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Message-ID: <20191031090621.GA8196@hori.linux.bs1.fc.nec.co.jp>
Date: Thu, 31 Oct 2019 09:06:21 +0000
From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To: Li Xinhai <lixinhai.lxh@...il.com>
CC: "linux-mm@...ck.org" <linux-mm@...ck.org>,
akpm <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
Michal Hocko <mhocko@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
"Hugh Dickins" <hughd@...gle.com>
Subject: Re: [PATCH v2] mm: Fix checking unmapped holes for mbind
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@...il.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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