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De Francesco" , Ira Weiny , "Sebastian Andrzej Siewior" , Alexander Potapenko , Andrey Konovalov , Tony Luck , Bagas Sanjaya , David Sterba , Kees Cook , , , Thomas Gleixner , Helge Deller Subject: Re: [PATCH] mm/highmem: Align-down to page the address for kunmap_flush_on_unmap() Message-ID: <63d45a837b15b_7f63c294a2@iweiny-mobl.notmuch> References: <20230126143346.12086-1-fmdefrancesco@gmail.com> <20230126123858.42d9377ad6e352a58c81668b@linux-foundation.org> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: BYAPR08CA0034.namprd08.prod.outlook.com (2603:10b6:a03:100::47) To SA1PR11MB6733.namprd11.prod.outlook.com (2603:10b6:806:25c::17) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SA1PR11MB6733:EE_|PH0PR11MB5644:EE_ X-MS-Office365-Filtering-Correlation-Id: 18e59d7c-ab69-4d39-4b61-08db00bc0f48 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: Fz5p3w9QOZhZDBQGKoLmvMEXMB+F82Q60qlG/IqIni9i547tsWnlgdLN8k/VMW1H8cGDfLGJRhr12n1z1Nvq4FL6paWmeWmikB8w80OZCq/LEBpl8oYwzgboLBzY0WjWmK0uih7ZLFUbpiENz3pSxgvlNvcun7VDASvRJ1eDTM08/UZmVYUu67eqz22qk+vF40Mv2dv61YngyIob5RGBeOHtqhk4dxRFcXV9W2iniQvIf8FPDCyjlEVGszwTPfJK8fa0xP6G3vDVzxU6FsILthZxhaJm/J//vJG6HlzHQhnDh+W/2TNzFZYUzaubit+0ahSrY4AK3IMq6qejmBfP9xNp9bQVMLHBvz9DM1jLEdspXFegQm/lrvGwjWUO3QgReEvG017kKTLZQLOoPc24gjrX6F5Pw9krCBqbO1QJx8TVXRHh6Z71QFldJE6NXAXMLWLReCXFuFLw4njpyrGVxd8oPVjf52V8lnDSo5UAkRvsXbI34ozKT5esbLicyngt28ck1ayi5mo9swNet95aYfbbhRSx/KMQvnPO4OicU6bJU/E2rpT+CslDPlp2FQE1pCEpMujpD8N/olOkGK+7C8rsWYkZBYxcbu5yO8bjjNZHRwGJ+gDpDVTxIj/hd3MQ62r+MreYdJte0Vj7P09RqQ== X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:SA1PR11MB6733.namprd11.prod.outlook.com;PTR:;CAT:NONE;SFS:(13230025)(346002)(366004)(39860400002)(376002)(396003)(136003)(451199018)(2906002)(186003)(5660300002)(8676002)(4326008)(66946007)(7416002)(82960400001)(86362001)(8936002)(41300700001)(66476007)(66556008)(38100700002)(83380400001)(6486002)(110136005)(44832011)(54906003)(316002)(6666004)(478600001)(9686003)(26005)(6512007)(6506007);DIR:OUT;SFP:1102; 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De Francesco" wrote: > > > > > > > > > > > If ARCH_HAS_FLUSH_ON_KUNMAP is defined (PA-RISC case), __kunmap_local() > > > > > > calls kunmap_flush_on_unmap(). The latter currently flushes the wrong > > > > > > address (as confirmed by Matthew Wilcox and Helge Deller). Al Viro > > > > > > proposed to call kunmap_flush_on_unmap() on an aligned-down to page > > > > > > address in order to fix this issue. Consensus has been reached on this > > > > > > solution. > > > > > > > > > > What are the user-visible runtime effects of this flaw? > > > > > > > > The version of this patch I sent out includes this information, > > > > as well as the missed alignment for kunmap_atomic(). > > > > > > One point: AFAICS, the situation right now is > > > * all callers of kunmap_local() pass page-aligned pointers > > > > Ah, no. kmap_local_folio() accepts a byte offset within the folio > > and returns a pointer to that byte. I hadn't noticed the parisc > > case and thought it was already allowed to pass a misaligned pointer > > to kunmap_local() since it is allowed for the highmem case. It > > simplified the callers, so it looked like a good tradeoff. > > > > See, eg 338f379cf7c2: > > > > - src_addr = kmap_atomic(src_page); > > - dest_addr = kmap_atomic(dest_page); > > + src_addr = kmap_local_folio(src_folio, > > + offset_in_folio(src_folio, srcoff)); > > + dst_addr = kmap_local_folio(dst_folio, > > + offset_in_folio(dst_folio, dstoff)); > > > > - if (memcmp(src_addr + src_poff, dest_addr + dest_poff, cmp_len)) > > + if (memcmp(src_addr, dst_addr, cmp_len)) > > > > - kunmap_atomic(dest_addr); > > - kunmap_atomic(src_addr); > > + kunmap_local(dst_addr); > > + kunmap_local(src_addr); > > Argh... Missed that. > > OK, then Fixes: on the patch makes a lot of sense. And it still > needs a chunk in D/m/highmem.rst... > I believe this is taken care of because the kdocs for kunmap_{local,atomic}() are included there. And those kdocs indicate the ability to use any address in the page. Thanks for setting me straight on the parisc/highmem thing I missed that. Ira