Re: [PATCH v4 2/2] mm: Fix checking unmapped holes for mbind

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Fri, Nov 08, 2019 at 09:01:44PM +0800, Li Xinhai wrote:
> mbind() is required to report EFAULT if range, specified by addr and len,
> contains unmapped holes. In current implementation, below rules are applied
> for this checking:
> 1 Unmapped holes at any part of the specified range should be reported as
>   EFAULT if mbind() for none MPOL_DEFAULT cases;
> 2 Unmapped holes at any part of the specified range should be ignored (do
>   not reprot EFAULT) if mbind() for MPOL_DEFAULT case;
> 3 The whole range in an unmapped hole should be reported as EFAULT;
> Note that rule 2 does not fullfill the mbind() API definition, but since
> that behavior has existed for long days (the internal flag
> MPOL_MF_DISCONTIG_OK is for this purpose), this patch does not plan to
> change it.
> 
> In current code, application observed inconsistent behavior on rule 1 and
> rule 2 respectively. That inconsistency is fixed as below details.
> 
> Cases of rule 1:
> 1) Hole at head side of range. Current code reprot EFAULT, no change by
> this patch.
> [  vma  ][ hole ][  vma  ]
>             [  range  ]
> 2) Hole at middle of range. Current code report EFAULT, no change by
> this patch.
> [  vma  ][ hole ][ vma ]
>    [     range      ]
> 3) Hole at tail side of range. Current code do not report EFAULT, this
> patch fix it.
> [  vma  ][ hole ][ vma ]
>    [  range  ]
> 
> Cases of rule 2:
> 1) Hole at head side of range. Current code reprot EFAULT, this patch
> fix it.
> [  vma  ][ hole ][  vma  ]
>             [  range  ]
> 2) Hole at middle of range. Current code do not report EFAULT, no change
> by this patch.
> this patch.
> [  vma  ][ hole ][ vma]
>    [     range      ]
> 3) Hole at tail side of range. Current code do not report EFAULT, no
> change by this patch.
> [  vma  ][ hole ][ vma]
>    [  range  ]
> 
> This patch has no changes to rule 3.
> 
> The unmapped hole checking can also be handled by using .pte_hole(),
> instead of .test_walk(). But .pte_hole() is called for holes inside and
> outside vma, which causes more cost, so this patch keeps the original
> design with .test_walk().
> 
> Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: linux-man <linux-man@vger.kernel.org>
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> ---
>  mm/mempolicy.c | 40 +++++++++++++++++++++++++++-------------
>  1 file changed, 27 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 807f06f..c697b29 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -410,7 +410,9 @@ struct queue_pages {
>  	struct list_head *pagelist;
>  	unsigned long flags;
>  	nodemask_t *nmask;
> -	struct vm_area_struct *prev;
> +	unsigned long start;
> +	unsigned long end;
> +	struct vm_area_struct *first;
>  };
>  
>  /*
> @@ -619,14 +621,20 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>  	unsigned long flags = qp->flags;
>  
>  	/* range check first */
> -	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
> -		if (!vma->vm_next && vma->vm_end < end)
> -			return -EFAULT;
> -		if (qp->prev && qp->prev->vm_end < vma->vm_start)
> +	VM_BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
> +
> +	if (!qp->first) {
> +		qp->first = vma;
> +		if (!(flags & MPOL_MF_DISCONTIG_OK) &&
> +			(qp->start < vma->vm_start))
> +			/* hole at head side of range */
>  			return -EFAULT;
>  	}
> -
> -	qp->prev = vma;
> +	if (!(flags & MPOL_MF_DISCONTIG_OK) &&
> +		((vma->vm_end < qp->end) && 

You here have a trailing whitespace.

Otherwise, looks good to me.

Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>