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

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Sep 20, 2012 at 08:51:56AM +0900, Minchan Kim wrote:
> 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);

Adding a check and a comment is a good idea, but I'm not a big fan of
checking for MPOL_MF_STRICT in particular because it's one of the
invalid inputs, and so you need to extend this check when somebody
extends the spectrum of invalid inputs.  I would much prefer checking
directly for !(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) instead, which
would also make the possible inputs apparent without having to chase
up the call chain to find out what is usually passed in.

And how about

/*
 * This does not "check" the range but isolates all pages that
 * need migration.  Between passing in the full user address
 * space range and MPOL_MF_DISCONTIG_OK, this call can not fail.
 */

?

> +	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,
> -- 
> 1.7.9.5
> 
> -- 
> Kind regards,
> Minchan Kim

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