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From:   Kallol Biswas <nucleodyne@gmail.com>
Date:   Tue, 9 Aug 2022 14:28:29 -0700
Message-ID: <CA+QbAV6YEN1h=T1H3-+1vg+DpSe8Uxzz2wH9u00GXe6L6TB-Uw@mail.gmail.com>
Subject: mm/memory-failure: __get_any_page: unknown zero refcount
To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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While running a memory RAS test on a new platform I encountered the
following on the 5.10.117 kernel.

__get_any_page: 0x77df: unknown zero refcount page type 7fffe000000000

The address 0x77df000 is in a system ram area:
00100000-5c9c0017 : System RAM

The page is not a huge page, not on the free buddy list and not in use.

__get_any_page()
..................
 if (!get_hwpoison_page(p)) {
    if (PageHuge(p)) {
      pr_info("%s: %#lx free huge page\n", __func__, pfn);
      ret = 0;
    } else if (is_free_buddy_page(p)) {
      pr_info("%s: %#lx free buddy page\n", __func__, pfn);
      ret = 0;
    } else if (page_count(p)) {
      /* raced with allocation */
      ret = -EBUSY;
    } else {
      pr_info("%s: %#lx: unknown zero refcount page type %lx\n",
        __func__, pfn, p->flags);


Sparse mem configs are set:
cat /boot/config-5.10.117-2.el7.nutanix.20220304.1002776.x86_64 | grep -i sparse
CONFIG_SPARSE_IRQ=y
CONFIG_ARCH_SPARSEMEM_ENABLE=y
CONFIG_ARCH_SPARSEMEM_DEFAULT=y
CONFIG_SPARSEMEM_MANUAL=y
CONFIG_SPARSEMEM=y
CONFIG_SPARSEMEM_EXTREME=y
CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
CONFIG_SPARSEMEM_VMEMMAP=y
CONFIG_MEMORY_HOTPLUG_SPARSE=y

Can someone help understand why we have such a page in the system?
What the purpose is.

Thank you,
Kallol