Re: [PATCH] mm: swapfile: avoid split_swap_cluster() NULL pointer dereference

[Huang, Ying]

Rafael Aquini <aquini@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxx>
>> > 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