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Date: Mon, 08 Apr 2013 15:27:32 -0400
From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To: KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
Rik van Riel <riel@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Konstantin Khlebnikov <khlebnikov@...nvz.org>,
Michal Hocko <mhocko@...e.cz>,
HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] hugetlbfs: add swap entry check in
follow_hugetlb_page()
On Fri, Apr 05, 2013 at 02:59:43PM -0400, KOSAKI Motohiro wrote:
> (4/3/13 2:35 PM), Naoya Horiguchi wrote:
> > With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
> > initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
> > error happens on a hugepage and the affected processes try to access
> > the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
> > in get_page().
> >
> > The reason for this bug is that coredump-related code doesn't recognise
> > "hugepage hwpoison entry" with which a pmd entry is replaced when a memory
> > error occurs on a hugepage.
> > In other words, physical address information is stored in different bit layout
> > between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
> > which is called in get_dump_page() returns a wrong page from a given address.
> >
> > We need to filter out only hwpoison hugepages to have data on healthy
> > hugepages in coredump. So this patch makes follow_hugetlb_page() avoid
> > trying to get page when a pmd is in swap entry like format.
> >
> > ChangeLog v3:
> > - add comment about using is_swap_pte()
> >
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > Reviewed-by: Michal Hocko <mhocko@...e.cz>
> > Acked-by: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> > Cc: stable@...r.kernel.org
> > ---
> > mm/hugetlb.c | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git v3.9-rc3.orig/mm/hugetlb.c v3.9-rc3/mm/hugetlb.c
> > index 0d1705b..3bc20bd 100644
> > --- v3.9-rc3.orig/mm/hugetlb.c
> > +++ v3.9-rc3/mm/hugetlb.c
> > @@ -2966,9 +2966,15 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> > * Some archs (sparc64, sh*) have multiple pte_ts to
> > * each hugepage. We have to make sure we get the
> > * first, for the page indexing below to work.
> > + *
> > + * is_swap_pte test covers both is_hugetlb_entry_hwpoisoned
> > + * and hugepages under migration in which case
> > + * hugetlb_fault waits for the migration and bails out
> > + * properly for HWPosined pages.
> > */
> > pte = huge_pte_offset(mm, vaddr & huge_page_mask(h));
> > - absent = !pte || huge_pte_none(huge_ptep_get(pte));
> > + absent = !pte || huge_pte_none(huge_ptep_get(pte)) ||
> > + is_swap_pte(huge_ptep_get(pte));
>
> Hmmm...
>
> Now absent has two meanings. 1) skip hugetlb_fault() and return immediately if FOLL_DUMP is used.
> 2) call hugetlb_fault() if to be need page population or cow.
>
> The description of this patch only explain about (2). and I'm not convinced why we don't need to
> dump pages under migraion.
We can/should dump hugepages under migration, and to do that we have to
put is_swap_pte() in the check of the hugetlb_falut block.
I updated this patch like below.
# I suspended Reviewed and Acked given for the previous version, because
# it has a non-minor change. If you want to restore it, please let me know.
Thanks,
Naoya
------------------------------------------------------------------------
From: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Date: Thu, 28 Mar 2013 10:17:38 -0400
Subject: [PATCH v4] hugetlbfs: add swap entry check in follow_hugetlb_page()
With applying the previous patch "hugetlbfs: stop setting VM_DONTDUMP in
initializing vma(VM_HUGETLB)" to reenable hugepage coredump, if a memory
error happens on a hugepage and the affected processes try to access
the error hugepage, we hit VM_BUG_ON(atomic_read(&page->_count) <= 0)
in get_page().
The reason for this bug is that coredump-related code doesn't recognise
"hugepage hwpoison entry" with which a pmd entry is replaced when a memory
error occurs on a hugepage.
In other words, physical address information is stored in different bit layout
between hugepage hwpoison entry and pmd entry, so follow_hugetlb_page()
which is called in get_dump_page() returns a wrong page from a given address.
The expected behavior is like this:
absent is_swap_pte FOLL_DUMP Expected behavior
---------------------------------------------------------------------
true false false hugetlb_fault
false true false hugetlb_fault
false false false return page
true false true skip page (to avoid allocation)
false true true hugetlb_fault
false false true return page
With this patch, we can call hugetlb_fault() and take proper actions
(we can wait for migration entries, fail with VM_FAULT_HWPOISON_LARGE for
hwpoisoned entries,) and as the result we can dump all hugepages except
for hwpoisoned ones.
ChangeLog v4:
- move is_swap_page() to right place.
ChangeLog v3:
- add comment about using is_swap_pte()
Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: stable@...r.kernel.org
---
mm/hugetlb.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 0d1705b..f155e59 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2983,7 +2983,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
break;
}
- if (absent ||
+ /*
+ * is_swap_pte test covers both is_hugetlb_entry_hwpoisoned
+ * and hugepages under migration in which case
+ * hugetlb_fault waits for the migration and bails out
+ * properly for HWPosined pages.
+ */
+ if (absent || is_swap_pte(huge_ptep_get(pte)) ||
((flags & FOLL_WRITE) && !pte_write(huge_ptep_get(pte)))) {
int ret;
--
1.7.11.7
--
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