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dkim=none (message not signed) header.d=none;google.com; dmarc=none action=none header.from=oracle.com; Received: from CY4PR10MB2038.namprd10.prod.outlook.com (2603:10b6:903:122::20) by CY4PR10MB1591.namprd10.prod.outlook.com (2603:10b6:903:23::10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3846.30; Fri, 26 Feb 2021 02:03:03 +0000 Received: from CY4PR10MB2038.namprd10.prod.outlook.com ([fe80::842f:b854:7899:e759]) by CY4PR10MB2038.namprd10.prod.outlook.com ([fe80::842f:b854:7899:e759%8]) with mapi id 15.20.3868.033; Fri, 26 Feb 2021 02:03:03 +0000 Subject: Re: [patch 014/173] mm, tracing: record slab name for kmem_cache_free() To: Steven Rostedt , Linus Torvalds Cc: Andrew Morton , Joe Perches , Christoph Lameter , Joonsoo Kim , Linux-MM , mm-commits@vger.kernel.org, Paul McKenney , Pekka Enberg , David Rientjes References: <20210224115824.1e289a6895087f10c41dd8d6@linux-foundation.org> <20210224200055.U7Xz47kX5%akpm@linux-foundation.org> <20210224203708.4489755a@oasis.local.home> <20210224210740.73273c7a@oasis.local.home> <5a0b6fb4-6efd-e391-45fa-cd188f181d5d@oracle.com> <20210225093128.4cd86439@gandalf.local.home> <20210225125741.4fc7e43e@gandalf.local.home> From: Jacob Wen Message-ID: <81645797-baee-fb17-6432-6a5cc423f09b@oracle.com> Date: Fri, 26 Feb 2021 10:02:53 +0800 User-Agent: Mozilla/5.0 (X11; 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It uses the same code as >>> printk() and what other string formatters use. >>> >>> What is special is that the print is on data that is stored from a previous >>> time. >> Well, technically that's not actually limited to just %s, although >> that's the common case. >> >> All our magic "%p*" formats do the same thing: they print out not the >> argument _value_, but something that the pointer value points to. >> >> So "%pD" takes a "struct file *" pointer, and follows it to the >> dentry, and then from the dentry to the name. So it will in fact >> follow pointers even more than "%s" does. > Correct, as I've told people about that as well. I was thinking about this yesterday. Freeing a slab name generally means removing a kernel module. See the callers of kmem_cache_destroy. We need to fix some of the trace events that call TP_printk with %pS to get a symbol of a kernel module that maybe removed later. > >> It might indeed be worth having a warning for TP_printk() about any of >> the formats that follow a pointer, exactly because of the whole "by >> the time it actually prints, the pointer may be long gone". >> > Just a comment? Or should we add some check that gives a warning for when > one of these are used? That can be done at boot up or module load. (note, %s > can be OK for some cases, as mentioned in a previous email). > > -- Steve