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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Jun 2025 12:19:38.4176 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d2d1c17e-e143-49e2-652e-08dda8e23c5b X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB1PEPF000509E4.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM9PR08MB6242 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250611_051946_173592_2B4AFD7B X-CRM114-Status: GOOD ( 14.01 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 11/06/25 3:03 pm, Will Deacon wrote: > On Thu, Jun 05, 2025 at 01:46:01PM +0530, Dev Jain wrote: >> On 05/06/25 10:18 am, Dev Jain wrote: >>> On 30/05/25 7:06 pm, Will Deacon wrote: >>>> On Fri, May 30, 2025 at 02:11:36PM +0100, Ryan Roberts wrote: >>>>> On 30/05/2025 13:35, Will Deacon wrote: >>>>>> I really don't think we should be adding unconditional >>>>>> locking overhead >>>>>> to core mm routines purely to facilitate a rarely used debug option. >>>>>> >>>>>> Instead, can we either adopt something like the RCU-like walk used by >>>>>> fast GUP or stick the locking behind a static key that's only enabled >>>>>> when a ptdump walk is in progress (a bit like how >>>>>> hugetlb_vmemmap_optimize_folio() manipulates >>>>>> hugetlb_optimize_vmemmap_key)? >>>>> My sense is that the static key will be less effort and can be >>>>> contained fully >>>>> in arm64. I think we would need to enable the key around the call to >>>>> ptdump_walk_pgd() in both ptdump_walk() and ptdump_check_wx(). >>>>> Then where Dev is >>>>> currently taking the read lock, that would be contingent on the >>>>> key being >>>>> enabled and the unlock would be contingent on having taken the lock. >>>>> >>>>> Does that sound like an acceptable approach? >>>> Yup, and I think you'll probably need something like a synchronize_rcu() >>>> when flipping the key to deal with any pre-existing page-table freers. >>> IIUC, you mean to say that we need to ensure any existing readers having >>> a reference to the isolated table in pmd_free_pte_page and friend, have >>> exited. >>> But the problem is that we take an mmap_write_lock() around >>> ptdump_walk_pgd() so >>> this is a sleepable code path. >> The mmap_write_lock around ptdump_walk_pgd() is definitely restrictive. I >> think that >> >> was put because ptdump is rarely used, I think we could have done >> RCU-freeing of pagetables to >> >> synchronize between ptdump and vmalloc pagetable lazy freeing/ hotunplug >> pagetable freeing. > The other idea was to use a static key like the HVO code does, which > shouldn't place any RCU requirements on the debug walk. Thanks for your suggestion. I shall look into this. > > Will