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Tue, 30 Jul 2019 17:42:13 +0000 From: Song Liu To: Oleg Nesterov CC: lkml , "linux-mm@kvack.org" , "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 v10 3/4] mm, thp: introduce FOLL_SPLIT_PMD Thread-Topic: [PATCH v10 3/4] mm, thp: introduce FOLL_SPLIT_PMD Thread-Index: AQHVRpbyXlkLbIiwxECjtBJ+XLP3V6bjVaQAgAAZagA= Date: Tue, 30 Jul 2019 17:42:13 +0000 Message-ID: <1E2B5653-BA85-4A05-9B41-57CF9E48F14A@fb.com> References: <20190730052305.3672336-1-songliubraving@fb.com> <20190730052305.3672336-4-songliubraving@fb.com> <20190730161113.GC18501@redhat.com> In-Reply-To: <20190730161113.GC18501@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::3:5cb8] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 34279050-7834-401d-5ba5-08d71515417c x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:MWHPR15MB1903; 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charset="us-ascii" Content-ID: <6751533878403B42A328D71EE1165B34@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 34279050-7834-401d-5ba5-08d71515417c X-MS-Exchange-CrossTenant-originalarrivaltime: 30 Jul 2019 17:42:13.1318 (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: MWHPR15MB1903 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-30_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=898 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1907300185 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 Jul 30, 2019, at 9:11 AM, Oleg Nesterov wrote: >=20 > I don't understand this code, so I can't review, but. >=20 > On 07/29, Song Liu wrote: >>=20 >> This patches introduces a new foll_flag: FOLL_SPLIT_PMD. As the name say= s >> FOLL_SPLIT_PMD splits huge pmd for given mm_struct, the underlining huge >> page stays as-is. >>=20 >> FOLL_SPLIT_PMD is useful for cases where we need to use regular pages, >> but would switch back to huge page and huge pmd on. One of such example >> is uprobe. The following patches use FOLL_SPLIT_PMD in uprobe. >=20 > So after the next patch we have a single user of FOLL_SPLIT_PMD (uprobes) > and a single user of FOLL_SPLIT: arch/s390/mm/gmap.c:thp_split_mm(). >=20 > Hmm. I think this is what we want. :)=20 FOLL_SPLIT is the fallback solution for users who cannot handle THP. With more THP aware code, there will be fewer users of FOLL_SPLIT.=20 >=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); >=20 > I fail to understand why this differs from the is_huge_zero_page() case a= bove. split_huge_pmd() handles is_huge_zero_page() differently. In this case, we= =20 cannot use the pmd_trans_unstable() check.=20 >=20 > Anyway, ret =3D pte_alloc(mm, pmd) can't be correct. If __pte_alloc() fai= ls pte_alloc() > will return 1. This will fool the IS_ERR(page) check in __get_user_pages(= ). Great catch! Let me fix it. Thanks, Song