From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751298AbeDDJxd (ORCPT ); Wed, 4 Apr 2018 05:53:33 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:54770 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751216AbeDDJxc (ORCPT ); Wed, 4 Apr 2018 05:53:32 -0400 Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF 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 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 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 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 X-TM-AS-GCONF: 00 x-cbid: 18040409-0020-0000-0000-0000040DE728 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18040409-0021-0000-0000-000042A1F577 Message-Id: <04b7be03-b90b-c890-d1bb-6981412a6b10@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-04-04_02:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1804040104 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.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.