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

[Rafael Aquini]

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?

regards,