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He mostly works on Btrfs and the block layer, but he also tends to debugrandom kernel problems. Facebook has so many machines that there are"super rare, one-in-a-million bugs"showing up all the time. He often volunteers to take a look. In theprocess he got used to tools like GDB, crash, and eBPF, but found that heoften wanted to be able to do arbitrarily complex analysis of kernel datastructures, which is why he ended up building drgn.
The physical memory in a computer system is a precious resource, so alot of effort has been put into managing it effectively. This task is mademore difficult by the complexity of the memory architecture on contemporarysystems. There are severallayers of abstraction that deal with the details of how physical memoryis laid out; one of those is simply called the "memory model". Thereare three models supported in the kernel, but one of them is on its wayout. As a way of understanding this change, this article willtake a closer look at the evolution of the kernel's memory models,their current state, and their possible future.FLATMEMBack in the beginning of Linux, memory was flat: it was a simple linear sequencewith physical addresses starting at zero and endingat several megabytes. Each physical page frame had an entry in the kernel'smem_map array which, at that time, contained a single unsignedshort entry for each page that counted the number of references that pagehad. Soon enough, though, the mem_map entries grew to also includeage and dirty counters for the management of swapping. InLinux 1.3.50 the elements of mem_map were finally named structpage.The flat memory map provided easy and fast conversion between aphysical page-frame number (PFN) and its corresponding structpage; it was a simple matter of calculating an array index.There were changes in the layout of struct page to accommodate newusages (the page cache, for example) and to optimize cache performance forthe struct page accesses. The memory map remained a flat arraythat was neat and efficient, but it had a major drawback: it couldn'tdeal well with large holes in the physical address space. Either thepart of the memory map corresponding to a hole would be wastedor, as was done on ARM, the memory map would also have holes.DISCONTIGMEMSupport for the efficient handling of widely discontiguous physicalmemory was introduced into the memory-management subsystem in 1999 as apart of theeffort to make Linux work well on NUMA machines. This code was dependent on theCONFIG_DISCONTIGMEM configuration option, so the first memorymodel that had a name was DISCONTIGMEM.The DISCONTIGMEM model introduced the notion of a memory node,which remains the basis of NUMA memory management. Each node carries anindependent (well, mostly) memory-management subsystem with its ownfree-page lists, in-use page lists, least-recently-used (LRU) information, and usagestatistics. Among all these goodies, the node data represented bystruct pglist_data (or pg_data_t for short) contains anode-specific memory map. Assuming that each node has contiguous physicalmemory, having an array of page structures per node solves theproblem of large holes in the flat memory map. But this doesn't come for free. With DISCONTIGMEM, it's necessaryto determine which node holds a given page in memory to turn its PFN intothe associated struct page, forexample. Similarly, one must determinewhich node's memory map contains a struct page to calculate itsPFN. After a long evolution, starting with the mips64 architecture defining theKVADDR_TO_NID(), LOCAL_MAP_BASE(),ADDR_TO_MAPBASE(), and LOCAL_BASE_ADDR() macros for thefirst time, the conversion of a PFN to the struct page and viceversa came to rely on the relatively simple pfn_to_page() andpage_to_pfn() macros defined in include/asm-generic/memory_model.h.DISCONTIGMEM, however, had a weak point: memory hotplug and hotremove. The actual NUMA node granularity was too coarse for proper hotplugsupport, and splitting the node would have created a lot of unnecessaryfragmentation and overhead. Remember that each node has an independent memorymanagement structure with all the associated costs; splitting nodes further wouldincrease those costs considerably.SPARSEMEMThis limitation was resolved with the introduction ofSPARSEMEM. This model abstracted the memory map as a collection ofsections of arbitrary size defined by the architectures. Each section,represented by structmem_section, is (as described in the code): "logically, apointer to an array of struct pages. However, it is stored with some othermagic". The array of these sections is a meta-memory map which canbe efficiently chopped at SECTION_SIZE granularity. For efficientconversion between a PFN and struct page, several high bits of thePFN are used to index into the sections array. For the other direction, thesection number was encoded in the page flags.A few months after its introduction into the Linux kernel,SPARSEMEM was extended with SPARSEMEM_EXTREME, which issuitable for systems with an especially sparse physical address space.SPARSEMEM_EXTREME added a second dimension to the sections arrayand made this array, well, sparse. With SPARSEMEM_EXTREME, thefirst level became pointers to mem_section structures, and the actualmem_section objects were dynamically allocated based on the actuallypopulated physical memory.Another enhancement to SPARSEMEM was added in 2007; it wascalled generic virtualmemmap support for SPARSEMEM, orSPARSEMEM_VMEMMAP. The idea behind SPARSEMEM_VMEMMAP is that the entire memory map is mappedinto a virtually contiguous area, but only the active sections are backedwith physical pages. This model wouldn't work well with 32-bit systems,where the physical memory size might approach or even exceed the virtualaddress space. However, for 64-bit systems SPARSEMEM_VMEMMAP is aclear win. At the cost of additional page table entries,page_to_pfn(), and pfn_to_page() became as simple as withthe flat model.The last extension of the SPARSEMEM memory model is more recent(2016); it was driven by the increased use of persistent-memorydevices. To support storing the memory map directly on those devices ratherthan in main memory, the virtual memory map can use structvmem_altmap, which will provide page structures in persistentmemory.Back in 2005, SPARSEMEM was described as a "newer, andmore experimental alternative to 'discontiguous memory'". Thecommit that added SPARSEMEM_VMEMMAP noted that it "has the potentialto allow us to make SPARSEMEM the default (and even the only) option formost systems". And indeed, several architectures have switched overfrom DISCONTIGMEM to SPARSEMEM. In 2008,SPARSEMEM_VMEMMAP became the only supported memory model for x86-64, as it was onlyslightly more expensive than FLATMEM but more efficient thanDISCONTIGMEM.Recent memory-management developments, such as memory hotplug,persistent-memory support, and various performance optimizations, all targetthe SPARSEMEM model. But the older models still exist, which comes with thecost of numerous #ifdef blocks in the architecture andmemory-management code, and a peculiar maze of related configurationoptions. There is an ongoing work to completely switch the remaining usersof DISCONTIGMEM to SPARSEMEM, but making the change forsuch architectures as ia64 and mips64 won't be an easy task. And the ARCarchitecture'suse of DISCONTIGMEM to represent a "high memory" area thatresides below the "normal" memory will definitely be challenging to change.Comments (7 posted)Testing and the stable tree By Jake EdgeMay 28, 2019 LSFMMThe stable tree was the topic for a plenary session led by Sasha Levin atthe 2019 Linux Storage, Filesystem, and Memory-Management Summit (LSFMM).One of the main areas that needs attention is testing, according to Levin.He wanted to discuss how to do more and better testing as well as toaddress any concerns that attendees might have with regard to the stable tree.
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