Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Li Xinhai]

On 2019-10-29 at 17:56 Li Xinhai wrote:
>queue_pages_range() will check for unmapped holes besides queue pages for
>migration. The rules for checking unmapped holes are:
>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 if
>  mbind() for MPOL_DEFAULT case;
>Note that the second rule is the current implementation, but it seems
>conflicts the Linux API definition.
>
>queue_pages_test_walk() is fixed by introduce new fields in struct
>queue_pages which help to check:
>1 holes at head and tail side of specified range;
>2 the whole range is in a hole;
>
>Besides, queue_pages_test_walk() must update previous vma record no matter
>the current vma should be considered for queue pages or not.
> 

More details about current issue (which breaks the mbind() API definition):
1. In queue_pages_test_walk()
checking on (!vma->vm_next && vma->vm_end < end) would never success, 
because 'end' passed from walk_page_test() make sure "end <=  vma->vm_end". so hole 
beyond the last vma can't be detected. 

2. queue_pages_test_walk() only called for vma, and 'start', 'end' parameters are guaranteed
within vma. Then, if the range starting or ending in an unmapped hole,  
queue_pages_test_walk() don't have chance to be called to check. In other words, the 
current code can detect this case (range span over hole):
[  vma  ][ hole ][ vma]
   [     range      ]
but cant not detect these case :
[  vma  ][ hole ][ vma]
   [  range  ]
[  vma  ][ hole ][  vma  ]
            [  range  ]

3. a checking in mbind_range() try to recover if the hole is in head side, but can't 
recover if hole is in tail side of range.

- Xinhai

>Fixes: 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
>Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
>Fixes: 48684a65b4e3 ("mm: pagewalk: fix misbehavior of walk_page_range for vma(VM_PFNMAP)")
>Signed-off-by: Li Xinhai <lixinhai.li@gmail.com>
>---
>Changes in v2:
>  - Fix the unmapped holes checking in queue_pages_test_walk() instead of 
>    mbind_rnage().
>
> mm/mempolicy.c | 44 +++++++++++++++++++++++++++++---------------
> 1 file changed, 29 insertions(+), 15 deletions(-)
>
>diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>index 4ae967bcf954..24087dfa4dcd 100644
>--- a/mm/mempolicy.c
>+++ b/mm/mempolicy.c
>@@ -411,6 +411,9 @@ struct queue_pages {
> 	unsigned long flags;
> 	nodemask_t *nmask;
> 	struct vm_area_struct *prev;
>+	unsigned long start;
>+	unsigned long end;
>+	int in_hole;
> };
> 
> /*
>@@ -618,28 +621,31 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
> 	unsigned long endvma = vma->vm_end;
> 	unsigned long flags = qp->flags;
> 
>-	/*
>-	* Need check MPOL_MF_STRICT to return -EIO if possible
>-	* regardless of vma_migratable
>-	*/
>-	if (!vma_migratable(vma) &&
>-	   !(flags & MPOL_MF_STRICT))
>-	return 1;
>-
>+	/* range check first */
> 	if (endvma > end)
> 	endvma = end;
>-	if (vma->vm_start > start)
>-	start = vma->vm_start;
>+	BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
> 
>+	qp->in_hole = 0;
> 	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
>-	if (!vma->vm_next && vma->vm_end < end)
>+	if ((!vma->vm_next && vma->vm_end < qp->end) ||
>+	(vma->vm_next && qp->end < vma->vm_next->vm_start))
> 	return -EFAULT;
>-	if (qp->prev && qp->prev->vm_end < vma->vm_start)
>+	if ((qp->prev && qp->prev->vm_end < vma->vm_start) ||
>+	(!qp->prev && qp->start < vma->vm_start))
> 	return -EFAULT;
> 	}
> 
> 	qp->prev = vma;
> 
>+	/*
>+	* Need check MPOL_MF_STRICT to return -EIO if possible
>+	* regardless of vma_migratable
>+	*/
>+	if (!vma_migratable(vma) &&
>+	   !(flags & MPOL_MF_STRICT))
>+	return 1;
>+
> 	if (flags & MPOL_MF_LAZY) {
> 	/* Similar to task_numa_work, skip inaccessible VMAs */
> 	if (!is_vm_hugetlb_page(vma) &&
>@@ -679,14 +685,23 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
> 	nodemask_t *nodes, unsigned long flags,
> 	struct list_head *pagelist)
> {
>+	int err;
> 	struct queue_pages qp = {
> 	.pagelist = pagelist,
> 	.flags = flags,
> 	.nmask = nodes,
> 	.prev = NULL,
>+	.start = start,
>+	.end = end,
>+	.in_hole = 1,
> 	};
> 
>-	return walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
>+	err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
>+	/* whole range in unmapped hole */
>+	if (qp->in_hole && !(flags & MPOL_MF_DISCONTIG_OK))
>+	err = -EFAULT;
>+
>+	return err;
> }
> 
> /*
>@@ -738,8 +753,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> 	unsigned long vmend;
> 
> 	vma = find_vma(mm, start);
>-	if (!vma || vma->vm_start > start)
>-	return -EFAULT;
>+	BUG_ON(!vma);
> 
> 	prev = vma->vm_prev;
> 	if (start > vma->vm_start)
>-- 
>2.22.0

