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Thu, 8 Aug 2019 17:16:08 +0000 Received: from MWHPR15MB1165.namprd15.prod.outlook.com ([fe80::79c8:442d:b528:802d]) by MWHPR15MB1165.namprd15.prod.outlook.com ([fe80::79c8:442d:b528:802d%9]) with mapi id 15.20.2157.015; Thu, 8 Aug 2019 17:16:08 +0000 From: Song Liu To: Oleg Nesterov CC: Linux Kernel Mailing List , Linux MM , "akpm@linux-foundation.org" , "matthew.wilcox@oracle.com" , "kirill.shutemov@linux.intel.com" , Kernel Team , "william.kucharski@oracle.com" , "srikar@linux.vnet.ibm.com" Subject: Re: [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD Thread-Topic: [PATCH v12 3/6] mm, thp: introduce FOLL_SPLIT_PMD Thread-Index: AQHVTXk1mNBIE/jl/kWGntKwDXuhGKbxdEOAgAAKuIA= Date: Thu, 8 Aug 2019 17:16:08 +0000 Message-ID: <48316E06-10B2-439C-AD10-3EC8C86C259C@fb.com> References: <20190807233729.3899352-1-songliubraving@fb.com> <20190807233729.3899352-4-songliubraving@fb.com> <20190808163745.GC7934@redhat.com> In-Reply-To: <20190808163745.GC7934@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:c090:200::3099] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: b5e3fcc2-70df-48ea-c555-08d71c241a5c x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:MWHPR15MB1376; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: b5e3fcc2-70df-48ea-c555-08d71c241a5c X-MS-Exchange-CrossTenant-originalarrivaltime: 08 Aug 2019 17:16:08.0563 (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: lQWtUvIXPiJDMJozyL77Mq5vJnG5XrjzgXg3gCovQfXmBHlSOLFdEjEiqAzEh9bawOsCYL6/StzGD7hyXco5rg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR15MB1376 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-08_07:,, 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=959 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908080154 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Aug 8, 2019, at 9:37 AM, Oleg Nesterov wrote: >=20 > On 08/07, Song Liu wrote: >>=20 >> @@ -399,7 +399,7 @@ static struct page *follow_pmd_mask(struct vm_area_s= truct *vma, >> spin_unlock(ptl); >> return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap); >> } >> - if (flags & FOLL_SPLIT) { >> + if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) { >> int ret; >> page =3D pmd_page(*pmd); >> if (is_huge_zero_page(page)) { >> @@ -408,7 +408,7 @@ static struct page *follow_pmd_mask(struct vm_area_s= truct *vma, >> split_huge_pmd(vma, pmd, address); >> if (pmd_trans_unstable(pmd)) >> ret =3D -EBUSY; >> - } else { >> + } else if (flags & FOLL_SPLIT) { >> if (unlikely(!try_get_page(page))) { >> spin_unlock(ptl); >> return ERR_PTR(-ENOMEM); >> @@ -420,6 +420,10 @@ static struct page *follow_pmd_mask(struct vm_area_= struct *vma, >> put_page(page); >> if (pmd_none(*pmd)) >> return no_page_table(vma, flags); >> + } else { /* flags & FOLL_SPLIT_PMD */ >> + spin_unlock(ptl); >> + split_huge_pmd(vma, pmd, address); >> + ret =3D pte_alloc(mm, pmd) ? -ENOMEM : 0; >> } >=20 > Can't resist, let me repeat that I do not like this patch because imo > it complicates this code for no reason. Personally, I don't think this is more complicated than your version.=20 This patch is safe as it doesn't change any code for is_huge_zero_page()=20 case.=20 Also, if some code calls follow_pmd_mask() with flags contains both=20 FOLL_SPLIT and FOLL_SPLIT_PMD, we should honor FOLL_SPLIT and split the huge page. Of course, there is no code that sets both flags. Does this resolve your concern here? Thanks, Song