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

> On Thu, Oct 01, 2020 at 10:31:57AM -0400, Rafael Aquini wrote:
>> On Fri, Sep 25, 2020 at 11:21:58AM +0800, Huang, Ying wrote:
>> > 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
>> >
>>
>> I left it running for several days, on several systems that had seen the
>> crash hitting before, and no crashes were observed for either the upstream
>> kernel nor the distro build 4.18-based kernel.
>>
>> I guess we can comfortably go with your patch. Thanks!
>>
>>
> Ping
>
> Are you going to post this patchfix soon? Or do you rather have me
> posting it?

Sorry for late replying. I just come back from a long local holiday.
Thanks a lot for testing! I will prepare the formal fixing patch.

Best Regards,
Huang, Ying

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