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Naoya Horiguchi]

On Thu, Oct 31, 2019 at 11:15:09AM +0800, Li Xinhai wrote:
> On 2019-10-29 at 17:56 Li Xinhai wrote:
> >queue_pages_range() will check for unmapped holes besides queue pages for
> >migration. The rules for checking unmapped holes are:
> >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 if
> >  mbind() for MPOL_DEFAULT case;
> >Note that the second rule is the current implementation, but it seems
> >conflicts the Linux API definition.
> >
> >queue_pages_test_walk() is fixed by introduce new fields in struct
> >queue_pages which help to check:
> >1 holes at head and tail side of specified range;
> >2 the whole range is in a hole;
> >
> >Besides, queue_pages_test_walk() must update previous vma record no matter
> >the current vma should be considered for queue pages or not.
> > 
> 
> More details about current issue (which breaks the mbind() API definition):
> 1. In queue_pages_test_walk()
> checking on (!vma->vm_next && vma->vm_end < end) would never success, 
> because 'end' passed from walk_page_test() make sure "end <=  vma->vm_end". so hole 
> beyond the last vma can't be detected. 
> 
> 2. queue_pages_test_walk() only called for vma, and 'start', 'end' parameters are guaranteed
> within vma. Then, if the range starting or ending in an unmapped hole,  
> queue_pages_test_walk() don't have chance to be called to check. In other words, the 
> current code can detect this case (range span over hole):
> [  vma  ][ hole ][ vma]
>    [     range      ]
> but cant not detect these case :
> [  vma  ][ hole ][ vma]
>    [  range  ]
> [  vma  ][ hole ][  vma  ]
>             [  range  ]

IIRC, page table walker (walk_page_range()) should be designed to
handle these range inputs by separating into sub-ranges by vma
boundaries like below (with your notation):

  [  vma  ][ hole ][ vma  ]
     [    ][      ][  ]      // for your 1st case
     [    ][   ]             // for your 2nd case
              [   ][  ]      // for your 3rd case

And I found that .pte_hole is undefined in queue_pages_walk_ops.
So I'm guessing now that that's why hole regions are ignored and
the definition of EFAULT behavior in manpage is violated.
So providing proper .pte_hole callback could be another approach
for this issue which might fit better to the design.
IOW, queue_pages_test_walk() might not the right place to handle
hole regions by definition.

