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 2019-10-29Âat 17:56ÂLi XinhaiÂwrote:
> >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
> >Â EFAULT if mbind() for none MPOL_DEFAULT cases;
> >2 Unmapped holes at any part of the specified range should be ignored if
> >Â 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. InÂqueue_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 <= Âvma->vm_end". so holeÂ
> beyond the last vma can't be detected.Â
> 
> 2.Âqueue_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, Â
> queue_pages_test_walk() don't have chance to be called to check. In other words, theÂ
> current code can detect this case (range span over hole):
> [ Âvma Â][ hole ][ vma]
>  Â[   range   Â]
> but cant not detect these case :
> [ Âvma Â][ hole ][ vma]
> Â Â[ Ârange Â]
> [ Âvma Â][ hole ][ Âvma Â]
> Â Â Â Â Â Â [ Ârange Â]

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'tÂ
> 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@xxxxxxxxx>
> >---
> >Changes in v2:
> >Â - Fix the unmapped holes checking in queue_pages_test_walk() instead ofÂ
> >Â Â mbind_rnage().
> >
> >Âmm/mempolicy.c | 44 +++++++++++++++++++++++++++++---------------
> >Â1 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 {
> >Â	unsigned long flags;
> >Â	nodemask_t *nmask;
> >Â	struct vm_area_struct *prev;
> >+	unsigned long start;
> >+	unsigned long end;
> >+	int in_hole;
> >Â};
> >Â
> >Â/*
> >@@ -618,28 +621,31 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
> >Â	unsigned long endvma = vma->vm_end;
> >Â	unsigned long flags = qp->flags;
> >Â
> >-	/*
> >-	* Need check MPOL_MF_STRICT to return -EIO if possible
> >-	* regardless of vma_migratable
> >-	*/
> >-	if (!vma_migratable(vma) &&
> >-	Â Â!(flags & MPOL_MF_STRICT))
> >-	return 1;
> >-
> >+	/* range check first */
> >Â	if (endvma > end)
> >Â	endvma = end;
> >-	if (vma->vm_start > start)
> >-	start = vma->vm_start;
> >+	BUG_ON((vma->vm_start > start) || (vma->vm_end < end));
> >Â
> >+	qp->in_hole = 0;
> >Â	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))
> >Â	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))
> >Â	return -EFAULT;
> >Â	}
> >Â
> >Â	qp->prev = vma;
> >Â
> >+	/*
> >+	* Need check MPOL_MF_STRICT to return -EIO if possible
> >+	* regardless of vma_migratable
> >+	*/
> >+	if (!vma_migratable(vma) &&
> >+	Â Â!(flags & MPOL_MF_STRICT))
> >+	return 1;
> >+
> >Â	if (flags & MPOL_MF_LAZY) {
> >Â	/* Similar to task_numa_work, skip inaccessible VMAs */
> >Â	if (!is_vm_hugetlb_page(vma) &&
> >@@ -679,14 +685,23 @@ queue_pages_range(struct mm_struct *mm, unsigned long start, unsigned long end,
> >Â	nodemask_t *nodes, unsigned long flags,
> >Â	struct list_head *pagelist)
> >Â{
> >+	int err;
> >Â	struct queue_pages qp = {
> >Â	.pagelist = pagelist,
> >Â	.flags = flags,
> >Â	.nmask = nodes,
> >Â	.prev = NULL,
> >+	.start = start,
> >+	.end = end,
> >+	.in_hole = 1,
> >Â	};
> >Â
> >-	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;
> >Â}
> >Â
> >Â/*
> >@@ -738,8 +753,7 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> >Â	unsigned long vmend;
> >Â
> >Â	vma = find_vma(mm, start);
> >-	if (!vma || vma->vm_start > start)
> >-	return -EFAULT;
> >+	BUG_ON(!vma);
> >Â
> >Â	prev = vma->vm_prev;
> >Â	if (start > vma->vm_start)
> >--Â
> >2.22.0