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

[Bob Liu <lliubbo@gmail.com>]

Hi Michal,

On Mon, Jan 6, 2014 at 7:24 PM, Michal Hocko <mhocko@suse.cz> 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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