Thanks,
Naoya Horiguchi

> 
> 3. a checking in mbind_range() try to recover if the hole is in head side, but can't 
> recover if hole is in tail side of range.
> 
> - Xinhai
> 
> >Fixes: 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
> >Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
> >Fixes: 48684a65b4e3 ("mm: pagewalk: fix misbehavior of walk_page_range for vma(VM_PFNMAP)")
> >Signed-off-by: Li Xinhai <lixinhai.li@gmail.com>
> >---
> >Changes in v2:
> >  - Fix the unmapped holes checking in queue_pages_test_walk() instead of 
> >    mbind_rnage().
> >
> > mm/mempolicy.c | 44 +++++++++++++++++++++++++++++---------------
> > 1 file changed, 29 insertions(+), 15 deletions(-)
> >
> >diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> >index 4ae967bcf954..24087dfa4dcd 100644
> >--- a/mm/mempolicy.c
> >+++ b/mm/mempolicy.c
> >@@ -411,6 +411,9 @@ struct queue_pages {
> > 	unsigned long flags;
> > 	nodemask_t *nmask;
> > 	struct vm_area_struct *prev;
> >+	unsigned long start;
> >+	unsigned long end;
> >+	int in_hole;
> > };
> > 
> > /*
> >@@ -618,28 +621,31 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
> > 	unsigned long endvma = vma->vm_end;
> > 	unsigned long flags = qp->flags;
> > 
> >-	/*
> >-	* Need check MPOL_MF_STRICT to return -EIO if possible
> >-	* regardless of vma_migratable
> >-	*/
> >-	if (!vma_migratable(vma) &&
> >-	   !(flags & MPOL_MF_STRICT))
> >-	return 1;
> >-
> >+	/* range check first */
> > 	if (endvma > end)
> > 	endvma = end;
> >-	if (vma->vm_start > start)
> >-	start = vma->vm_start;
> >+	BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
> > 
> >+	qp->in_hole = 0;
> > 	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
> >-	if (!vma->vm_next && vma->vm_end < end)
> >+	if ((!vma->vm_next && vma->vm_end < qp->end) ||
> >+	(vma->vm_next && qp->end < vma->vm_next->vm_start))
> > 	return -EFAULT;
> >-	if (qp->prev && qp->prev->vm_end < vma->vm_start)
> >+	if ((qp->prev && qp->prev->vm_end < vma->vm_start) ||
> >+	(!qp->prev && qp->start < vma->vm_start))
> > 	return -EFAULT;
> > 	}
> > 
> > 	qp->prev = vma;
> > 
> >+	/*
> >+	* Need check MPOL_MF_STRICT to return -EIO if possible
> >+	* regardless of vma_migratable
> >+	*/
> >+	if (!vma_migratable(vma) &&
> >+	   !(flags & MPOL_MF_STRICT))
> >+	return 1;
> >+
> > 	if (flags & MPOL_MF_LAZY) {
> > 	/* Similar to task_numa_work, skip inaccessible VMAs */
> > 	if (!is_vm_hugetlb_page(vma) &&
> >@@ -679,14 +685,23 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
> > 	nodemask_t *nodes, unsigned long flags,
> > 	struct list_head *pagelist)
> > {
> >+	int err;
> > 	struct queue_pages qp = {
> > 	.pagelist = pagelist,
> > 	.flags = flags,
> > 	.nmask = nodes,
> > 	.prev = NULL,
> >+	.start = start,
> >+	.end = end,
> >+	.in_hole = 1,
> > 	};
> > 
> >-	return walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
> >+	err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
> >+	/* whole range in unmapped hole */
> >+	if (qp->in_hole && !(flags & MPOL_MF_DISCONTIG_OK))
> >+	err = -EFAULT;
> >+
> >+	return err;
> > }
> > 
> > /*
> >@@ -738,8 +753,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> > 	unsigned long vmend;
> > 
> > 	vma = find_vma(mm, start);
> >-	if (!vma || vma->vm_start > start)
> >-	return -EFAULT;
> >+	BUG_ON(!vma);
> > 
> > 	prev = vma->vm_prev;
> > 	if (start > vma->vm_start)
> >-- 
> >2.22.0

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Li Xinhai]

On 2019/10/31 17:06, Naoya Horiguchi wrote:
> On Thu, Oct 31, 2019 at 11:15:09AM +0800, Li Xinhai wrote:
>> On 2019-10-29 at 17:56 Li Xinhai wrote:
>>> queue_pages_range() will check for unmapped holes besides queue pages for
>>> migration. The rules for checking unmapped holes are:
>>> 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 if
>>>    mbind() for MPOL_DEFAULT case;
>>> Note that the second rule is the current implementation, but it seems
>>> conflicts the Linux API definition.
>>>
>>> queue_pages_test_walk() is fixed by introduce new fields in struct
>>> queue_pages which help to check:
>>> 1 holes at head and tail side of specified range;
>>> 2 the whole range is in a hole;
>>>
>>> Besides, queue_pages_test_walk() must update previous vma record no matter
>>> the current vma should be considered for queue pages or not.
>>>
>>
>> More details about current issue (which breaks the mbind() API definition):
>> 1. In queue_pages_test_walk()
>> checking on (!vma->vm_next && vma->vm_end < end) would never success,
>> because 'end' passed from walk_page_test() make sure "end <=  vma->vm_end". so hole
>> beyond the last vma can't be detected.
>>
>> 2. queue_pages_test_walk() only called for vma, and 'start', 'end' parameters are guaranteed
>> within vma. Then, if the range starting or ending in an unmapped hole,
>> queue_pages_test_walk() don't have chance to be called to check. In other words, the
>> current code can detect this case (range span over hole):
>> [  vma  ][ hole ][ vma]
>>     [     range      ]
>> but cant not detect these case :
>> [  vma  ][ hole ][ vma]
>>     [  range  ]
>> [  vma  ][ hole ][  vma  ]
>>              [  range  ]
> 
> IIRC, page table walker (walk_page_range()) should be designed to
> handle these range inputs by separating into sub-ranges by vma
> boundaries like below (with your notation):
> 
>    [  vma  ][ hole ][ vma  ]
>       [    ][      ][  ]      // for your 1st case
>       [    ][   ]             // for your 2nd case
>                [   ][  ]      // for your 3rd case
> 
Yes, pagewalk works exactly as your description.

