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Message-ID: <20190924163542.GI41214@arrakis.emea.arm.com>
Date: Tue, 24 Sep 2019 17:35:42 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Jia He <hejianet@...il.com>
Cc: "Justin He (Arm Technology China)" <Justin.He@....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>,
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>,
nd <nd@....com>
Subject: Re: [PATCH v8 3/3] mm: fix double page fault on arm64 if PTE_AF is
cleared
On Tue, Sep 24, 2019 at 11:29:07PM +0800, Jia He wrote:
> On 2019/9/24 18:33, Catalin Marinas wrote:
> > On Tue, Sep 24, 2019 at 06:43:06AM +0000, Justin He (Arm Technology China) wrote:
> > > Catalin Marinas wrote:
> > > > On Sat, Sep 21, 2019 at 09:50:54PM +0800, Jia He wrote:
> > > > > /*
> > > > > * 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.
> > > > > */
> > > > > - if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> > > > > + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
> > > > > + /* Give a warn in case there can be some obscure
> > > > > + * use-case
> > > > > + */
> > > > > + WARN_ON_ONCE(1);
> > > > That's more of a question for the mm guys: at this point we do the
> > > > copying with the ptl released; is there anything else that could have
> > > > made the pte old in the meantime? I think unuse_pte() is only called on
> > > > anonymous vmas, so it shouldn't be the case here.
> >
> > If we need to hold the ptl here, you could as well have an enclosing
> > kmap/kunmap_atomic (option 2) with some goto instead of "return false".
>
> I am not 100% sure that I understand your suggestion well, so I
> drafted the patch
Well, however you think the code is cleaner really.
The copy/paste didn't work well, tabs disappeared (or rather the
Exchange server corrupting outgoing emails) but I'll try to comment
below:
> -static inline void cow_user_page(struct page *dst, struct page *src,
> unsigned long va, struct vm_area_struct *vma)
> +static inline bool cow_user_page(struct page *dst, struct page *src,
> + struct vm_fault *vmf)
> {
> + struct vm_area_struct *vma = vmf->vma;
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long addr = vmf->address;
> + bool ret;
> + pte_t entry;
> + void *kaddr;
> + void __user *uaddr;
> +
> debug_dma_assert_idle(src);
>
> + if (likely(src)) {
> + copy_user_highpage(dst, src, addr, vma);
> + return true;
> + }
> +
> /*
> * If the source page was a PFN mapping, we don't have
> * a "struct page" for it. We do a best-effort copy by
> * just copying from the original user address. If that
> * fails, we just zero-fill it. Live with it.
> */
> - if (unlikely(!src)) {
> - void *kaddr = kmap_atomic(dst);
> - void __user *uaddr = (void __user *)(va & PAGE_MASK);
> + kaddr = kmap_atomic(dst);
> + uaddr = (void __user *)(addr & PAGE_MASK);
> +
> + /*
> + * On architectures with software "accessed" bits, we would
> + * take a double page fault, so mark it accessed here.
> + */
> + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
> + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
I'd move the pte_offset_map_lock() inside the 'if' block as we don't
want to affect architectures that handle old ptes automatically.
> + if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> + /*
> + * 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.
> + */
> + ret = false;
> + goto pte_unlock;
> + }
> +
> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vma, addr, vmf->pte, entry, 0))
> + update_mmu_cache(vma, addr, vmf->pte);
> + }
>
> + /*
> + * 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.
> + */
> + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
> /*
> - * 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.
> + * Give a warn in case there can be some obscure
> + * use-case
> */
> - if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> - clear_page(kaddr);
> - kunmap_atomic(kaddr);
> - flush_dcache_page(dst);
> - } else
> - copy_user_highpage(dst, src, va, vma);
> + WARN_ON_ONCE(1);
> + clear_page(kaddr);
> + }
> +
> + ret = true;
> +
> +pte_unlock:
> + pte_unmap_unlock(vmf->pte, vmf->ptl);
Since the locking would be moved in the 'if' block above, we need
another check here before unlocking:
if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
pte_unmap_unlock(vmf->pte, vmf->ptl);
You could probably replace the two calls to arch_faults_on_old_pte()
with a single bool variable initialisation, something like:
force_mkyoung = arch_faults_on_old_pte() &&
!pte_young(vmf->orig_pte)
and only check for "if (force_mkyoung)" in both cases.
> + kunmap_atomic(kaddr);
> + flush_dcache_page(dst);
> +
> + return ret;
> }
--
Catalin
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