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    SubjectRe: [PATCH] mm/debug_vm_pgtable: Fix corrupted PG_arch_1 by set_pmd_at()
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    Hello Gavin,

    On 7/2/21 4:02 PM, Gavin Shan wrote:
    > There are two addresses selected: random virtual address and physical
    > address corresponding to kernel symbol @start_kernel. During the PMD
    > tests in pmd_advanced_tests(), the physical address is aligned down
    > to the starting address of the huge page, whose size is 512MB on ARM64
    > when we have 64KB base page size. After that, set_pmd_at() is called
    > to populate the PMD entry. PG_arch_1, PG_dcache_clean on ARM64, is
    > set to the page flags. Unforunately, the page, corresponding to the
    > starting address of the huge page could be owned by buddy. It means
    > PG_arch_1 can be unconditionally set to page owned by buddy.
    >
    > Afterwards, the page with PG_arch_1 set is fetched from buddy's free
    > area list, but fails the checking. It leads to the following warning
    > on ARM64:
    >
    > BUG: Bad page state in process memhog pfn:08000
    > page:0000000015c0a628 refcount:0 mapcount:0 \
    > mapping:0000000000000000 index:0x1 pfn:0x8000
    > flags: 0x7ffff8000000800(arch_1|node=0|zone=0|lastcpupid=0xfffff)
    > raw: 07ffff8000000800 dead000000000100 dead000000000122 0000000000000000
    > raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
    > page dumped because: PAGE_FLAGS_CHECK_AT_PREP flag(s) set

    Does this problem happen right after the boot ? OR you ran some tests
    and workloads to trigger this ? IIRC never seen this before on arm64.
    Does this happen on other archs too ?

    >
    > This fixes the issue by calling flush_dcache_page() after each call
    > to set_{pud, pmd, pte}_at() because PG_arch_1 isn't needed in any case.

    This (arm64 specific solution) might cause some side effects on other
    platforms ? The solution here needs to be generic enough. I will take
    a look into this patch but probably later this week or next week.

    - Anshuman

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