> And I found that .pte_hole is undefined in queue_pages_walk_ops.
> So I'm guessing now that that's why hole regions are ignored and
> the definition of EFAULT behavior in manpage is violated.
> So providing proper .pte_hole callback could be another approach
> for this issue which might fit better to the design.
Using the .pte_hole() can be one option, we may stop detecting the
unmapped holes when .pte_hole() been first called for a hole which
is outside vma. But, .pte_hole() would be called many times during
the walking, because it is called for holes inside and outside VMA.
.test_walk() is called once for one vma, and not called for other
situation, so better for cost.

> IOW, queue_pages_test_walk() might not the right place to handle
> hole regions by definition.
I guess the original design was to avoid walking those vma twice, one
for test holes and one for queue pages. Maybe we can find better
choice?

> 
> Thanks,
> Naoya Horiguchi
> 
>>
>> 3. a checking in mbind_range() try to recover if the hole is in head side, but can't
>> recover if hole is in tail side of range.
>>
>> - Xinhai
>>
>>> Fixes: 9d8cebd4bcd7 ("mm: fix mbind vma merge problem")
>>> Fixes: 6f4576e3687b ("mempolicy: apply page table walker on queue_pages_range()")
>>> Fixes: 48684a65b4e3 ("mm: pagewalk: fix misbehavior of walk_page_range for vma(VM_PFNMAP)")
>>> Signed-off-by: Li Xinhai <lixinhai.li@gmail.com>
>>> ---
>>> Changes in v2:
>>>    - Fix the unmapped holes checking in queue_pages_test_walk() instead of
>>>      mbind_rnage().
>>>
>>>   mm/mempolicy.c | 44 +++++++++++++++++++++++++++++---------------
>>>   1 file changed, 29 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
>>> index 4ae967bcf954..24087dfa4dcd 100644
>>> --- a/mm/mempolicy.c
>>> +++ b/mm/mempolicy.c
>>> @@ -411,6 +411,9 @@ struct queue_pages {
>>>   	unsigned long flags;
>>>   	nodemask_t *nmask;
>>>   	struct vm_area_struct *prev;
>>> +	unsigned long start;
>>> +	unsigned long end;
>>> +	int in_hole;
>>>   };
>>>   
>>>   /*
>>> @@ -618,28 +621,31 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
>>>   	unsigned long endvma = vma->vm_end;
>>>   	unsigned long flags = qp->flags;
>>>   
>>> -	/*
>>> -	* Need check MPOL_MF_STRICT to return -EIO if possible
>>> -	* regardless of vma_migratable
>>> -	*/
>>> -	if (!vma_migratable(vma) &&
>>> -	   !(flags & MPOL_MF_STRICT))
>>> -	return 1;
>>> -
>>> +	/* range check first */
>>>   	if (endvma > end)
>>>   	endvma = end;
>>> -	if (vma->vm_start > start)
>>> -	start = vma->vm_start;
>>> +	BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
>>>   
>>> +	qp->in_hole = 0;
>>>   	if (!(flags & MPOL_MF_DISCONTIG_OK)) {
>>> -	if (!vma->vm_next && vma->vm_end < end)
>>> +	if ((!vma->vm_next && vma->vm_end < qp->end) ||
>>> +	(vma->vm_next && qp->end < vma->vm_next->vm_start))
>>>   	return -EFAULT;
>>> -	if (qp->prev && qp->prev->vm_end < vma->vm_start)
>>> +	if ((qp->prev && qp->prev->vm_end < vma->vm_start) ||
>>> +	(!qp->prev && qp->start < vma->vm_start))
>>>   	return -EFAULT;
>>>   	}
>>>   
>>>   	qp->prev = vma;
>>>   
>>> +	/*
>>> +	* Need check MPOL_MF_STRICT to return -EIO if possible
>>> +	* regardless of vma_migratable
>>> +	*/
>>> +	if (!vma_migratable(vma) &&
>>> +	   !(flags & MPOL_MF_STRICT))
>>> +	return 1;
>>> +
>>>   	if (flags & MPOL_MF_LAZY) {
>>>   	/* Similar to task_numa_work, skip inaccessible VMAs */
>>>   	if (!is_vm_hugetlb_page(vma) &&
>>> @@ -679,14 +685,23 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
>>>   	nodemask_t *nodes, unsigned long flags,
>>>   	struct list_head *pagelist)
>>>   {
>>> +	int err;
>>>   	struct queue_pages qp = {
>>>   	.pagelist = pagelist,
>>>   	.flags = flags,
>>>   	.nmask = nodes,
>>>   	.prev = NULL,
>>> +	.start = start,
>>> +	.end = end,
>>> +	.in_hole = 1,
>>>   	};
>>>   
>>> -	return walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
>>> +	err = walk_page_range(mm, start, end, &queue_pages_walk_ops, &qp);
>>> +	/* whole range in unmapped hole */
>>> +	if (qp->in_hole && !(flags & MPOL_MF_DISCONTIG_OK))
>>> +	err = -EFAULT;
>>> +
>>> +	return err;
>>>   }
>>>   
>>>   /*
>>> @@ -738,8 +753,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
>>>   	unsigned long vmend;
>>>   
>>>   	vma = find_vma(mm, start);
>>> -	if (!vma || vma->vm_start > start)
>>> -	return -EFAULT;
>>> +	BUG_ON(!vma);
>>>   
>>>   	prev = vma->vm_prev;
>>>   	if (start > vma->vm_start)
>>> -- 
>>> 2.22.0

