On Tue 07-01-14 13:29:31, Bob Liu wrote:
> On Mon, Jan 6, 2014 at 10:18 PM, Michal Hocko <mho...@suse.cz> 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. For
some reason I thought that migration for non-linear mappings is not
allowed. This is not the case and it would explain why the BUG_ON
triggered.
> Anyway, some debug code is needed to verify what really happened here.
That would be prefferable before the patch had been submitted and
merged...
> alloc_page_vma() can handle the vma=NULL case while
> alloc_huge_page_noerr() can't, so we return NULL instead of call down
> to alloc_huge_page().
OK, I see.
Thanks!
--
Michal Hocko
SUSE Labs
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