On Fri, 1 Aug 2014, Naoya Horiguchi wrote:
> There is a race condition between hugepage migration and change_protection(),
> where hugetlb_change_protection() doesn't care about migration entries and
> wrongly overwrites them. That causes unexpected results like kernel crash.
>
> This patch adds is_hugetlb_entry_(migration|hwpoisoned) check in this
> function and skip all such entries.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # [3.12+]
> ---
> mm/hugetlb.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git mmotm-2014-07-22-15-58.orig/mm/hugetlb.c mmotm-2014-07-22-15-58/mm/hugetlb.c
> index 863f45f63cd5..1da7ca2e2a02 100644
> --- mmotm-2014-07-22-15-58.orig/mm/hugetlb.c
> +++ mmotm-2014-07-22-15-58/mm/hugetlb.c
> @@ -3355,7 +3355,13 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> spin_unlock(ptl);
> continue;
> }
> - if (!huge_pte_none(huge_ptep_get(ptep))) {
> + pte = huge_ptep_get(ptep);
> + if (unlikely(is_hugetlb_entry_migration(pte) ||
> + is_hugetlb_entry_hwpoisoned(pte))) {
Another instance of this pattern. Oh well, perhaps we have to continue
this way while backporting fixes, but the repetition irritates me.
Or use is_swap_pte() as follow_hugetlb_page() does?
More importantly, the regular change_pte_range() has to
make_migration_entry_read() if is_migration_entry_write():
why is that not necessary here?
And have you compared hugetlb codepaths with normal codepaths, to see
if there are other huge places which need to check for a migration entry
now? If you have checked, please reassure us in the commit message:
we would prefer not to have these fixes coming in one by one.
(I first thought __unmap_hugepage_range() would need it, but since
zap_pte_range() only checks it for rss stats, and hugetlb does not
participate in rss stats, it looks like no need.)
Hugh
> + spin_unlock(ptl);
> + continue;
> + }
> + if (!huge_pte_none(pte)) {
> pte = huge_ptep_get_and_clear(mm, address, ptep);
> pte = pte_mkhuge(huge_pte_modify(pte, newprot));
> pte = arch_make_huge_pte(pte, vma, NULL, 0);
> --
> 1.9.3
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