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Wed, 2 Sep 2020 03:58:09 +0000 (GMT) Received: from [9.199.61.124] (unknown [9.199.61.124]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 2 Sep 2020 03:58:09 +0000 (GMT) Subject: Re: [PATCH v3 13/13] mm/debug_vm_pgtable: populate a pte entry before fetching it To: Anshuman Khandual , linux-mm@kvack.org, akpm@linux-foundation.org References: <20200827080438.315345-1-aneesh.kumar@linux.ibm.com> <20200827080438.315345-14-aneesh.kumar@linux.ibm.com> <7ef7c302-e7e6-570e-3100-5dd1bf9551be@linux.ibm.com> <4ba15b8f-ac90-17ec-9b95-0451e2a38e98@arm.com> From: "Aneesh Kumar K.V" Message-ID: <7de344c3-26ea-be34-4435-8842360ed565@linux.ibm.com> Date: Wed, 2 Sep 2020 09:28:08 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <4ba15b8f-ac90-17ec-9b95-0451e2a38e98@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235, 18.0.687 definitions=2020-09-02_02:2020-09-01, 2020-09-02 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 bulkscore=0 malwarescore=0 suspectscore=0 mlxlogscore=999 phishscore=0 lowpriorityscore=0 mlxscore=0 impostorscore=0 adultscore=0 priorityscore=1501 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2006250000 definitions=main-2009020028 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, linux-s390@vger.kernel.org, Christophe Leroy , x86@kernel.org, Mike Rapoport , Qian Cai , Gerald Schaefer , Vineet Gupta , linux-snps-arc@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 9/2/20 9:19 AM, Anshuman Khandual wrote: > > > On 09/01/2020 03:28 PM, Aneesh Kumar K.V wrote: >> On 9/1/20 1:08 PM, Anshuman Khandual wrote: >>> >>> >>> On 09/01/2020 12:07 PM, Aneesh Kumar K.V wrote: >>>> On 9/1/20 8:55 AM, Anshuman Khandual wrote: >>>>> >>>>> >>>>> On 08/27/2020 01:34 PM, Aneesh Kumar K.V wrote: >>>>>> pte_clear_tests operate on an existing pte entry. Make sure that is not a none >>>>>> pte entry. >>>>>> >>>>>> Signed-off-by: Aneesh Kumar K.V >>>>>> --- >>>>>>    mm/debug_vm_pgtable.c | 6 ++++-- >>>>>>    1 file changed, 4 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c >>>>>> index 21329c7d672f..8527ebb75f2c 100644 >>>>>> --- a/mm/debug_vm_pgtable.c >>>>>> +++ b/mm/debug_vm_pgtable.c >>>>>> @@ -546,7 +546,7 @@ static void __init pgd_populate_tests(struct mm_struct *mm, pgd_t *pgdp, >>>>>>    static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep, >>>>>>                       unsigned long vaddr) >>>>>>    { >>>>>> -    pte_t pte = ptep_get(ptep); >>>>>> +    pte_t pte =  ptep_get_and_clear(mm, vaddr, ptep); >>>>> >>>>> Seems like ptep_get_and_clear() here just clears the entry in preparation >>>>> for a following set_pte_at() which otherwise would have been a problem on >>>>> ppc64 as you had pointed out earlier i.e set_pte_at() should not update an >>>>> existing valid entry. So the commit message here is bit misleading. >>>>> >>>> >>>> and also fetch the pte value which is used further. >>>> >>>> >>>>>>          pr_debug("Validating PTE clear\n"); >>>>>>        pte = __pte(pte_val(pte) | RANDOM_ORVALUE); >>>>>> @@ -944,7 +944,7 @@ static int __init debug_vm_pgtable(void) >>>>>>        p4d_t *p4dp, *saved_p4dp; >>>>>>        pud_t *pudp, *saved_pudp; >>>>>>        pmd_t *pmdp, *saved_pmdp, pmd; >>>>>> -    pte_t *ptep; >>>>>> +    pte_t *ptep, pte; >>>>>>        pgtable_t saved_ptep; >>>>>>        pgprot_t prot, protnone; >>>>>>        phys_addr_t paddr; >>>>>> @@ -1049,6 +1049,8 @@ static int __init debug_vm_pgtable(void) >>>>>>         */ >>>>>>          ptep = pte_alloc_map_lock(mm, pmdp, vaddr, &ptl); >>>>>> +    pte = pfn_pte(pte_aligned, prot); >>>>>> +    set_pte_at(mm, vaddr, ptep, pte); >>>>> >>>>> Not here, creating and populating an entry must be done in respective >>>>> test functions itself. Besides, this seems bit redundant as well. The >>>>> test pte_clear_tests() with the above change added, already >>>>> >>>>> - Clears the PTEP entry with ptep_get_and_clear() >>>> >>>> and fetch the old value set previously. >>> >>> In that case, please move above two lines i.e >>> >>> pte = pfn_pte(pte_aligned, prot); >>> set_pte_at(mm, vaddr, ptep, pte); >>> >>> from debug_vm_pgtable() to pte_clear_tests() and update it's arguments >>> as required. >>> >> >> Frankly, I don't understand what these tests are testing. It all looks like some random clear and set. > > The idea here is to have some value with some randomness preferably, in > a given PTEP before attempting to clear the entry, in order to make sure > that pte_clear() is indeed clearing something of non-zero value. > >> >> static void __init pte_clear_tests(struct mm_struct *mm, pte_t *ptep, >>                    unsigned long vaddr, unsigned long pfn, >>                    pgprot_t prot) >> { >> >>     pte_t pte = pfn_pte(pfn, prot); >>     set_pte_at(mm, vaddr, ptep, pte); >> >>     pte =  ptep_get_and_clear(mm, vaddr, ptep); > > Looking at this again, this preceding pfn_pte() followed by set_pte_at() > is not really required. Its reasonable to start with what ever was there > in the PTEP as a seed value which anyway gets added with RANDOM_ORVALUE. > s/ptep_get/ptep_get_and_clear is sufficient to take care of the powerpc > set_pte_at() constraint. > But the way test is written we had none pte before. That is why I added that set_pte_at to put something there. With none pte the below sequence fails. pte = __pte(pte_val(pte) | RANDOM_ORVALUE); set_pte_at(mm, vaddr, ptep, pte); because nobody is marking a _PAGE_PTE there. pte_t pte = pfn_pte(pfn, prot); pr_debug("Validating PTE clear\n"); pte = __pte(pte_val(pte) | RANDOM_ORVALUE); set_pte_at(mm, vaddr, ptep, pte); barrier(); pte_clear(mm, vaddr, ptep); pte = ptep_get(ptep); WARN_ON(!pte_none(pte)); will that work for you? -aneesh