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Message-ID: <VI1SPR01MB0218FB365FE518F4FCB37ADF7890@VI1SPR01MB021.eurprd08.prod.outlook.com>
Date: Thu, 19 Sep 2019 15:02:29 +0000
From: "Justin He (Arm Technology China)" <Justin.He@....com>
To: "Kirill A. Shutemov" <kirill@...temov.name>,
Jia He <hejianet@...il.com>
CC: Catalin Marinas <Catalin.Marinas@....com>,
Will Deacon <will@...nel.org>,
Mark Rutland <Mark.Rutland@....com>,
James Morse <James.Morse@....com>,
Marc Zyngier <maz@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
Suzuki Poulose <Suzuki.Poulose@....com>,
Punit Agrawal <punitagrawal@...il.com>,
Anshuman Khandual <Anshuman.Khandual@....com>,
Jun Yao <yaojun8558363@...il.com>,
Alex Van Brunt <avanbrunt@...dia.com>,
Robin Murphy <Robin.Murphy@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Jérôme Glisse <jglisse@...hat.com>,
Ralph Campbell <rcampbell@...dia.com>,
"Kaly Xin (Arm Technology China)" <Kaly.Xin@....com>
Subject: RE: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is
cleared
Hi Kirill
Thanks for the detailed explanation.
--
Cheers,
Justin (Jia He)
> -----Original Message-----
> From: Kirill A. Shutemov <kirill@...temov.name>
> Sent: 2019年9月19日 22:58
> To: Jia He <hejianet@...il.com>
> Cc: Justin He (Arm Technology China) <Justin.He@....com>; Catalin
> Marinas <Catalin.Marinas@....com>; Will Deacon <will@...nel.org>; Mark
> Rutland <Mark.Rutland@....com>; James Morse
> <James.Morse@....com>; Marc Zyngier <maz@...nel.org>; Matthew
> Wilcox <willy@...radead.org>; Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com>; linux-arm-kernel@...ts.infradead.org;
> linux-kernel@...r.kernel.org; linux-mm@...ck.org; Suzuki Poulose
> <Suzuki.Poulose@....com>; Punit Agrawal <punitagrawal@...il.com>;
> Anshuman Khandual <Anshuman.Khandual@....com>; Jun Yao
> <yaojun8558363@...il.com>; Alex Van Brunt <avanbrunt@...dia.com>;
> Robin Murphy <Robin.Murphy@....com>; Thomas Gleixner
> <tglx@...utronix.de>; Andrew Morton <akpm@...ux-foundation.org>;
> Jérôme Glisse <jglisse@...hat.com>; Ralph Campbell
> <rcampbell@...dia.com>; Kaly Xin (Arm Technology China)
> <Kaly.Xin@....com>
> Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is
> cleared
>
> On Thu, Sep 19, 2019 at 10:16:34AM +0800, Jia He wrote:
> > Hi Kirill
> >
> > [On behalf of justin.he@....com because some mails are filted...]
> >
> > On 2019/9/18 22:00, Kirill A. Shutemov wrote:
> > > On Wed, Sep 18, 2019 at 09:19:14PM +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()
> > > >
> > > > [1]
> https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork
> > > >
> > > > Reported-by: Yibo Cai <Yibo.Cai@....com>
> > > > Signed-off-by: Jia He <justin.he@....com>
> > > > ---
> > > > mm/memory.c | 35 ++++++++++++++++++++++++++++++-----
> > > > 1 file changed, 30 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index e2bb51b6242e..d2c130a5883b 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
> > > > 2;
> > > > #endif
> > > > +#ifndef arch_faults_on_old_pte
> > > > +static inline bool arch_faults_on_old_pte(void)
> > > > +{
> > > > + return false;
> > > > +}
> > > > +#endif
> > > > +
> > > > static int __init disable_randmaps(char *s)
> > > > {
> > > > randomize_va_space = 0;
> > > > @@ -2140,8 +2147,12 @@ static inline int pte_unmap_same(struct
> mm_struct *mm, pmd_t *pmd,
> > > > return same;
> > > > }
> > > > -static inline void cow_user_page(struct page *dst, struct page *src,
> unsigned long va, struct vm_area_struct *vma)
> > > > +static inline void cow_user_page(struct page *dst, struct page *src,
> > > > + struct vm_fault *vmf)
> > > > {
> > > > + struct vm_area_struct *vma = vmf->vma;
> > > > + unsigned long addr = vmf->address;
> > > > +
> > > > debug_dma_assert_idle(src);
> > > > /*
> > > > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct
> page *dst, struct page *src, unsigned lo
> > > > */
> > > > if (unlikely(!src)) {
> > > > void *kaddr = kmap_atomic(dst);
> > > > - void __user *uaddr = (void __user *)(va & PAGE_MASK);
> > > > + void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> > > > + pte_t entry;
> > > > /*
> > > > * This really shouldn't fail, because the page is there
> > > > * in the page tables. But it might just be unreadable,
> > > > * in which case we just give up and fill the result with
> > > > - * zeroes.
> > > > + * zeroes. On architectures with software "accessed" bits,
> > > > + * we would take a double page fault here, so mark it
> > > > + * accessed here.
> > > > */
> > > > + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
> {
> > > > + spin_lock(vmf->ptl);
> > > > + if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> > > > + entry = pte_mkyoung(vmf->orig_pte);
> > > > + if (ptep_set_access_flags(vma, addr,
> > > > + vmf->pte, entry, 0))
> > > > + update_mmu_cache(vma, addr, vmf-
> >pte);
> > > > + }
> > > I don't follow.
> > >
> > > So if pte has changed under you, you don't set the accessed bit, but
> never
> > > the less copy from the user.
> > >
> > > What makes you think it will not trigger the same problem?
> > >
> > > I think we need to make cow_user_page() fail in this case and caller --
> > > wp_page_copy() -- return zero. If the fault was solved by other thread,
> we
> > > are fine. If not userspace would re-fault on the same address and we
> will
> > > handle the fault from the second attempt.
> >
> > Thanks for the pointing. How about make cow_user_page() be returned
> >
> > VM_FAULT_RETRY? Then in do_page_fault(), it can retry the page fault?
>
> No. VM_FAULT_RETRY has different semantics: we have to drop
> mmap_sem(), so
> let's try to take it again and handle the fault. In this case the more
> likely scenario is that other thread has already handled the fault and we
> don't need to do anything. If it's not the case, the fault will be
> triggered again on the same address.
>
> --
> Kirill A. Shutemov
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