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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id y3sm19257860pfr.140.2021.10.05.12.21.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Oct 2021 12:21:27 -0700 (PDT) Date: Tue, 5 Oct 2021 12:21:26 -0700 From: Kees Cook To: Suren Baghdasaryan Cc: Pavel Machek , Andrew Morton , Colin Cross , Sumit Semwal , Michal Hocko , Dave Hansen , Matthew Wilcox , "Kirill A . Shutemov" , Vlastimil Babka , Johannes Weiner , Jonathan Corbet , Al Viro , Randy Dunlap , Kalesh Singh , Peter Xu , rppt@kernel.org, Peter Zijlstra , Catalin Marinas , vincenzo.frascino@arm.com, Chinwen Chang =?utf-8?B?KOW8temMpuaWhyk=?= , Axel Rasmussen , Andrea Arcangeli , Jann Horn , apopple@nvidia.com, John Hubbard , Yu Zhao , Will Deacon , fenghua.yu@intel.com, thunder.leizhen@huawei.com, Hugh Dickins , feng.tang@intel.com, Jason Gunthorpe , Roman Gushchin , Thomas Gleixner , krisman@collabora.com, chris.hyser@oracle.com, Peter Collingbourne , "Eric W. Biederman" , Jens Axboe , legion@kernel.org, Rolf Eike Beer , Cyrill Gorcunov , Muchun Song , Viresh Kumar , Thomas Cedeno , sashal@kernel.org, cxfcosmos@gmail.com, Rasmus Villemoes , LKML , linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm , kernel-team Subject: Re: [PATCH v10 3/3] mm: add anonymous vma name refcounting Message-ID: <202110051220.FFD4C604EF@keescook> References: <20211001205657.815551-1-surenb@google.com> <20211001205657.815551-3-surenb@google.com> <20211005184211.GA19804@duo.ucw.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 05, 2021 at 12:14:59PM -0700, Suren Baghdasaryan wrote: > On Tue, Oct 5, 2021 at 11:42 AM Pavel Machek wrote: > > > > On Fri 2021-10-01 13:56:57, Suren Baghdasaryan wrote: > > > While forking a process with high number (64K) of named anonymous vmas the > > > overhead caused by strdup() is noticeable. Experiments with ARM64 > > Android > > > > I still believe you should simply use numbers and do the > > numbers->strings mapping in userspace. We should not need to optimize > > strdups in kernel... > > Here are complications with mapping numbers to strings in the userspace: > Approach 1: hardcode number->string in some header file and let all > tools use that mapping. The issue is that whenever that mapping > changes all the tools that are using it (including 3rd party ones) > have to be rebuilt. This is not really maintainable since we don't > control 3rd party tools and even for the ones we control, it will be a > maintenance issue figuring out which version of the tool used which > header file. > Approach 2: have a centralized facility (a process or a DB) > maintaining number->string mapping. This would require an additional > request to this facility whenever we want to make a number->string > conversion. Moreover, when we want to name a VMA, we would have to > register a new VMA name in that facility or check that one already > exists and get its ID. So each prctl() call to name a VMA will be > preceded by such a request (IPC call), maybe with some optimizations > to cache already known number->string pairs. This would be quite > expensive performance-wise. Additional issue with this approach is > that this mapping will have to be persistent to handle a case when the > facility crashes and has to be restored. > > As I said before, it complicates userspace quite a bit. Is that a good > enough reason to store the names in the kernel and pay a little more > memory for that? IMHO yes, but I might be wrong. FWIW, I prefer the strings. It's more human-readable, which is important for the kinds of cases where the maps are being used for diagnostics, etc. -- Kees Cook