Re: [PATCH] mm: mempolicy: make mbind() return -EIO when MPOL_MF_STRICT is specified

[Yang Shi]

On 3/20/19 1:16 AM, Oscar Salvador wrote:
> On Wed, Mar 20, 2019 at 02:35:56AM +0800, Yang Shi wrote:
> > Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
> > Reported-by: Cyril Hrubis <chrubis@xxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Suggested-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> Hi Yang, thanks for the patch.
>
> Some observations below.
>
> >   	}
> >   	page = pmd_page(*pmd);
> > @@ -473,8 +480,15 @@ static int queue_pages_pmd(pmd_t *pmd, spinlock_t *ptl, unsigned long addr,
> >   	ret = 1;
> >   	flags = qp->flags;
> >   	/* go to thp migration */
> > -	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL))
> > +	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> > +		if (!vma_migratable(walk->vma)) {
> > +			ret = -EIO;
> > +			goto unlock;
> > +		}
> > +
> >   		migrate_page_add(page, qp->pagelist, flags);
> > +	} else
> > +		ret = -EIO;
> 	if (!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
>         	        !vma_migratable(walk->vma)) {
>                 	ret = -EIO;
>                  goto unlock;
>          }
>
> 	migrate_page_add(page, qp->pagelist, flags);
> unlock:
>          spin_unlock(ptl);
> out:
>          return ret;
>
> seems more clean to me?

Yes, it sounds so.

> >   unlock:
> >   	spin_unlock(ptl);
> >   out:
> > @@ -499,8 +513,10 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
> >   	ptl = pmd_trans_huge_lock(pmd, vma);
> >   	if (ptl) {
> >   		ret = queue_pages_pmd(pmd, ptl, addr, end, walk);
> > -		if (ret)
> > +		if (ret > 0)
> >   			return 0;
> > +		else if (ret < 0)
> > +			return ret;
> I would go with the following, but that's a matter of taste I guess.
>
> if (ret < 0)
> 	return ret;
> else
> 	return 0;

No, this is not correct. queue_pages_pmd() may return 0, which means THP 
gets split. If it returns 0 the code should just fall through instead of 
returning.

> >   	}
> >   
> >   	if (pmd_trans_unstable(pmd))
> > @@ -521,11 +537,16 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
> >   			continue;
> >   		if (!queue_pages_required(page, qp))
> >   			continue;
> > -		migrate_page_add(page, qp->pagelist, flags);
> > +		if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
> > +			if (!vma_migratable(vma))
> > +				break;
> > +			migrate_page_add(page, qp->pagelist, flags);
> > +		} else
> > +			break;
> I might be missing something, but AFAICS neither vma nor flags is going to change
> while we are in queue_pages_pte_range(), so, could not we move the check just
> above the loop?
> In that way, 1) we only perform the check once and 2) if we enter the loop
> we know that we are going to do some work, so, something like:
>
> index af171ccb56a2..7c0e44389826 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -487,6 +487,9 @@ static int queue_pages_pte_range(pmd_t *pmd, unsigned long addr,
>          if (pmd_trans_unstable(pmd))
>                  return 0;
>   
> +       if (!(flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) || !vma_migratable(vma))
> +               return -EIO;

It sounds not correct to me. We need check if there is existing page on 
the node which is not allowed by the policy. This is what 
queue_pages_required() does.

Thanks,
Yang

> +
>          pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>          for (; addr != end; pte++, addr += PAGE_SIZE) {
>                  if (!pte_present(*pte))
>
>
> >   	}
> >   	pte_unmap_unlock(pte - 1, ptl);
> >   	cond_resched();
> > -	return 0;
> > +	return addr != end ? -EIO : 0;
> If we can do the above, we can leave the return value as it was.