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Andrew Morton]

(cc linux-man@vger.kernel.org)

On Tue, 29 Oct 2019 17:56:06 +0800 "Li Xinhai" <lixinhai.lxh@gmail.com> wrote:

> queue_pages_range() will check for unmapped holes besides queue pages for
> migration. The rules for checking unmapped holes are:
> 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 if
>   mbind() for MPOL_DEFAULT case;
> Note that the second rule is the current implementation, but it seems
> conflicts the Linux API definition.

Can you quote the part of the API definition which you're looking at?

My mbind(2) manpage says

ERRORS
       EFAULT Part or all of the memory range specified by nodemask and maxn-
              ode points outside your accessible address space.  Or, there was
              an unmapped hole in the specified memory range specified by addr
              and len.

(I assume the first sentence meant to say "specified by addr and len")

I agree with your interpretation, but there's no mention here that
MPOL_DEFAULT is treated differently and I don't see why it should be.


More broadly, I worry that it's too late to change this - existing
applications might fail if we change the implementation in the proposed
fashion.  So perhaps what we should do here is to change the manpage to
match reality?

Is the current behavior causing you any problems in a real-world use
case?

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Li Xinhai]

On 2019-10-31 at 12:08 Andrew Morton wrote:
>(cc linux-man@vger.kernel.org)
>
>On Tue, 29 Oct 2019 17:56:06 +0800 "Li Xinhai" <lixinhai.lxh@gmail.com> wrote:
>
>> queue_pages_range() will check for unmapped holes besides queue pages for
>> migration. The rules for checking unmapped holes are:
>> 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 if
>>   mbind() for MPOL_DEFAULT case;
>> Note that the second rule is the current implementation, but it seems
>> conflicts the Linux API definition.
>
>Can you quote the part of the API definition which you're looking at?
>
>My mbind(2) manpage says
>
>ERRORS
>       EFAULT Part or all of the memory range specified by nodemask and maxn-
>              ode points outside your accessible address space.  Or, there was
>              an unmapped hole in the specified memory range specified by addr
>              and len.
>
>(I assume the first sentence meant to say "specified by addr and len")
> 

this part: 
"Or, there was an unmapped hole in the specified memory range specified by addr 
and len."
is concerned by my patch.

>I agree with your interpretation, but there's no mention here that
>MPOL_DEFAULT is treated differently and I don't see why it should be.
> 
The first rule match the manpage, but the current mempolicy implementation only 
reports EFAULT if holes are within range, or at the head side of range. No EFAULT 
reported if hole at tail side of range. I suppose the first rule has to be fixed.

