From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f197.google.com (mail-qt0-f197.google.com [209.85.216.197]) by kanga.kvack.org (Postfix) with ESMTP id A7B776B0003 for ; Tue, 3 Apr 2018 15:10:15 -0400 (EDT) Received: by mail-qt0-f197.google.com with SMTP id h4so13951968qtj.11 for ; Tue, 03 Apr 2018 12:10:15 -0700 (PDT) Received: from mx1.redhat.com (mx3-rdu2.redhat.com. [66.187.233.73]) by mx.google.com with ESMTPS id n189si738316qkc.450.2018.04.03.12.10.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 03 Apr 2018 12:10:14 -0700 (PDT) Date: Tue, 3 Apr 2018 15:10:05 -0400 From: Jerome Glisse Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF Message-ID: <20180403191005.GC5935@redhat.com> References: <1520963994-28477-1-git-send-email-ldufour@linux.vnet.ibm.com> <1520963994-28477-7-git-send-email-ldufour@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1520963994-28477-7-git-send-email-ldufour@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Laurent Dufour Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org, akpm@linux-foundation.org, kirill@shutemov.name, ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , Sergey Senozhatsky , Andrea Arcangeli , Alexei Starovoitov , kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com, Daniel Jordan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, bsingharora@gmail.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org On Tue, Mar 13, 2018 at 06:59:36PM +0100, Laurent Dufour wrote: > pte_unmap_same() is making the assumption that the page table are still > around because the mmap_sem is held. > This is no more the case when running a speculative page fault and > additional check must be made to ensure that the final page table are still > there. > > This is now done by calling pte_spinlock() to check for the VMA's > consistency while locking for the page tables. > > This is requiring passing a vm_fault structure to pte_unmap_same() which is > containing all the needed parameters. > > As pte_spinlock() may fail in the case of a speculative page fault, if the > VMA has been touched in our back, pte_unmap_same() should now return 3 > cases : > 1. pte are the same (0) > 2. pte are different (VM_FAULT_PTNOTSAME) > 3. a VMA's changes has been detected (VM_FAULT_RETRY) > > The case 2 is handled by the introduction of a new VM_FAULT flag named > VM_FAULT_PTNOTSAME which is then trapped in cow_user_page(). > If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the > page fault while holding the mmap_sem. > > Signed-off-by: Laurent Dufour > --- > include/linux/mm.h | 1 + > mm/memory.c | 29 +++++++++++++++++++---------- > 2 files changed, 20 insertions(+), 10 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 2f3e98edc94a..b6432a261e63 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1199,6 +1199,7 @@ static inline void clear_page_pfmemalloc(struct page *page) > #define VM_FAULT_NEEDDSYNC 0x2000 /* ->fault did not modify page tables > * and needs fsync() to complete (for > * synchronous page faults in DAX) */ > +#define VM_FAULT_PTNOTSAME 0x4000 /* Page table entries have changed */ > > #define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \ > VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \ > diff --git a/mm/memory.c b/mm/memory.c > index 21b1212a0892..4bc7b0bdcb40 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf) > * parts, do_swap_page must check under lock before unmapping the pte and > * proceeding (but do_wp_page is only called after already making such a check; > * and do_anonymous_page can safely check later on). > + * > + * pte_unmap_same() returns: > + * 0 if the PTE are the same > + * VM_FAULT_PTNOTSAME if the PTE are different > + * VM_FAULT_RETRY if the VMA has changed in our back during > + * a speculative page fault handling. > */ > -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd, > - pte_t *page_table, pte_t orig_pte) > +static inline int pte_unmap_same(struct vm_fault *vmf) > { > - int same = 1; > + int ret = 0; > + > #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT) > if (sizeof(pte_t) > sizeof(unsigned long)) { > - spinlock_t *ptl = pte_lockptr(mm, pmd); > - spin_lock(ptl); > - same = pte_same(*page_table, orig_pte); > - spin_unlock(ptl); > + if (pte_spinlock(vmf)) { > + if (!pte_same(*vmf->pte, vmf->orig_pte)) > + ret = VM_FAULT_PTNOTSAME; > + spin_unlock(vmf->ptl); > + } else > + ret = VM_FAULT_RETRY; > } > #endif > - pte_unmap(page_table); > - return same; > + pte_unmap(vmf->pte); > + return ret; > } > > static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) > @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf) > int exclusive = 0; > int ret = 0; > > - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) > + ret = pte_unmap_same(vmf); > + if (ret) > goto out; > This change what do_swap_page() returns ie before it was returning 0 when locked pte lookup was different from orig_pte. After this patch it returns VM_FAULT_PTNOTSAME but this is a new return value for handle_mm_fault() (the do_swap_page() return value is what ultimately get return by handle_mm_fault()) Do we really want that ? This might confuse some existing user of handle_mm_fault() and i am not sure of the value of that information to caller. Note i do understand that you want to return retry if anything did change from underneath and thus need to differentiate from when the pte value are not the same. Cheers, Jerome