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Wed, 1 Sep 2021 17:55:48 +0000 Received: from BL0PR12MB5506.namprd12.prod.outlook.com ([fe80::e8af:232:915e:2f95]) by BL0PR12MB5506.namprd12.prod.outlook.com ([fe80::e8af:232:915e:2f95%8]) with mapi id 15.20.4478.020; Wed, 1 Sep 2021 17:55:48 +0000 Date: Wed, 1 Sep 2021 14:55:47 -0300 From: Jason Gunthorpe To: David Hildenbrand Cc: Qi Zheng , akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages Message-ID: <20210901175547.GP1721383@nvidia.com> References: <20210819031858.98043-1-zhengqi.arch@bytedance.com> <20210819031858.98043-7-zhengqi.arch@bytedance.com> <20210901135314.GA1859446@nvidia.com> <0c9766c9-6e8b-5445-83dc-9f2b71a76b4f@redhat.com> <20210901153247.GJ1721383@nvidia.com> <7789261d-6a64-c47b-be6c-c9be680e5d33@redhat.com> <20210901161613.GN1721383@nvidia.com> <20210901171039.GO1721383@nvidia.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?goSZ4bVgcP/qif56Xwniq5nthqj0KpQRzXhTpSyqv3peEQrLW9gtIauqWEIs?= =?us-ascii?Q?h2xPsC3NpheJoAf0BvJfzgJFZ0zjWDTgl+jb7KB2vthU3fsAosFCcVHZceK7?= =?us-ascii?Q?Ivoj9ZLdSz5UB9Uu4rA8yEpoBcgv8/svYxwnKE5ho+Dizag7A8qW00ElVvzf?= =?us-ascii?Q?YD/3oQYu+EfjTCEwyCnTzk7g7wNInOoUpV8Pwnbq6F4ymHj++tbUw9ZEFJDB?= =?us-ascii?Q?EkzFCXSA8CdfqMA8ndksJ7A2JhiDehNnKij/fVnYH2ojxiJuJlFeDxyVPTlz?= =?us-ascii?Q?lVcq+wFNfG0ddSvFUPuUdDVbc3nCjzcjOyY7uZ50ehm12XQTimcm2QmOaw5f?= =?us-ascii?Q?XWs/Ht4keFVrbXM+ikoiIEsq8VaRjQ7lkp6KldKOEsIcvkSxPEyOs7S0+Vqi?= =?us-ascii?Q?ZI8OYBGJBmHLRq6bc/gFa0Vbe8tlulEVBn3sSGvzU9BoLhAx5ZLMqtdku7SS?= =?us-ascii?Q?2Us13PtL3PpINadqls6zMoU27up0DXVHAM5gxW/e8I0EEy0FoKesbGBoyBUK?= =?us-ascii?Q?sC4UNWV08UhZIMiX9rqLAxhEXxDbsse+rYj1Sa1AVC9i1N6rza7h35KmR98p?= =?us-ascii?Q?o9HWuyftHbQ7ItBSaKg2GDvkN/W1RYjKse7wt0jM6Q1a8yg0/LRCY67U6IkM?= =?us-ascii?Q?r680PDisr77ersrKmAOAtJSasv8dOqTnMRmEHORsbjqO6aIZCggqkZB1ESRA?= =?us-ascii?Q?ytJbTQw29EKUgYml/bKmy3a4Om10zMI8WTLLgVVPbkqV8ezskY3h5eLW1WUT?= =?us-ascii?Q?GKxxCD7s7Jt+6e+e4cahcMazZjCnes6zb/d/AQefSdo0lsLKtjYtV+z8MuHz?= =?us-ascii?Q?U32+evoIGvDZ4y4Nu+OW7HWbV29dZ/jyfPbn/hzlMS2kvD2MtJJh4rcfoXDm?= =?us-ascii?Q?GFnZ15jM+431VfV0lWI5J/MbTyYTeFUr8sx9g91RQ3D155PxZgaGO8Dc7XqB?= =?us-ascii?Q?cED6ZoLa8EUV2dQ1yR1r9F5wBeOKOKpMJ0wyVtPOw6cwmBNeok1aVb5Ux51w?= =?us-ascii?Q?aAyF47ZMlI36QaEh0iagAlD3GXIGck+eLH4KjvQPBS4h2wRFNnhyelluWL7n?= =?us-ascii?Q?11exIC6mqWdtSzdGxk08ZgbFR1L+EFIN0xzbLJeiZsFk7gwU8gZp65EgnOQj?= =?us-ascii?Q?CPhMaXUkSz+MVBM2toufmIO9Q1dbVWBkDPejBJdFCBU57gcZyn2/0cMkKDY2?= =?us-ascii?Q?GbXNS0vIlm+q0RSc9ObVXNXrcpsvoRdeYgts53swpwfCaWejGgklVoV3kTyG?= =?us-ascii?Q?VPoKu4hkjB+uw3MefsGCHZOzvRi06rNWQNMFJimOgSicTqkp17SXr4IbcDdh?= =?us-ascii?Q?O8PJDEs/GVPJUkjCD8UVB6xr?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 6b510e26-6c83-42c8-4a0e-08d96d71ba7e X-MS-Exchange-CrossTenant-AuthSource: BL0PR12MB5506.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Sep 2021 17:55:47.9108 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 33cm9+02uRm807fewe3NhgW3tcHsuOTGZES7xTwryDpaYM5AyAoiH7pierP32ZUo X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL1PR12MB5238 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 01, 2021 at 07:49:23PM +0200, David Hildenbrand wrote: > On 01.09.21 19:10, Jason Gunthorpe wrote: > > On Wed, Sep 01, 2021 at 06:19:03PM +0200, David Hildenbrand wrote: > > > > > > I wouldn't think it works everywhere, bit it works in a lot of places, > > > > and it is a heck of a lot better than what is proposed here. I'd > > > > rather see the places that can use it be moved, and the few places > > > > that can't be opencoded. > > > > > > Well, I used ptep_get_map_lock() and friends. But hacking directly into > > > ptep_map_lock() and friends wasn't possible due to all the corner cases. > > > > Sure, I'm not surprised you can't get every single case, but that just > > suggest we need two API families, today's to support the special cases > > and a different one for the other regular simple cases. > > > > A new function family pte_try_map/_locked() and paired unmap that can > > internally do the recounting and THP trickery and convert the easy > > callsites. > > > > Very rough counting suggest at least half of the pte_offset_map_lock() > > call sites can trivially use the simpler API. > > > > The other cases can stay as is and get open coded refcounts, or maybe > > someone will have a better idea once they are more clearly identified. > > > > But I don't think we should take a performance hit of additional > > atomics in cases like GUP where this is trivially delt with by using a > > better API. > > Right, but as I said in the cover letter, we can happily optimize once we > have the basic infrastructure in place and properly reviewed. Getting rid of > some unnecessary atomics by introducing additional fancy helpers falls under > that category. I'm not sure I agree given how big and wide this patch series is. It would be easier to review if it was touching less places. The helpers are not fancy, it is a logical re-arrangement of existing code that shrinks the LOC of this series and makes it more reviewable. Or stated another way, a niche feature like this try much harder not to add more complexity everywhere. Jason