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Shutemov" , Kernel Team , "William Kucharski" , "srikar@linux.vnet.ibm.com" Subject: Re: [PATCH v3 1/2] khugepaged: enable collapse pmd for pte-mapped THP Thread-Topic: [PATCH v3 1/2] khugepaged: enable collapse pmd for pte-mapped THP Thread-Index: AQHVSJnBcC4K72VM4ESRnUeFDmyc+aboC4IAgABGwoA= Date: Fri, 2 Aug 2019 20:34:52 +0000 Message-ID: References: <20190801184823.3184410-1-songliubraving@fb.com> <20190801184823.3184410-2-songliubraving@fb.com> <20190802162136.GA2539@redhat.com> In-Reply-To: <20190802162136.GA2539@redhat.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3445.104.11) x-originating-ip: [2620:10d:c091:480::5569] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: c7c0e2d4-9c71-46bd-4a12-08d71788df83 x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:MWHPR15MB1710; 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charset="us-ascii" Content-ID: <4B97E34F69EA784386431931B1CAC4C9@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: c7c0e2d4-9c71-46bd-4a12-08d71788df83 X-MS-Exchange-CrossTenant-originalarrivaltime: 02 Aug 2019 20:34:52.7127 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: songliubraving@fb.com X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR15MB1710 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-02_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=705 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908020219 X-FB-Internal: deliver X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Aug 2, 2019, at 9:21 AM, Oleg Nesterov wrote: >=20 > On 08/01, Song Liu wrote: >>=20 >> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm, >> + unsigned long addr) >> +{ >> + struct mm_slot *mm_slot; >> + int ret =3D 0; >> + >> + /* hold mmap_sem for khugepaged_test_exit() */ >> + VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm); >> + VM_BUG_ON(addr & ~HPAGE_PMD_MASK); >> + >> + if (unlikely(khugepaged_test_exit(mm))) >> + return 0; >> + >> + if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) && >> + !test_bit(MMF_DISABLE_THP, &mm->flags)) { >> + ret =3D __khugepaged_enter(mm); >> + if (ret) >> + return ret; >> + } >=20 > see my reply to v2 >=20 >> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long haddr) >> +{ >> + struct vm_area_struct *vma =3D find_vma(mm, haddr); >> + pmd_t *pmd =3D mm_find_pmd(mm, haddr); >> + struct page *hpage =3D NULL; >> + unsigned long addr; >> + spinlock_t *ptl; >> + int count =3D 0; >> + pmd_t _pmd; >> + int i; >> + >> + VM_BUG_ON(haddr & ~HPAGE_PMD_MASK); >> + >> + if (!vma || !vma->vm_file || !pmd) > ^^^^^^^^^^^^^ >=20 > I am not sure this is enough, >=20 >> + return; >> + >> + /* step 1: check all mapped PTEs are to the right huge page */ >> + for (i =3D 0, addr =3D haddr; i < HPAGE_PMD_NR; i++, addr +=3D PAGE_SI= ZE) { >> + pte_t *pte =3D pte_offset_map(pmd, addr); >> + struct page *page; >> + >> + if (pte_none(*pte)) >> + continue; >> + >> + page =3D vm_normal_page(vma, addr, *pte); >=20 > Why can't vm_normal_page() return NULL? Again, we do not if this vm_file > is the same shmem_file() or something else. Hmm.. I guess we also need to check page !=3D NULL.=20 For vm_file, I guess it is OK for it to be another file. As long as all pages in the pmd map to the same THP, we should be fine.=20 >=20 > And in fact I don't think it is safe to use vm_normal_page(vma, addr) > unless you know that vma includes this addr. Yeah, we need to check vma includes this address.=20 >=20 > to be honest, I am not even sure that unconditional mm_find_pmd() is safe > if this "something else" is really special. I cannot imagine why mm_find_pmd() could be unsafe.=20 Thanks, Song