Hi Michal,
On Mon, Jan 6, 2014 at 7:24 PM, Michal Hocko <mhocko@xxxxxxx> wrote:
> Hi Wanpeng Li,
> I have just noticed 11c731e81bb0 (mm/mempolicy: fix !vma in
> new_vma_page()) and I am not sure I understand it. Your changelog claims
> "
> page_address_in_vma() may still return -EFAULT because of many other
> conditions in it. As a result the while loop in new_vma_page() may end
> with vma=NULL.
> "
>
> And the patch handles hugetlb case only. I was wondering what are those
> "other conditions" that failed in the BUG_ON mentioned in the changelog?
> Could you be more specific please?
>
Sorry for the confusion caused.
The code of new_vma_page() used to like this:
1193 while (vma) {
1194 address = page_address_in_vma(page, vma);
1195 if (address != -EFAULT)
1196 break;
1197 vma = vma->vm_next;
1198 }
1199 /*
1200 * queue_pages_range() confirms that @page belongs to some vma,
1201 * so vma shouldn't be NULL.
1202 */
1203 BUG_ON(!vma);
1204
1205 if (PageHuge(page))
1206 return alloc_huge_page_noerr(vma, address, 1);
1207 return alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
The BUG_ON() was triggered and my idea was that even
queue_pages_range() confirms @page belongs to some vma,
page_address_in_vma() may still return -EFAULT because of below checks
in page_address_in_vma().
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().
--
Regards,
--Bob
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