Hi Catalin
Please see an important comment inline, thanks
> -----Original Message-----
> From: Catalin Marinas <catalin.mari...@arm.com>
> Sent: 2019年9月24日 1:05
> To: Justin He (Arm Technology China) <justin...@arm.com>
> Cc: Will Deacon <w...@kernel.org>; Mark Rutland
> <mark.rutl...@arm.com>; James Morse <james.mo...@arm.com>; Marc
> Zyngier <m...@kernel.org>; Matthew Wilcox <wi...@infradead.org>; Kirill A.
> Shutemov <kirill.shute...@linux.intel.com>; linux-arm-
> ker...@lists.infradead.org; linux-kernel@vger.kernel.org; linux-
> m...@kvack.org; Suzuki Poulose <suzuki.poul...@arm.com>; Punit
> Agrawal <punitagra...@gmail.com>; Anshuman Khandual
> <anshuman.khand...@arm.com>; Alex Van Brunt
> <avanbr...@nvidia.com>; Robin Murphy <robin.mur...@arm.com>;
> Thomas Gleixner <t...@linutronix.de>; Andrew Morton <akpm@linux-
> foundation.org>; Jérôme Glisse <jgli...@redhat.com>; Ralph Campbell
> <rcampb...@nvidia.com>; hejia...@gmail.com; Kaly Xin (Arm Technology
> China) <kaly....@arm.com>; nd <n...@arm.com>
> Subject: Re: [PATCH v8 3/3] mm: fix double page fault on arm64 if PTE_AF
> is cleared
>
> On Sat, Sep 21, 2019 at 09:50:54PM +0800, Jia He wrote:
> > @@ -2151,21 +2163,53 @@ static inline void cow_user_page(struct page
> *dst, struct page *src, unsigned lo
> > * 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);
> > + void *kaddr;
> > + pte_t entry;
> > + void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> >
> > + /* On architectures with software "accessed" bits, we would
> > + * take a double page fault, so mark it accessed here.
> > + */
>
> Nitpick: please follow the kernel coding style for multi-line comments
> (above and the for the rest of the patch):
>
> /*
> * Your multi-line comment.
> */
>
> > + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
> {
> > + vmf->pte = pte_offset_map_lock(mm, vmf->pmd,
> addr,
> > + &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);
> > + } else {
> > + /* 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.
> > + */
> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
> > + return false;
> > + }
> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
> > + }
>
> Another nit, you could rewrite this block slightly to avoid too much
> indentation. Something like (untested):
>
> if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
> {
> vmf->pte = pte_offset_map_lock(mm, vmf->pmd,
> addr,
> &vmf->ptl);
> if (unlikely(!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.
> */
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return false;
> }
> entry = pte_mkyoung(vmf->orig_pte);
> if (ptep_set_access_flags(vma, addr,
> vmf->pte, entry, 0))
> update_mmu_cache(vma, addr, vmf->pte);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> }
>
> > +
> > + kaddr = kmap_atomic(dst);
>
> Since you moved the kmap_atomic() here, could the above
> arch_faults_on_old_pte() run in a preemptible context? I suggested to
> add a WARN_ON in patch 2 to be sure.
Should I move kmap_atomic back to the original line? Thus, we can make sure
that arch_faults_on_old_pte() is in the context of preempt_disabled?
Otherwise, arch_faults_on_old_pte() may cause plenty of warning if I add
a WARN_ON in arch_faults_on_old_pte. I tested it when I enable the PREEMPT=y
on a ThunderX2 qemu guest.
--
Cheers,
Justin (Jia He)
>
> > /*
> > * 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.
>
> > clear_page(kaddr);
> > + }
> > kunmap_atomic(kaddr);
> > flush_dcache_page(dst);
> > } else
> > - copy_user_highpage(dst, src, va, vma);
> > + copy_user_highpage(dst, src, addr, vma);
> > +
> > + return true;
> > }
>
> --
> Catalin
