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SubjectRe: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference
Date
Rafael Aquini <aquini@redhat.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@intel.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
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