Re: could you clarify mm/mempolicy: fix !vma in new_vma_page()

[Michal Hocko]

On Tue 07-01-14 11:22:12, Michal Hocko wrote:
> On Tue 07-01-14 13:29:31, Bob Liu wrote:
> > On Mon, Jan 6, 2014 at 10:18 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
> > > On Mon 06-01-14 20:45:54, Bob Liu wrote:
> > > [...]
> > >>  544         if (PageAnon(page)) {
> > >>  545                 struct anon_vma *page__anon_vma = page_anon_vma(page);
> > >>  546                 /*
> > >>  547                  * Note: swapoff's unuse_vma() is more efficient with this
> > >>  548                  * check, and needs it to match anon_vma when KSM is active.
> > >>  549                  */
> > >>  550                 if (!vma->anon_vma || !page__anon_vma ||
> > >>  551                     vma->anon_vma->root != page__anon_vma->root)
> > >>  552                         return -EFAULT;
> > >>  553         } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
> > >>  554                 if (!vma->vm_file ||
> > >>  555                     vma->vm_file->f_mapping != page->mapping)
> > >>  556                         return -EFAULT;
> > >>  557         } else
> > >>  558                 return -EFAULT;
> > >>
> > >> That's the "other conditions" and the reason why we can't use
> > >> BUG_ON(!vma) in new_vma_page().
> > >
> > > Sorry, I wasn't clear with my question. I was interested in which of
> > > these triggered and why only for hugetlb pages?
> > >
> > 
> > Sorry I didn't analyse the root cause. They are several checks in
> > page_address_in_vma() so I think it might be not difficult to hit one
> > of them.
> 
> I would be really curious when anon_vma or f_mapping would be out of
> sync, that's why I've asked in the first place.
> 
> > For example, if the page was mapped to vma by nonlinear
> > mapping?
> 
> Hmm, ok !private shmem/hugetlbfs might be remapped as non-linear.

OK, it didn't let go away from my head so I had to check. hugetlbfs
cannot be remmaped as non-linear because it is missing its vm_ops is
missing remap_pages implementation. So this case is impossible for these
pages. So at least the PageHuge part of the patch is bogus AFAICS.

We still have shmem and even then I am curious whether we are doing the
right thing. The loop is inteded to handle range spanning multiple VMAs
(as per 3ad33b2436b54 (Migration: find correct vma in new_vma_page()))
and it doesn't seem to be VM_NONLINEAR aware. It will always fail for
shared shmem and so we always fallback to task/system default mempolicy.
Whether somebody uses mempolicy on VM_NONLINEAR mappings is hard to
tell. I am not familiar with this feature much.

That being said. The BUG_ON(!vma) was bogus for VM_NONLINEAR cases.
The changed code could keep it for hugetlbfs path because we shouldn't
see NULL vma there AFAICS.

What is the right(tm) thing to do for VM_NONLINEAR is hard to tell and I
would leave it to those who are more familiar with the usage.

-- 
Michal Hocko
SUSE Labs
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