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