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[34.78.140.88]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3502b79bca6sm4238664f8f.12.2024.05.10.03.57.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 10 May 2024 03:57:31 -0700 (PDT) Date: Fri, 10 May 2024 11:57:27 +0100 From: Vincent Donnefort To: David Hildenbrand Cc: Steven Rostedt , mhiramat@kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com, kernel-team@android.com, rdunlap@infradead.org, rppt@kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v22 2/5] ring-buffer: Introducing ring-buffer mapping functions Message-ID: References: <20240430111354.637356-1-vdonnefort@google.com> <20240430111354.637356-3-vdonnefort@google.com> <20240507223402.206d6ddc@rorschach.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 6kf8i8ucpa5q9zke3k7xqcsr9a7y8z34 X-Rspam-User: X-Rspamd-Queue-Id: 709A1100010 X-Rspamd-Server: rspam05 X-HE-Tag: 1715338653-647750 X-HE-Meta: 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 65KxLGcr 8ifyHl8WLM9j49Z/stD6BaP3chodssQRiNxDgtcSMHkzvJyqh788VzH3TnIOK5GAfhotlR95XQl0HLfq69Z3gSZMpTpu1mWUOaNsJW7LnpbqZIE+THhKbro2x4nikHlZZJ7hhNNKsqGpQ72mk1bGCSh5SwwNRonnzQIStlEjQPJDLqcbLRjAkhM3kwC5xgXdMTCz9XS/TAMCQWqNar7DRoByAQg3WmuIFKJUMOSgLx/j5I5ssdVTmwmy6TYZxaDuduY/C+GNGPaiu1Ec0IK9nm8XIlXFfSWI9kwxEVsj+B7bm9oqmqTMq+EXcPVCgrZd2RD702etB9Rqxdw74YZa+F3yuJ5LuIS5m0aoXvCZtnnePsV7sKcsG5R9pqCLSpy3JQy2O 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 Fri, May 10, 2024 at 11:15:59AM +0200, David Hildenbrand wrote: > On 09.05.24 13:05, Vincent Donnefort wrote: > > On Tue, May 07, 2024 at 10:34:02PM -0400, Steven Rostedt wrote: > > > On Tue, 30 Apr 2024 12:13:51 +0100 > > > Vincent Donnefort wrote: > > > > > > > +#ifdef CONFIG_MMU > > > > +static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer, > > > > + struct vm_area_struct *vma) > > > > +{ > > > > + unsigned long nr_subbufs, nr_pages, vma_pages, pgoff = vma->vm_pgoff; > > > > + unsigned int subbuf_pages, subbuf_order; > > > > + struct page **pages; > > > > + int p = 0, s = 0; > > > > + int err; > > > > + > > > > + /* Refuse MP_PRIVATE or writable mappings */ > > > > + if (vma->vm_flags & VM_WRITE || vma->vm_flags & VM_EXEC || > > > > + !(vma->vm_flags & VM_MAYSHARE)) > > > > + return -EPERM; > > > > + > > > > + /* > > > > + * Make sure the mapping cannot become writable later. Also tell the VM > > > > + * to not touch these pages (VM_DONTCOPY | VM_DONTEXPAND). Finally, > > > > + * prevent migration, GUP and dump (VM_IO). > > > > + */ > > > > + vm_flags_mod(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_IO, VM_MAYWRITE); > > > > > > Do we really need the VM_IO? > > > > > > When testing this in gdb, I would get: > > > > > > (gdb) p tmap->map->subbuf_size > > > Cannot access memory at address 0x7ffff7fc2008 > > > > > > It appears that you can't ptrace IO memory. When I removed that flag, > > > gdb has no problem reading that memory. > > > > Yeah, VM_IO indeed implies DONTDUMP. VM_IO was part of Linus recommendations. > > Yes, the VM should recognize that memory to some degree as being special > already due to VM_MIXEDMAP and VM_DONTEXPAND. > > #define VM_SPECIAL (VM_IO | VM_DONTEXPAND | VM_PFNMAP | VM_MIXEDMAP) > > So any of these flag achieve that (e.g., mlock_fixup() checks VM_SPECIAL). > KSM similarly skips VM_DONTEXPAND and VM_MIXEDMAP (likely we should be using > VM_SPECIAL in vma_ksm_compatible()). Not sure about page migration, likely > its fine. > > Thinking about MADV_DONTNEED, I can spot in > madvise_dontneed_free_valid_vma() only that we disallow primarily VM_PFNMAP. > > ... I assume if user space MADV_DONTNEED's some pages we'll simply get a > page fault later on access that will SIGBUS, handling that gracefully (we > should double-check!). I've just tested and indeed, I get a SIGBUS! All good there. > > > > But perhaps, VM_DONTEXPAND and MIXEDMAP (implicitely set by vm_insert_pages) are > > enough protection? > > Do we want to dump these pages? VM_DONTDUMP might be reasonabe then. Somehow I thought this would prevent ptrace as well, but I've just tested it and this is not the case as well. So let's keep DONTDUMP. Thanks! > > > > > I don't see how anything could use GUP there and as David pointed-out on the > > previous version, it doesn't event prevent the GUP-fast path. > > Yes, GUP-fast would still have worked under some conditions. > > -- > Cheers, > > David / dhildenb >