Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

Kirill A. Shutemov
2019-09-20T14:21:17Z

On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote:
> When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest, there
> will be a double page fault in __copy_from_user_inatomic of cow_user_page.
>
> Below call trace is from arm64 do_page_fault for debugging purpose
> [ 110.016195] Call trace:
> [ 110.016826] do_page_fault+0x5a4/0x690
> [ 110.017812] do_mem_abort+0x50/0xb0
> [ 110.018726] el1_da+0x20/0xc4
> [ 110.019492] __arch_copy_from_user+0x180/0x280
> [ 110.020646] do_wp_page+0xb0/0x860
> [ 110.021517] __handle_mm_fault+0x994/0x1338
> [ 110.022606] handle_mm_fault+0xe8/0x180
> [ 110.023584] do_page_fault+0x240/0x690
> [ 110.024535] do_mem_abort+0x50/0xb0
> [ 110.025423] el0_da+0x20/0x24
>
> The pte info before __copy_from_user_inatomic is (PTE_AF is cleared):
> [ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003, pmd=000000023d4b3003, pte=360000298607bd3
>
> As told by Catalin: "On arm64 without hardware Access Flag, copying from
> user will fail because the pte is old and cannot be marked young. So we
> always end up with zeroed page after fork() + CoW for pfn mappings. we
> don't always have a hardware-managed access flag on arm64."
>
> This patch fix it by calling pte_mkyoung. Also, the parameter is
> changed because vmf should be passed to cow_user_page()
>
> Add a WARN_ON_ONCE when __copy_from_user_inatomic() returns error
> in case there can be some obscure use-case.(by Kirill)
>
> [1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
>
> Reported-by: Yibo Cai <Yibo.Cai@arm.com>
> Signed-off-by: Jia He <justin.he@arm.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

--
Kirill A. Shutemov