From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f69.google.com (mail-pl0-f69.google.com [209.85.160.69]) by kanga.kvack.org (Postfix) with ESMTP id 4E64B6B0003 for ; Tue, 3 Apr 2018 16:40:22 -0400 (EDT) Received: by mail-pl0-f69.google.com with SMTP id t8-v6so9059274ply.22 for ; Tue, 03 Apr 2018 13:40:22 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id i190sor851824pge.333.2018.04.03.13.40.20 for (Google Transport Security); Tue, 03 Apr 2018 13:40:20 -0700 (PDT) Date: Tue, 3 Apr 2018 13:40:18 -0700 (PDT) From: David Rientjes Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF In-Reply-To: <20180403191005.GC5935@redhat.com> Message-ID: References: <1520963994-28477-1-git-send-email-ldufour@linux.vnet.ibm.com> <1520963994-28477-7-git-send-email-ldufour@linux.vnet.ibm.com> <20180403191005.GC5935@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse Cc: Laurent Dufour , 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, 3 Apr 2018, Jerome Glisse wrote: > > 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. > I think VM_FAULT_RETRY should be handled appropriately for any user of handle_mm_fault() already, and would be surprised to learn differently. Khugepaged has the appropriate handling. I think the concern is whether a user is handling anything other than VM_FAULT_RETRY and VM_FAULT_ERROR (which VM_FAULT_PTNOTSAME is not set in)? I haven't done a full audit of the users.