
> "
> 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
> "
>
> And I add new VM_BUG_ON for checking migrate_to_node's future usecase
> which might pass to MPOL_MF_STRICT.
>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Vasiliy Kulikov <segooon@gmail.com>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/mempolicy.c |    9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 3d64b36..9ec87bd 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -946,15 +946,16 @@ static int migrate_to_node(struct mm_struct *mm, int source, int dest,
>         nodemask_t nmask;
>         LIST_HEAD(pagelist);
>         int err = 0;
> -       struct vm_area_struct *vma;
>
>         nodes_clear(nmask);
>         node_set(source, nmask);
>
> -       vma = check_range(mm, mm->mmap->vm_start, mm->task_size, &nmask,
> +       /*
> +        * Collect migrate pages and it shoudn't be failed.
> +        */
> +       VM_BUG_ON(flags & MPOL_MF_STRICT);
> +       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)) {
>                 err = migrate_pages(&pagelist, new_node_page, dest,

Looks good. thank you.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

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