[PATCH v2 3/7] mm/hwpoison: fix race against poison thp

[Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>]

v1 -> v2:
	* unpoison thp fail  

There is a race between hwpoison page and unpoison page, memory_failure
set the page hwpoison and increase num_poisoned_pages without hold page
lock, and one page count will be accounted against thp for num_poisoned_pages.
However, unpoison can occur before memory_failure hold page lock and
split transparent hugepage, unpoison will decrease num_poisoned_pages
by 1 << compound_order since memory_failure has not yet split transparent
hugepage with page lock held. That means we account one page for hwpoison
and 1 << compound_order for unpoison. This patch fix it by inserting a 
PageTransHuge check before doing TestClearPageHWPoison, unpoison failed 
without clearing PageHWPoison and decreasing num_poisoned_pages.


            A                                                 	B
    	memory_failue
        TestSetPageHWPoison(p);
        if (PageHuge(p))
            nr_pages = 1 << compound_order(hpage);
        else
            nr_pages = 1;
        atomic_long_add(nr_pages, &num_poisoned_pages);
                                                            unpoison_memory
	                                                        nr_pages = 1<< compound_trans_order(page);
                                                            if(TestClearPageHWPoison(p))
                                                            atomic_long_sub(nr_pages, &num_poisoned_pages);
        lock page
        if (!PageHWPoison(p))
        	unlock page and return
        hwpoison_user_mappings
        if (PageTransHuge(hpage))
        	split_huge_page(hpage);


Suggested-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
---
 mm/memory-failure.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 5a4f4d6..a6c4752 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1339,6 +1339,16 @@ int unpoison_memory(unsigned long pfn)
 		return 0;
 	}
 
+	/*
+	 * unpoison_memory() can encounter thp only when the thp is being
+	 * worked by memory_failure() and the page lock is not held yet.
+	 * In such case, we yield to memory_failure() and make unpoison fail.
+	 */
+	if (PageTransHuge(page)) {
+		pr_info("MCE: Memory failure is now running on %#lx\n", pfn);
+			return 0;
+	}
+
 	nr_pages = 1 << compound_order(page);
 
 	if (!get_page_unless_zero(page)) {
-- 
1.8.1.2

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