The seconde rule, when MPOL_DEFAULT is used, was summarized by me according 
to mempolicy implementation. Actually, this rule does not follow manpage and exsits 
for long days. In my understanding, this rule is reasonable (in code,  the internal flag 
MPOL_MF_DISCONTIG_OK is used for that purpose, there is comments for reason) 
and we'd better keep it.

>
>More broadly, I worry that it's too late to change this - existing
>applications might fail if we change the implementation in the proposed
>fashion.  So perhaps what we should do here is to change the manpage to
>match reality?
> 
I prefer add description in manpage for the second rule, so no change to our code. 
Only fix for first rule.

>Is the current behavior causing you any problems in a real-world use
>case? 
I was using mbind() with MPOL_DEFAULT(or MPOL_BIND) to reset a range of address 
(which maybe contiguous or not in the whole range) to the default policy (to a specific 
node), and observed this issue. If mbind() call for each mapping one by one, we don't see the 
issue.

- Xinhai

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Michal Hocko]

On Wed 30-10-19 21:08:36, Andrew Morton wrote:
> (cc linux-man@vger.kernel.org)
> 
> On Tue, 29 Oct 2019 17:56:06 +0800 "Li Xinhai" <lixinhai.lxh@gmail.com> wrote:
> 
> > queue_pages_range() will check for unmapped holes besides queue pages for
> > migration. The rules for checking unmapped holes are:
> > 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 if
> >   mbind() for MPOL_DEFAULT case;
> > Note that the second rule is the current implementation, but it seems
> > conflicts the Linux API definition.
> 
> Can you quote the part of the API definition which you're looking at?
> 
> My mbind(2) manpage says
> 
> ERRORS
>        EFAULT Part or all of the memory range specified by nodemask and maxn-
>               ode points outside your accessible address space.  Or, there was
>               an unmapped hole in the specified memory range specified by addr
>               and len.
> 
> (I assume the first sentence meant to say "specified by addr and len")

My understanding is that this really refers to area pointed to by nodemask.
Btw. why there is any special casing around the unmapped holes with the
address space range? This looks like an antipattern to other address
space operations to me. E.g. munmap simply unmaps all existing vmas in
the given range, mprotect, madvise etc. behave the same.

So my question is, do we want to remove that weird restriction and
simply act on all existing VMAs in the range? The only situation this
could regress would be if somebody used mbind to probe for existing VMAs
and that sounds a more than sensible to me. Or am I missing anything?
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm: Fix checking unmapped holes for mbind

[Li Xinhai]

On 2019-10-31 at 19:26 Michal Hocko wrote:
>On Wed 30-10-19 21:08:36, Andrew Morton wrote:
>> (cc linux-man@vger.kernel.org)
>>
>> On Tue, 29 Oct 2019 17:56:06 +0800 "Li Xinhai" <lixinhai.lxh@gmail.com> wrote:
>>
>> > queue_pages_range() will check for unmapped holes besides queue pages for
>> > migration. The rules for checking unmapped holes are:
>> > 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 if
>> >   mbind() for MPOL_DEFAULT case;
>> > Note that the second rule is the current implementation, but it seems
>> > conflicts the Linux API definition.
>>
>> Can you quote the part of the API definition which you're looking at?
>>
>> My mbind(2) manpage says
>>
>> ERRORS
>>        EFAULT Part or all of the memory range specified by nodemask and maxn-
>>               ode points outside your accessible address space.  Or, there was
>>               an unmapped hole in the specified memory range specified by addr
>>               and len.
>>
>> (I assume the first sentence meant to say "specified by addr and len")
>
>My understanding is that this really refers to area pointed to by nodemask.
>Btw. why there is any special casing around the unmapped holes with the
>address space range? This looks like an antipattern to other address
>space operations to me. E.g. munmap simply unmaps all existing vmas in
>the given range, mprotect, madvise etc. behave the same.
>
>So my question is, do we want to remove that weird restriction and
>simply act on all existing VMAs in the range? The only situation this
>could regress would be if somebody used mbind to probe for existing VMAs
>and that sounds a more than sensible to me. Or am I missing anything?
>--
>Michal Hocko
>SUSE Labs 

yes, mbind() care about the unmapped holes for non MPOL_DEFAULT cases, but 
other operations don't care those holes. It seems no clues about why that 
restriction was decided.

At present, if it is hard to decide to remove this restriction, we may keep the 
current behavior. New patch posted for this purpose,  
"[PATCH v3] mm: Fix checking unmapped holes for mbind".
Thanks.

Xinhai