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


I finally could grab a good crashdump and confirm that head is really
not locked. I still need to dig into it to figure out more about the
crash. I guess that your patch will guarantee that lock on head, but
it still doesn't help on explaining how did we get the THP marked as
PG_swapcache, given that it should fail add_to_swap()->get_swap_page()
right?

I'll give your patch a run over the weekend, hopefully we'll have more
info on this next week.


> 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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