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    SubjectRe: [PATCH v4 55/66] mm/mempolicy: Use maple tree iterators instead of vma linked list
    Date
    * Vlastimil Babka <vbabka@suse.cz> [220126 04:23]:
    > On 1/26/22 03:48, Liam Howlett wrote:
    > > * Vlastimil Babka <vbabka@suse.cz> [220120 06:58]:
    > >> On 12/1/21 15:30, Liam Howlett wrote:
    > >> > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
    > >> >
    > >> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    > >> > Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
    > >> > ---
    > >> > mm/mempolicy.c | 53 ++++++++++++++++++++++++++------------------------
    > >> > 1 file changed, 28 insertions(+), 25 deletions(-)
    > >> >
    > >> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
    > >> > index 10e9c87260ed..0e2d918f4f30 100644
    > >> > --- a/mm/mempolicy.c
    > >> > +++ b/mm/mempolicy.c
    > >> > @@ -377,9 +377,10 @@ void mpol_rebind_task(struct task_struct *tsk, const nodemask_t *new)
    > >> > void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
    > >> > {
    > >> > struct vm_area_struct *vma;
    > >> > + MA_STATE(mas, &mm->mm_mt, 0, 0);
    > >>
    > >> VMA_ITERATOR?
    > >
    > > Yes, I will make this change. Thanks.
    > >
    > >>
    > >> >
    > >> > mmap_write_lock(mm);
    > >> > - for (vma = mm->mmap; vma; vma = vma->vm_next)
    > >> > + mas_for_each(&mas, vma, ULONG_MAX)
    > >> > mpol_rebind_policy(vma->vm_policy, new);
    > >> > mmap_write_unlock(mm);
    > >> > }
    > >> > @@ -652,7 +653,7 @@ static unsigned long change_prot_numa(struct vm_area_struct *vma,
    > >> > static int queue_pages_test_walk(unsigned long start, unsigned long end,
    > >> > struct mm_walk *walk)
    > >> > {
    > >> > - struct vm_area_struct *vma = walk->vma;
    > >> > + struct vm_area_struct *next, *vma = walk->vma;
    > >> > struct queue_pages *qp = walk->private;
    > >> > unsigned long endvma = vma->vm_end;
    > >> > unsigned long flags = qp->flags;
    > >> > @@ -667,9 +668,10 @@ static int queue_pages_test_walk(unsigned long start, unsigned long end,
    > >> > /* hole at head side of range */
    > >> > return -EFAULT;
    > >> > }
    > >> > + next = find_vma(vma->vm_mm, vma->vm_end);
    > >> > if (!(flags & MPOL_MF_DISCONTIG_OK) &&
    > >> > ((vma->vm_end < qp->end) &&
    > >> > - (!vma->vm_next || vma->vm_end < vma->vm_next->vm_start)))
    > >> > + (!next || vma->vm_end < next->vm_start)))
    > >> > /* hole at middle or tail of range */
    > >> > return -EFAULT;
    > >> >
    > >> > @@ -783,28 +785,24 @@ static int vma_replace_policy(struct vm_area_struct *vma,
    > >> > static int mbind_range(struct mm_struct *mm, unsigned long start,
    > >> > unsigned long end, struct mempolicy *new_pol)
    > >> > {
    > >> > - struct vm_area_struct *next;
    > >> > + MA_STATE(mas, &mm->mm_mt, start - 1, start - 1);
    > >> > struct vm_area_struct *prev;
    > >> > struct vm_area_struct *vma;
    > >> > int err = 0;
    > >> > pgoff_t pgoff;
    > >> > - unsigned long vmstart;
    > >> > - unsigned long vmend;
    > >> > -
    > >> > - vma = find_vma(mm, start);
    > >> > - VM_BUG_ON(!vma);
    > >> >
    > >> > - prev = vma->vm_prev;
    > >> > - if (start > vma->vm_start)
    > >> > - prev = vma;
    > >> > + prev = mas_find_rev(&mas, 0);
    > >> > + if (prev && (start < prev->vm_end))
    > >> > + vma = prev;
    > >> > + else
    > >> > + vma = mas_next(&mas, end - 1);
    > >> >
    > >> > - for (; vma && vma->vm_start < end; prev = vma, vma = next) {
    > >> > - next = vma->vm_next;
    > >> > - vmstart = max(start, vma->vm_start);
    > >> > - vmend = min(end, vma->vm_end);
    > >> > + do {
    > >> > + unsigned long vmstart = max(start, vma->vm_start);
    > >> > + unsigned long vmend = min(end, vma->vm_end);
    > >>
    > >> What if vma is null? Shouldn't this rather be a "for (; vma; vma =
    > >> mas_next(...)"
    > >
    > > We have already found the vma above.
    >
    > AFAICS if the range intersects no vmas, we might have found a 'prev', but
    > 'vma' might be NULL after the "vma = mas_next(&mas, end - 1);"?

    Yes, I agree. I was going to restore VM_BUG_ON(!vma) after mas_next(),
    but that's not the same as the existing code. The VM_BUG_ON(!vma)
    only catches if we search for 'start' above any VMA. It seems
    mbind_range() would have returned without error if there were no VMAs
    within the range but would error if the 'start' was sufficiently large.

    I think it is better to write it as you suggested to restore the
    functionality of not failing on an empty list. I don't see a decent
    way of checking if we searched for an address above any VMA to restore
    the VM_BUG_ON() which existed - although I see little value in it to
    begin with. I will mention this in the commit message.
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