Re: [PATCH] mm/debug_vm_pgtable: Fix corrupted PG_arch_1 by set_pmd_at()

[Anshuman Khandual]

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