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

regards,

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