Linux如何为其物理分配器分配内存? [英] How does Linux allocate memory for its physical allocator?

问题描述

我最近想研究Linux内存管理的细节，因为我想为自己的玩具内核实现类似的东西，所以我希望熟悉这些细节的人能帮助我理解一件事.显然，物理内存管理器是一种伙伴算法，该算法还专门用于返回特定顺序的页面块(0至9，其中0只是单个页面).对于每个顺序，块都存储为链接列表.假设如果请求了5阶块，但未在5阶列表中找到该算法，则算法将搜索6阶块，将其分成两部分，给出所请求的一半，然后将另一半移低一个阶(如它只有一半大小). 我没有得到的是内核如何存储这些结构，或者它如何为它们分配空间.由于对于订单0页面，您将需要1M条目(每个页面都是4KiB页面)，这是否意味着内核分配了1MiB * sizeof(结构页面)?那么1级以上的块呢?内核是否通过将已分配的块标记为更高的顺序来重用分配的块，并且当需要将其分成两部分时，只需返回该块并获得未使用的块?

I was recently delving into the details of Linux's memory management as I want to implement something similar for my own toy kernel, so I was hoping if someone who's familiar with the details could help me understand one thing. Apparently the physical memory manager is a buddy algorithm, which is further specialised to return blocks of pages of a particular order (0 to 9, with 0 being just a single page). For each order the blocks are stored as a linked list. Say if a block of order 5 is requested but is not found on the list of order 5 blocks, the algorithm searches for a block in order 6, splits it into two, gives the requested half and moves the other half an order lower (as it is half in size). What I don't get is how the kernel stores these structures, or how it allocates space for them. Since for order 0 pages you would need 1M entries (each is a 4KiB page), does it mean that the kernel allocates 1MiB * sizeof(struct page)? What about the blocks of order 1 and above? Does the kernel reuse allocated blocks by marking them as a higher order, and when it needs to split it in two just return the block and get one that is unused?

推荐答案

我没有得到的是内核如何存储这些结构，或者它如何为它们分配空间.由于对于订单0页面，您将需要1M条目(每个页面都是4KiB页面)，这是否意味着内核分配了1MiB * sizeof(结构页面)?

What I don't get is how the kernel stores these structures, or how it allocates space for them. Since for order 0 pages you would need 1M entries (each is a 4KiB page), does it mean that the kernel allocates 1MiB * sizeof(struct page)?

通过调用 paging_init() (arch/x86/mm/init_32.c；一些说明- https://www.kernel.org/doc/gorman/html/understand/understand005.html 2.3区域初始化和 setup_arch() 通过( native_pagetable_init() 和间接调用1166 x86_init.paging.pagetable_init();):

Initialization of zones is done by calling paging_init() (arch/x86/mm/init_32.c; some descriptions - https://www.kernel.org/doc/gorman/html/understand/understand005.html 2.3 Zone Initialisation and http://repo.hackerzvoice.net/depot_madchat/ebooks/Mem_virtuelle/linux-mm/vminit.html Initializing the Kernel Page Tables) from setup_arch() via (native_pagetable_init() and indirect call 1166 x86_init.paging.pagetable_init();):

690 /*
691  * paging_init() sets up the page tables - note that the first 8MB are
692  * already mapped by head.S.
...*/
697 void __init paging_init(void)
698 {
699         pagetable_init();
...
711         zone_sizes_init();
712 }

pagetable_init()在1024个pgd_t s的swapper_pg_dir数组中创建内核页表.

pagetable_init() creates kernel page tables in swapper_pg_dir array of 1024 pgd_ts.

zone_sizes_init() 实际上定义了物理内存区域，并调用 free_area_init_nodes()来初始化它们，并在for_each_online_node(nid) {...}) "rel =" nofollow> free_area_init_node() ，它调用三个函数:

zone_sizes_init() actually defines zones of physical memory and calls free_area_init_nodes() to initialize them with actual work done (for each NUMA node for_each_online_node(nid) {...}) in free_area_init_node() which calls three functions:

• calculate_node_totalpages()打印dmesg中每个节点的页数
• alloc_node_mem_map() 做实际的工作为该节点中的每个物理页面分配struct page的方法；它们的内存由bootmem分配器 doc1 doc2 (您可以使用bootmem_debug=1内核启动选项查看其调试) :

• calculate_node_totalpages() prints page counts for every node in dmesg
• alloc_node_mem_map() does actual job of allocating struct page for every physical page in this node; memory for them is allocated by bootmem allocator doc1 doc2 (you can see its debug with bootmem_debug=1 kernel boot option):

4936 size = (end - start) * sizeof(struct page);

4937 map = alloc_remap(pgdat->node_id, size);

if (!map) map = memblock_virt_alloc_node_nopanic(size, pgdat->node_id);

• free_area_init_core() (带填充 struct zone 中的位图数量) .在 http:/中为较早版本的内核描述的free_area_init_core功能. /repo.hackerzvoice.net/depot_madchat/ebooks/Mem_virtuelle/linux-mm/zonealloc.html#INITIALIZE 为:

• free_area_init_core() (with filling of bitmaps in struct zone). Functionality of free_area_init_core described for older kernels in http://repo.hackerzvoice.net/depot_madchat/ebooks/Mem_virtuelle/linux-mm/zonealloc.html#INITIALIZE as:

free_area_init_core()在free_area_init_core()中构建内存映射，并初始化空闲列表和伙伴位图.

free_area_init_core() The memory map is built, and the freelists and buddy bitmaps initialized, in free_area_init_core().

初始化每个区域中的免费订单列表，并将订单标记为没有任何空闲页面:free_area_init_core()-> zone_init_free_lists :

Free lists of orders in each zone are initialized and orders are marked as having no any free page: free_area_init_core() -> init_currently_empty_zone() -> zone_init_free_lists:

4147 static void __meminit zone_init_free_lists(struct zone *zone)
4148 {
4149         unsigned int order, t;
4150         for_each_migratetype_order(order, t) {
4151                 INIT_LIST_HEAD(&zone->free_area[order].free_list[t]);
4152                 zone->free_area[order].nr_free = 0;
4153         }
4154 }

PS:内核中有 个init()，它称为

PS: There is init() in kernel, it is called start_kernel(), and LXR (Linux cross-reference) will help you to navigate between functions (I posted links to lxr.free-electrons.com, but there are several online LXRs):

501 asmlinkage __visible void __init start_kernel(void)
...
528         boot_cpu_init();
529         page_address_init();
530         pr_notice("%s", linux_banner);
531         setup_arch(&command_line);

这篇关于Linux如何为其物理分配器分配内存?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！