From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f200.google.com (mail-qk0-f200.google.com [209.85.220.200]) by kanga.kvack.org (Postfix) with ESMTP id 108E16B000C for ; Wed, 4 Apr 2018 05:53:34 -0400 (EDT) Received: by mail-qk0-f200.google.com with SMTP id v6so4700128qkd.23 for ; Wed, 04 Apr 2018 02:53:34 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id c6si3610986qtg.258.2018.04.04.02.53.32 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 04 Apr 2018 02:53:33 -0700 (PDT) Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w349qDPP143750 for ; Wed, 4 Apr 2018 05:53:32 -0400 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0a-001b2d01.pphosted.com with ESMTP id 2h4uw1shv4-1 (version=TLSv1.2 cipher=AES256-SHA256 bits=256 verify=NOT) for ; Wed, 04 Apr 2018 05:53:31 -0400 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 4 Apr 2018 10:53:28 +0100 Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF 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> From: Laurent Dufour Date: Wed, 4 Apr 2018 11:53:18 +0200 MIME-Version: 1.0 In-Reply-To: <20180403191005.GC5935@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <04b7be03-b90b-c890-d1bb-6981412a6b10@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Jerome Glisse 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 03/04/2018 21:10, Jerome Glisse wrote: > 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. You're right, do_swap_page() should still return 0 in the case the lookup pte is different from orig_pte, assuming that the swap operation has been handled in our back by another concurrent thread. So in do_swap_page(), VM_FAULT_PTNOTSAME should be translated in ret = 0. ret = pte_unmap_same(vmf); if (ret) { /* * If pte != orig_pte, this means another thread did the * swap operation in our back. * So nothing else to do. */ if (ret == VM_FAULT_PTNOTSAME) ret = 0; goto out; } This means that VM_FAULT_PTNOTSAME will never been reported up and limited to do_swap_page(). Doing this will make easier to understand why when pte_unmap_same() is returning 0, do_swap_page() is done. Cheers, Laurent.