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

["lixinhai.lxh@gmail.com" <lixinhai.lxh@gmail.com>]

On 2019-11-14 at 17:30 Naoya Horiguchi wrote:
>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> 

Thanks for review, I'v sent out new patch with same title
- Xinhai