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Shutemov" , Kernel Team , "William Kucharski" , Andrew Morton , "hdanton@sina.com" Subject: Re: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS Thread-Topic: [PATCH v9 5/6] mm,thp: add read-only THP support for (non-shmem) FS Thread-Index: AQHVKurnm0pZpZevHki8fTgvnJjHiqbESjWAgBMt6IA= Date: Mon, 22 Jul 2019 23:41:25 +0000 Message-ID: References: <20190625001246.685563-1-songliubraving@fb.com> <20190625001246.685563-6-songliubraving@fb.com> <20190710184811.GF11197@cmpxchg.org> In-Reply-To: <20190710184811.GF11197@cmpxchg.org> 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::2:c799] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: fc390eae-4e6a-472e-f34c-08d70efe1c86 x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:MWHPR15MB1198; x-ms-traffictypediagnostic: MWHPR15MB1198: x-microsoft-antispam-prvs: x-ms-oob-tlc-oobclassifiers: OLM:9508; x-forefront-prvs: 01068D0A20 x-forefront-antispam-report: SFV:NSPM;SFS:(10019020)(39860400002)(376002)(366004)(136003)(396003)(346002)(189003)(199004)(71190400001)(6246003)(81166006)(81156014)(316002)(76176011)(6506007)(6116002)(53546011)(305945005)(71200400001)(4326008)(64756008)(6436002)(6512007)(66476007)(66946007)(76116006)(66556008)(7736002)(66446008)(256004)(6486002)(14444005)(476003)(186003)(2906002)(486006)(68736007)(14454004)(8936002)(8676002)(229853002)(57306001)(86362001)(54906003)(102836004)(6916009)(25786009)(478600001)(46003)(50226002)(99286004)(5660300002)(53936002)(33656002)(446003)(11346002)(36756003)(2616005)(142933001);DIR:OUT;SFP:1102;SCL:1;SRVR:MWHPR15MB1198;H:MWHPR15MB1165.namprd15.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:1; received-spf: None (protection.outlook.com: fb.com does not designate permitted sender hosts) x-ms-exchange-senderadcheck: 1 x-microsoft-antispam-message-info: fTR1i/LKTvXLh742Ol9lB4spEwORWJUnasJXAlxSnNj1yggM53uXyCAykMd0ZBzPF2ZsD1sBzmYVuhgflfnSdyw0G9U7b3hf0KvHcecD3BSufR5KsS+XhB+C6O+IqLrrI1Po36Eux3CVo0qCg6BfcxK5RKrLjJZFyMPUwJWZzLab5ke1JbNHpa7C342towggOB4LOg8P8sfeSiA2R51AAi6zhVQYo1AodRhuytY2zq474WY5OLMVzrdZMy+CV3Wz0i+o7w1JV9wsZSrhM0C2pkN1uAXjGQ2e4VpKPvFVe7ptVflYl0dsoJBFQ6q1pqlJJvzjbsZHkNixXDlKP8tctUAViKQJU5oFiVuKp8TxWl8oP14HUVaocCTHpuskJsAZq4rxqyuoI0WaXtKKlg709F8JvjK6aRTDsKuWYtsxbmY= Content-Type: text/plain; charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: fc390eae-4e6a-472e-f34c-08d70efe1c86 X-MS-Exchange-CrossTenant-originalarrivaltime: 22 Jul 2019 23:41:25.5489 (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: MWHPR15MB1198 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-22_16:,, 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=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1907220250 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 10, 2019, at 11:48 AM, Johannes Weiner wrote: >=20 > On Mon, Jun 24, 2019 at 05:12:45PM -0700, Song Liu wrote: >> This patch is (hopefully) the first step to enable THP for non-shmem >> filesystems. >>=20 >> This patch enables an application to put part of its text sections to TH= P >> via madvise, for example: >>=20 >> madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE); >>=20 >> We tried to reuse the logic for THP on tmpfs. >>=20 >> Currently, write is not supported for non-shmem THP. khugepaged will onl= y >> process vma with VM_DENYWRITE. sys_mmap() ignores VM_DENYWRITE requests >> (see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is >> execve(). This requirement limits non-shmem THP to text sections. >>=20 >> The next patch will handle writes, which would only happen when the all >> the vmas with VM_DENYWRITE are unmapped. >>=20 >> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this >> feature. >>=20 >> Acked-by: Rik van Riel >> Signed-off-by: Song Liu >=20 > This is really cool, and less invasive than I anticipated. Nice work. >=20 > I only have one concern and one question: >=20 >> @@ -1392,6 +1401,29 @@ static void collapse_file(struct mm_struct *mm, >> result =3D SCAN_FAIL; >> goto xa_unlocked; >> } >> + } else if (!page || xa_is_value(page)) { >> + xas_unlock_irq(&xas); >> + page_cache_sync_readahead(mapping, &file->f_ra, file, >> + index, PAGE_SIZE); >> + /* drain pagevecs to help isolate_lru_page() */ >> + lru_add_drain(); >> + page =3D find_lock_page(mapping, index); >> + if (unlikely(page =3D=3D NULL)) { >> + result =3D SCAN_FAIL; >> + goto xa_unlocked; >> + } >> + } else if (!PageUptodate(page)) { >> + VM_BUG_ON(is_shmem); >> + xas_unlock_irq(&xas); >> + wait_on_page_locked(page); >> + if (!trylock_page(page)) { >> + result =3D SCAN_PAGE_LOCK; >> + goto xa_unlocked; >> + } >> + get_page(page); >> + } else if (!is_shmem && PageDirty(page)) { >> + result =3D SCAN_FAIL; >> + goto xa_locked; >> } else if (trylock_page(page)) { >> get_page(page); >> xas_unlock_irq(&xas); >=20 > The many else ifs here check fairly complex page state and are hard to > follow and verify mentally. In fact, it's a bit easier now in the > patch when you see how it *used* to work with just shmem, but the end > result is fragile from a maintenance POV. >=20 > The shmem and file cases have little in common - basically only the > trylock_page(). Can you please make one big 'if (is_shmem) {} {}' > structure instead that keeps those two scenarios separate? Good point! Will fix in next version.=20 >=20 >> @@ -1426,6 +1458,12 @@ static void collapse_file(struct mm_struct *mm, >> goto out_unlock; >> } >>=20 >> + if (page_has_private(page) && >> + !try_to_release_page(page, GFP_KERNEL)) { >> + result =3D SCAN_PAGE_HAS_PRIVATE; >> + break; >> + } >> + >> if (page_mapped(page)) >> unmap_mapping_pages(mapping, index, 1, false); >=20 >> @@ -1607,6 +1658,17 @@ static void khugepaged_scan_file(struct mm_struct= *mm, >> break; >> } >>=20 >> + if (page_has_private(page) && trylock_page(page)) { >> + int ret; >> + >> + ret =3D try_to_release_page(page, GFP_KERNEL); >> + unlock_page(page); >> + if (!ret) { >> + result =3D SCAN_PAGE_HAS_PRIVATE; >> + break; >> + } >> + } >> + >> if (page_count(page) !=3D 1 + page_mapcount(page)) { >> result =3D SCAN_PAGE_COUNT; >> break; >=20 > There is already a try_to_release() inside the page lock section in > collapse_file(). I'm assuming you added this one because private data > affects the refcount. But it seems a bit overkill just for that; we > could also still fail the check, in which case we'd have dropped the > buffers in vain. Can you fix the check instead? >=20 > There is an is_page_cache_freeable() function in vmscan.c that handles > private fs references: >=20 > static inline int is_page_cache_freeable(struct page *page) > { > /* > * A freeable page cache page is referenced only by the caller > * that isolated the page, the page cache and optional buffer > * heads at page->private. > */ > int page_cache_pins =3D PageTransHuge(page) && PageSwapCache(page) ? > HPAGE_PMD_NR : 1; > return page_count(page) - page_has_private(page) =3D=3D 1 + page_cache_p= ins; > } >=20 > Wouldn't this work here as well? Good point! Let me try fix this.=20 Thanks, Song