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Wed, 31 Jul 2019 17:10:11 +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+XLP3V6bjVaQAgAAZagCAAWo+gIAAHyQA Date: Wed, 31 Jul 2019 17:10:11 +0000 Message-ID: <04FB43C3-6E2B-4868-B9D5-C00342DA5C6F@fb.com> References: <20190730052305.3672336-1-songliubraving@fb.com> <20190730052305.3672336-4-songliubraving@fb.com> <20190730161113.GC18501@redhat.com> <1E2B5653-BA85-4A05-9B41-57CF9E48F14A@fb.com> <20190731151842.GB25078@redhat.com> In-Reply-To: <20190731151842.GB25078@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:c090:180::1:70cb] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: bf4a40bf-da5a-4e1d-e2bc-08d715d9f256 x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:MWHPR15MB1309; 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charset="us-ascii" Content-ID: <440364F82D87CB439B1BD89C43F6E8EB@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: bf4a40bf-da5a-4e1d-e2bc-08d715d9f256 X-MS-Exchange-CrossTenant-originalarrivaltime: 31 Jul 2019 17:10:11.2360 (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: MWHPR15MB1309 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-31_09:,, 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=859 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1907310190 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 31, 2019, at 8:18 AM, Oleg Nesterov wrote: >=20 > On 07/30, Song Liu wrote: >>=20 >>=20 >>> On Jul 30, 2019, at 9:11 AM, Oleg Nesterov wrote: >>>=20 >>> So after the next patch we have a single user of FOLL_SPLIT_PMD (uprobe= s) >>> and a single user of FOLL_SPLIT: arch/s390/mm/gmap.c:thp_split_mm(). >>>=20 >>> Hmm. >>=20 >> I think this is what we want. :) >=20 > We? I don't ;) >=20 >> FOLL_SPLIT is the fallback solution for users who cannot handle THP. >=20 > and again, we have a single user: thp_split_mm(). I do not know if it > can use FOLL_SPLIT_PMD or not, may be you can take a look... I haven't played with s390, so it gonna take me some time to ramp up.=20 I will add it to my to-do list.=20 >=20 >> With >> more THP aware code, there will be fewer users of FOLL_SPLIT. >=20 > Fewer than 1? Good ;) Yes! It will be great if thp_split_mm() can use FOLL_SPLIT_PMD=20 instead.=20 >=20 >>>> @@ -399,7 +399,7 @@ static struct page *follow_pmd_mask(struct vm_area= _struct *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= _struct *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_are= a_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= above. >>=20 >> split_huge_pmd() handles is_huge_zero_page() differently. In this case, = we >> cannot use the pmd_trans_unstable() check. >=20 > Please correct me, but iiuc the problem is not that split_huge_pmd() hand= les > is_huge_zero_page() differently, the problem is that __split_huge_pmd_loc= ked() > handles the !vma_is_anonymous(vma) differently and returns with pmd_none(= ) =3D T > after pmdp_huge_clear_flush_notify(). This means that pmd_trans_unstable(= ) will > fail. Agreed.=20 >=20 > Now, I don't understand why do we need pmd_trans_unstable() after > split_huge_pmd(huge-zero-pmd), but whatever reason we have, why can't we > unify both cases? >=20 > IOW, could you explain why the path below is wrong? I _think_ the following patch works (haven't fully tested yet). But I am no= t=20 sure whether this is the best. By separating the two cases, we don't duplic= ate=20 much code. And it is clear that the two cases are handled differently.=20 Therefore, I would prefer to keep these separate for now.=20 Thanks, Song >=20 >=20 > --- x/mm/gup.c > +++ x/mm/gup.c > @@ -399,14 +399,16 @@ static struct page *follow_pmd_mask(struct vm_area_= struct *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)) { > + if ((flags & FOLL_SPLIT_PMD) || is_huge_zero_page(page)) { > spin_unlock(ptl); > - ret =3D 0; > split_huge_pmd(vma, pmd, address); > - if (pmd_trans_unstable(pmd)) > + ret =3D 0; > + if (pte_alloc(mm, pmd)) > + ret =3D -ENOMEM; > + else if (pmd_trans_unstable(pmd)) > ret =3D -EBUSY; > } else { > if (unlikely(!try_get_page(page))) { >=20