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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 May 2025 06:33:54.2432 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 351dc43c-d2c2-4119-cab6-08dd98fa9f9a 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: DU6PEPF0000B622.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: AS8PR08MB6278 X-Rspam-User: X-Stat-Signature: hn9f8jhq73qkd3n7sedq196kd4wuqfb6 X-Rspamd-Queue-Id: 213934000C X-Rspamd-Server: rspam11 X-HE-Tag: 1747895637-763676 X-HE-Meta: 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 H1EchRsw 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 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: List-Subscribe: List-Unsubscribe: On 21/05/25 5:15 pm, Ryan Roberts wrote: > On 21/05/2025 12:16, Ryan Roberts wrote: >> On 19/05/2025 08:48, Dev Jain wrote: >>> Batch ptep_modify_prot_start/commit in preparation for optimizing mprotect. >>> Architecture can override these helpers; in case not, they are implemented >>> as a simple loop over the corresponding single pte helpers. >>> >>> Signed-off-by: Dev Jain > [...] > >> I have some general concerns about the correctness of batching these functions. >> The support was originally added by Commit 1ea0704e0da6 ("mm: add a >> ptep_modify_prot transaction abstraction"), and the intent was to make it easier >> to defer the pte updates for XEN on x86. >> >> Your default implementations of the batched versions will match the number of >> ptep_modify_prot_start() calls with the same number of ptep_modify_prot_commit() >> calls, even if modify_prot_commit_ptes() is called incrementally for sub-batches >> of the batch used for modify_prot_start_ptes(). That's a requirement and you've >> met it. But in the batched case, there are 2 differences; >> >> - You can now have multiple PTEs within a start-commit block at one time. I >> hope none of the specialized implementations care about that (i.e. XEN). > I had a look; this isn't a problem. > >> - when calling ptep_modify_prot_commit(), old_pte may not be exactly what >> ptep_modify_prot_start() returned for that pte. You have collected the A/D bits, >> and according to your docs "PTE bits in the PTE range besides the PFN can >> differ" when calling modify_prot_start_ptes() so R/W and other things could >> differ here. > It looks like powerpc will break if you provide old_pte which has different > permissions to the "real" old_pte, see radix__ptep_modify_prot_commit(). So I > think you need to at least spec modify_prot_start_ptes() to require that all > bits of the PTE except the PFN, access and dirty are identical. And perhaps you > can VM_WARN if found to be otherwise? And perhaps modify > ptep_modify_prot_commit()'s documentation to explcitly allow old_pte's > access/dirty to be "upgraded" from what was actually read in > ptep_modify_prot_start()? Got it, so we just need to document that, the permissions for all ptes must be identical when using modify_prot_start_ptes(). And that we may be smearing extra a/d bits in modify_prot_commit_ptes(). > > XEN/x86 and arm64 don't care about old_pte. > > Thanks, > Ryan > >> I'm not sure if these are problems in practice; they probably are not. But have >> you checked the XEN implementation (and any other specialized implementations) >> are definitely compatible with your batched semantics? >>