linux-kernel - Re: [PATCH] mm: swapfile: avoid split_swap

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Date:   Fri, 25 Sep 2020 11:21:58 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Rafael Aquini <aquini@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference

Rafael Aquini <aquini@...hat.com> writes:
>> Or, can you help to run the test with a debug kernel based on upstream
>> kernel.  I can provide some debug patch.
>> 
>
> Sure, I can set your patches to run with the test cases we have that tend to 
> reproduce the issue with some degree of success.

Thanks!

I found a race condition.  During THP splitting, "head" may be unlocked
before calling split_swap_cluster(), because head != page during
deferred splitting.  So we should call split_swap_cluster() before
unlocking.  The debug patch to do that is as below.  Can you help to
test it?

Best Regards,
Huang, Ying

------------------------8<----------------------------
>From 24ce0736a9f587d2dba12f12491c88d3e296a491 Mon Sep 17 00:00:00 2001
From: Huang Ying <ying.huang@...el.com>
Date: Fri, 25 Sep 2020 11:10:56 +0800
Subject: [PATCH] dbg: Call split_swap_clsuter() before unlock page during
 split THP

---
 mm/huge_memory.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index faadc449cca5..8d79e5e6b46e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2444,6 +2444,12 @@ static void __split_huge_page(struct page *page, struct list_head *list,
 
 	remap_page(head);
 
+	if (PageSwapCache(head)) {
+		swp_entry_t entry = { .val = page_private(head) };
+
+		split_swap_cluster(entry);
+	}
+
 	for (i = 0; i < HPAGE_PMD_NR; i++) {
 		struct page *subpage = head + i;
 		if (subpage == page)
@@ -2678,12 +2684,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 		}
 
 		__split_huge_page(page, list, end, flags);
-		if (PageSwapCache(head)) {
-			swp_entry_t entry = { .val = page_private(head) };
-
-			ret = split_swap_cluster(entry);
-		} else
-			ret = 0;
+		ret = 0;
 	} else {
 		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
 			pr_alert("total_mapcount: %u, page_count(): %u\n",
-- 
2.28.0