[RFC,v1,02/11] mm: soft-offline: add missing error check of set_hwpoison_free_buddy_page()
diff mbox series

Message ID 1541746035-13408-3-git-send-email-n-horiguchi@ah.jp.nec.com
State New
Headers show
Series
  • hwpoison improvement part 1
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Commit Message

Naoya Horiguchi Nov. 9, 2018, 6:47 a.m. UTC
set_hwpoison_free_buddy_page() could fail, then the target page is
finally not isolated, so it's better to report -EBUSY for userspace
to know the failure and chance of retry.

And for consistency, this patch moves set_hwpoison_free_buddy_page()
in unmap_and_move() to __soft_offline_page().

Fixes: 6bc9b56433b7 ("mm: fix race on soft-offlining free huge pages")
Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 mm/memory-failure.c | 15 ++++++++++++---
 mm/migrate.c        |  9 ---------
 2 files changed, 12 insertions(+), 12 deletions(-)

Comments

Anshuman Khandual Nov. 9, 2018, 10:20 a.m. UTC | #1
On 11/09/2018 12:17 PM, Naoya Horiguchi wrote:
> set_hwpoison_free_buddy_page() could fail, then the target page is
> finally not isolated, so it's better to report -EBUSY for userspace
> to know the failure and chance of retry.
> 

IIUC set_hwpoison_free_buddy_page() could only fail if the page is not
free in the buddy. At least for soft_offline_huge_page() that wont be
the case otherwise dissolve_free_huge_page() would have returned non
zero -EBUSY. Is there any other reason set_hwpoison_free_buddy_page()
would not succeed ?

> And for consistency, this patch moves set_hwpoison_free_buddy_page()
> in unmap_and_move() to __soft_offline_page().

Yeah this check should be handled in soft offline functions not inside
migrations they trigger.
Naoya Horiguchi Nov. 13, 2018, 12:16 a.m. UTC | #2
Hi Anshuman,

On Fri, Nov 09, 2018 at 03:50:41PM +0530, Anshuman Khandual wrote:
> 
> 
> On 11/09/2018 12:17 PM, Naoya Horiguchi wrote:
> > set_hwpoison_free_buddy_page() could fail, then the target page is
> > finally not isolated, so it's better to report -EBUSY for userspace
> > to know the failure and chance of retry.
> > 
> 
> IIUC set_hwpoison_free_buddy_page() could only fail if the page is not
> free in the buddy. At least for soft_offline_huge_page() that wont be
> the case otherwise dissolve_free_huge_page() would have returned non
> zero -EBUSY. Is there any other reason set_hwpoison_free_buddy_page()
> would not succeed ?

There is a race window between page freeing (after successful soft-offline
-> page migration case) and the containment by set_hwpoison_free_buddy_page().
Or a target page can be allocated just after get_any_page() decided that
the target page is a free page.
So set_hwpoison_free_buddy_page() would safely fail in such cases.

Thanks,
Naoya Horiguchi

> 
> > And for consistency, this patch moves set_hwpoison_free_buddy_page()
> > in unmap_and_move() to __soft_offline_page().
> 
> Yeah this check should be handled in soft offline functions not inside
> migrations they trigger.
>
Anshuman Khandual Nov. 14, 2018, 8:53 a.m. UTC | #3
On 11/13/2018 05:46 AM, Naoya Horiguchi wrote:
> Hi Anshuman,
> 
> On Fri, Nov 09, 2018 at 03:50:41PM +0530, Anshuman Khandual wrote:
>>
>> On 11/09/2018 12:17 PM, Naoya Horiguchi wrote:
>>> set_hwpoison_free_buddy_page() could fail, then the target page is
>>> finally not isolated, so it's better to report -EBUSY for userspace
>>> to know the failure and chance of retry.
>>>
>> IIUC set_hwpoison_free_buddy_page() could only fail if the page is not
>> free in the buddy. At least for soft_offline_huge_page() that wont be
>> the case otherwise dissolve_free_huge_page() would have returned non
>> zero -EBUSY. Is there any other reason set_hwpoison_free_buddy_page()
>> would not succeed ?
> There is a race window between page freeing (after successful soft-offline
> -> page migration case) and the containment by set_hwpoison_free_buddy_page().
> Or a target page can be allocated just after get_any_page() decided that
> the target page is a free page.
> So set_hwpoison_free_buddy_page() would safely fail in such cases.

Makes sense. Thanks.

Patch
diff mbox series

diff --git v4.19-mmotm-2018-10-30-16-08/mm/memory-failure.c v4.19-mmotm-2018-10-30-16-08_patched/mm/memory-failure.c
index 9f09bf3..11e283e 100644
--- v4.19-mmotm-2018-10-30-16-08/mm/memory-failure.c
+++ v4.19-mmotm-2018-10-30-16-08_patched/mm/memory-failure.c
@@ -1719,14 +1719,18 @@  static int soft_offline_huge_page(struct page *page, int flags)
 		/*
 		 * We set PG_hwpoison only when the migration source hugepage
 		 * was successfully dissolved, because otherwise hwpoisoned
-		 * hugepage remains on free hugepage list, then userspace will
-		 * find it as SIGBUS by allocation failure. That's not expected
-		 * in soft-offlining.
+		 * hugepage remains on free hugepage list. The allocator ignores
+		 * such a hwpoisoned page so it's never allocated, but it could
+		 * kill a process because of no-memory rather than hwpoison.
+		 * Soft-offline never impacts the userspace, so this is
+		 * undesired.
 		 */
 		ret = dissolve_free_huge_page(page);
 		if (!ret) {
 			if (set_hwpoison_free_buddy_page(page))
 				num_poisoned_pages_inc();
+			else
+				ret = -EBUSY;
 		}
 	}
 	return ret;
@@ -1804,6 +1808,11 @@  static int __soft_offline_page(struct page *page, int flags)
 				pfn, ret, page->flags, &page->flags);
 			if (ret > 0)
 				ret = -EIO;
+		} else {
+			if (set_hwpoison_free_buddy_page(page))
+				num_poisoned_pages_inc();
+			else
+				ret = -EBUSY;
 		}
 	} else {
 		pr_info("soft offline: %#lx: isolation failed: %d, page count %d, type %lx (%pGp)\n",
diff --git v4.19-mmotm-2018-10-30-16-08/mm/migrate.c v4.19-mmotm-2018-10-30-16-08_patched/mm/migrate.c
index f7e4bfd..1742372 100644
--- v4.19-mmotm-2018-10-30-16-08/mm/migrate.c
+++ v4.19-mmotm-2018-10-30-16-08_patched/mm/migrate.c
@@ -1199,15 +1199,6 @@  static ICE_noinline int unmap_and_move(new_page_t get_new_page,
 	 */
 	if (rc == MIGRATEPAGE_SUCCESS) {
 		put_page(page);
-		if (reason == MR_MEMORY_FAILURE) {
-			/*
-			 * Set PG_HWPoison on just freed page
-			 * intentionally. Although it's rather weird,
-			 * it's how HWPoison flag works at the moment.
-			 */
-			if (set_hwpoison_free_buddy_page(page))
-				num_poisoned_pages_inc();
-		}
 	} else {
 		if (rc != -EAGAIN) {
 			if (likely(!__PageMovable(page))) {