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Wed, 27 Oct 2021 13:01:22 -0700 (PDT) MIME-Version: 1.0 References: <20211019215511.3771969-1-surenb@google.com> <20211019215511.3771969-2-surenb@google.com> <89664270-4B9F-45E0-AC0B-8A185ED1F531@google.com> In-Reply-To: <89664270-4B9F-45E0-AC0B-8A185ED1F531@google.com> From: Suren Baghdasaryan Date: Wed, 27 Oct 2021 13:01:11 -0700 Message-ID: Subject: Re: [PATCH v11 2/3] mm: add a field to store names for private anonymous memory To: Alexey Alexandrov Cc: akpm@linux-foundation.org, ccross@google.com, sumit.semwal@linaro.org, mhocko@suse.com, dave.hansen@intel.com, keescook@chromium.org, willy@infradead.org, kirill.shutemov@linux.intel.com, vbabka@suse.cz, hannes@cmpxchg.org, corbet@lwn.net, viro@zeniv.linux.org.uk, rdunlap@infradead.org, kaleshsingh@google.com, peterx@redhat.com, rppt@kernel.org, peterz@infradead.org, catalin.marinas@arm.com, vincenzo.frascino@arm.com, chinwen.chang@mediatek.com, axelrasmussen@google.com, aarcange@redhat.com, jannh@google.com, apopple@nvidia.com, jhubbard@nvidia.com, yuzhao@google.com, will@kernel.org, fenghua.yu@intel.com, thunder.leizhen@huawei.com, hughd@google.com, feng.tang@intel.com, jgg@ziepe.ca, guro@fb.com, tglx@linutronix.de, krisman@collabora.com, chris.hyser@oracle.com, pcc@google.com, ebiederm@xmission.com, axboe@kernel.dk, legion@kernel.org, eb@emlix.com, gorcunov@gmail.com, pavel@ucw.cz, songmuchun@bytedance.com, viresh.kumar@linaro.org, thomascedeno@google.com, sashal@kernel.org, cxfcosmos@gmail.com, linux@rasmusvillemoes.dk, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: CF76EB0001A1 X-Stat-Signature: z5ahee739c8ibi81jk73rn18e6ur5fwr Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b="m/yQ0pD9"; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf19.hostedemail.com: domain of surenb@google.com designates 209.85.219.175 as permitted sender) smtp.mailfrom=surenb@google.com X-HE-Tag: 1635364878-252944 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: On Wed, Oct 27, 2021 at 11:35 AM Alexey Alexandrov wr= ote: > > > On Oct 19, 2021, at 2:55 PM, Suren Baghdasaryan wro= te: > > > > From: Colin Cross > > > > In many userspace applications, and especially in VM based applications > > like Android uses heavily, there are multiple different allocators in u= se. > > At a minimum there is libc malloc and the stack, and in many cases ther= e > > are libc malloc, the stack, direct syscalls to mmap anonymous memory, a= nd > > multiple VM heaps (one for small objects, one for big objects, etc.). > > Each of these layers usually has its own tools to inspect its usage; > > malloc by compiling a debug version, the VM through heap inspection too= ls, > > and for direct syscalls there is usually no way to track them. > > > > On Android we heavily use a set of tools that use an extended version o= f > > the logic covered in Documentation/vm/pagemap.txt to walk all pages map= ped > > in userspace and slice their usage by process, shared (COW) vs. unique > > mappings, backing, etc. This can account for real physical memory usag= e > > even in cases like fork without exec (which Android uses heavily to sha= re > > as many private COW pages as possible between processes), Kernel SamePa= ge > > Merging, and clean zero pages. It produces a measurement of the pages > > that only exist in that process (USS, for unique), and a measurement of > > the physical memory usage of that process with the cost of shared pages > > being evenly split between processes that share them (PSS). > > > > If all anonymous memory is indistinguishable then figuring out the real > > physical memory usage (PSS) of each heap requires either a pagemap walk= ing > > tool that can understand the heap debugging of every layer, or for ever= y > > layer's heap debugging tools to implement the pagemap walking logic, in > > which case it is hard to get a consistent view of memory across the who= le > > system. > > > > Tracking the information in userspace leads to all sorts of problems. > > It either needs to be stored inside the process, which means every > > process has to have an API to export its current heap information upon > > request, or it has to be stored externally in a filesystem that > > somebody needs to clean up on crashes. It needs to be readable while > > the process is still running, so it has to have some sort of > > synchronization with every layer of userspace. Efficiently tracking > > the ranges requires reimplementing something like the kernel vma > > trees, and linking to it from every layer of userspace. It requires > > more memory, more syscalls, more runtime cost, and more complexity to > > separately track regions that the kernel is already tracking. > > > > This patch adds a field to /proc/pid/maps and /proc/pid/smaps to show a > > userspace-provided name for anonymous vmas. The names of named anonymo= us > > vmas are shown in /proc/pid/maps and /proc/pid/smaps as [anon:]. > > > > Userspace can set the name for a region of memory by calling > > prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, start, len, (unsigned long)name= ); > > Setting the name to NULL clears it. The name length limit is 80 bytes > > including NUL-terminator and is checked to contain only printable ascii > > characters (including space), except '[',']','\','$' and '`'. Ascii > > strings are being used to have a descriptive identifiers for vmas, whic= h > > can be understood by the users reading /proc/pid/maps or /proc/pid/smap= s. > > Names can be standardized for a given system and they can include some > > variable parts such as the name of the allocator or a library, tid of > > the thread using it, etc. > > > > The name is stored in a pointer in the shared union in vm_area_struct > > that points to a null terminated string. Anonymous vmas with the same > > name (equivalent strings) and are otherwise mergeable will be merged. > > The name pointers are not shared between vmas even if they contain the > > same name. The name pointer is stored in a union with fields that are > > only used on file-backed mappings, so it does not increase memory usage= . > > > > CONFIG_ANON_VMA_NAME kernel configuration is introduced to enable this > > feature. It keeps the feature disabled by default to prevent any > > additional memory overhead and to avoid confusing procfs parsers on > > systems which are not ready to support named anonymous vmas. > > > > The patch is based on the original patch developed by Colin Cross, more > > specifically on its latest version [1] posted upstream by Sumit Semwal. > > It used a userspace pointer to store vma names. In that design, name > > pointers could be shared between vmas. However during the last upstream= ing > > attempt, Kees Cook raised concerns [2] about this approach and suggeste= d > > to copy the name into kernel memory space, perform validity checks [3] > > and store as a string referenced from vm_area_struct. > > One big concern is about fork() performance which would need to strdup > > anonymous vma names. Dave Hansen suggested experimenting with worst-cas= e > > scenario of forking a process with 64k vmas having longest possible nam= es > > [4]. I ran this experiment on an ARM64 Android device and recorded a > > worst-case regression of almost 40% when forking such a process. This > > regression is addressed in the followup patch which replaces the pointe= r > > to a name with a refcounted structure that allows sharing the name poin= ter > > between vmas of the same name. Instead of duplicating the string during > > fork() or when splitting a vma it increments the refcount. > > > > [1] https://lore.kernel.org/linux-mm/20200901161459.11772-4-sumit.semwa= l@linaro.org/ > > [2] https://lore.kernel.org/linux-mm/202009031031.D32EF57ED@keescook/ > > [3] https://lore.kernel.org/linux-mm/202009031022.3834F692@keescook/ > > [4] https://lore.kernel.org/linux-mm/5d0358ab-8c47-2f5f-8e43-23b89d6a8e= 95@intel.com/ > > > > Changes for prctl(2) manual page (in the options section): > > > > PR_SET_VMA > > Sets an attribute specified in arg2 for virtual memory areas > > starting from the address specified in arg3 and spanning the > > size specified in arg4. arg5 specifies the value of the attribut= e > > to be set. Note that assigning an attribute to a virtual memory > > area might prevent it from being merged with adjacent virtual > > memory areas due to the difference in that attribute's value. > > > > Currently, arg2 must be one of: > > > > PR_SET_VMA_ANON_NAME > > Set a name for anonymous virtual memory areas. arg5 shoul= d > > be a pointer to a null-terminated string containing the > > name. The name length including null byte cannot exceed > > 80 bytes. If arg5 is NULL, the name of the appropriate > > anonymous virtual memory areas will be reset. The name > > can contain only printable ascii characters (including > > space), except '[',']','\','$' and '`'. > > > > This feature is available only if the kernel is built wi= th > > the CONFIG_ANON_VMA_NAME option enabled. > > For what it=E2=80=99s worth, it=E2=80=99s definitely interesting to see t= his going upstream. > In particular, we would use it for high-level grouping of the data in > production profiling when proper symbolization is not available: > > * JVM could associate a name with the memory regions it uses for the JIT > code so that Linux perf data are associated with a high level name like > "Java JIT" even if the proper Java JIT profiling is not enabled. > * Similar for other JIT engines like v8 - they could annotate the memory > regions they manage and use as well. > * Traditional memory allocators like tcmalloc can use this as well so > that the associated name is used in data access profiling via Linux per= f. Hi Alexey, Thanks for providing your feedback! Nice to hear that this can be useful outside of Android.