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

["Justin He (Arm Technology China)" <Justin.He@arm.com>]

Thanks for your patent review 😊

--
Cheers,
Justin (Jia He)



> -----Original Message-----
> From: Kirill A. Shutemov <kirill@shutemov.name>
> Sent: 2019年9月20日 22:21
> To: Justin He (Arm Technology China) <Justin.He@arm.com>
> Cc: Catalin Marinas <Catalin.Marinas@arm.com>; Will Deacon
> <will@kernel.org>; Mark Rutland <Mark.Rutland@arm.com>; James Morse
> <James.Morse@arm.com>; Marc Zyngier <maz@kernel.org>; Matthew
> Wilcox <willy@infradead.org>; Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; linux-mm@kvack.org; Suzuki Poulose
> <Suzuki.Poulose@arm.com>; Punit Agrawal <punitagrawal@gmail.com>;
> Anshuman Khandual <Anshuman.Khandual@arm.com>; Alex Van Brunt
> <avanbrunt@nvidia.com>; Robin Murphy <Robin.Murphy@arm.com>;
> Thomas Gleixner <tglx@linutronix.de>; Andrew Morton <akpm@linux-
> foundation.org>; Jérôme Glisse <jglisse@redhat.com>; Ralph Campbell
> <rcampbell@nvidia.com>; hejianet@gmail.com; Kaly Xin (Arm Technology
> China) <Kaly.Xin@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v7 3/3] mm: fix double page fault on arm64 if PTE_AF is
> cleared
> 
> 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