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gatieme
@@ -628,6 +628,8 @@ not_early:
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还有一组kmalloc类型的函数, 用于分配小于一整页的内存区. 其实现
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将在本章后续的几节分别讨论。
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## 释放函数
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-------
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@@ -1060,13 +1062,16 @@ static inline enum zone_type gfp_zone(gfp_t flags)
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与内存域修饰符相反, 这些额外的标志并不限制从哪个物理内存段分配内存, 但确实可以改变分配器的行为. 例如, 它们可以修改查找空闲内存时的积极程度.
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##4.3 分配页
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-------
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##4.3 内存分配统一到alloc_pages接口
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###4.3.1 内存分配统一到alloc_pages接口
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-------
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通过使用标志、内存域修饰符和各个分配函数，内核提供了一种非常灵活的内存分配体系.尽管如此, 所有接口函数都可以追溯到一个简单的基本函数(alloc_pages_node)
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分配单页的函数[`alloc_page`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L483)和[`__get_free_page`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L500), 还有[`__get_dma_pages`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L503)是借助于宏定义的
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分配单页的函数[`alloc_page`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L483)和[`__get_free_page`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L500), 还有[`__get_dma_pages`](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L503)是借助于宏定义的.
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```cpp
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// http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L483
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@@ -1081,7 +1086,7 @@ static inline enum zone_type gfp_zone(gfp_t flags)
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__get_free_pages((gfp_mask) | GFP_DMA, (order))
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```
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[`get_zeroed_page`](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3900)的实现也没什么困难, 对__get_free_pages使用__GFP_ZERO标志，即可分配填充字节0的页. 再返回与页关联的内存区地址即可.
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[`get_zeroed_page`](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3900)的实现也没什么困难, 对`__get_free_pages`使用`__GFP_ZERO`标志，即可分配填充字节0的页. 再返回与页关联的内存区地址即可.
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```cpp
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@@ -1094,7 +1099,7 @@ EXPORT_SYMBOL(get_zeroed_page);
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```
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[`__get_free_pages`](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3883)调用alloc_pages完成内存分配, 而alloc_pages又借助于alloc_pages_node
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[`__get_free_pages`](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3883)调用`alloc_pages`完成内存分配, 而alloc_pages又借助于alloc_pages_node
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[`__get_free_pages`](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3883)函数的定义在[mm/page_alloc.c?v=4.7, line 3883](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3883)
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@@ -1118,10 +1123,34 @@ unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
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EXPORT_SYMBOL(__get_free_pages);
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```
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在这种情况下， 使用了一个普通函数而不是宏， 因为alloc_pages返回的page实例需要使用辅助
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函数page_address转换为内存地址. 在这里，只要知道该函数可根据page实例计算相关页的线性内存地址即可. 对高端内存页这是有问题的
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在这种情况下， 使用了一个普通函数而不是宏， 因为`alloc_pages`返回的`page`实例需要使用辅助
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函数`page_address`转换为内存地址. 在这里，只要知道该函数可根据`page`实例计算相关页的线性内存地址即可. 对高端内存页这是有问题的
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<font color = 0x00ffff>
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这样, 就完成了所有分配内存的API函数到公共的基础函数`alloc_pages`的统一
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</font>
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所有体系结构都必须实现的标准函数`clear_page`, 可帮助alloc_pages对页填充字节0, 实现如下表所示
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| x86 | arm |
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|:----:|:-----:|
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| [arch/x86/include/asm/page_32.h?v=4.7, line 24](http://lxr.free-electrons.com/source/arch/x86/include/asm/page_32.h?v=4.7#L24) | [arch/arm/include/asm/page.h?v=4.7#L14](http://lxr.free-electrons.com/source/arch/arm/include/asm/page.h?v=4.7#L142)<br>[arch/arm/include/asm/page-nommu.h](http://lxr.free-electrons.com/source/arch/arm/include/asm/page-nommu.h?v=4.7#L20) |
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###4.3.2 alloc_pages函数分配页
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-------
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既然所有的内存分配API函数都可以追溯掉`alloc_page`函数, 从某种意义上说，该函数是伙伴系统主要实现的"发射台".
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`alloc_pages`函数的定义是依赖于NUMA或者UMA架构的, 定义如下
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alloc_pages_node函数的定义是依赖于NUMA或者UMA架构的, 定义如下
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```cpp
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#ifdef CONFIG_NUMA
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@@ -1141,21 +1170,225 @@ alloc_pages(gfp_t gfp_mask, unsigned int order)
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#endif
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```
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<font color = 0x00ffff>
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这样, 就完成了所有分配内存的API函数到公共的基础函数alloc_pages的统一
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</font>
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UMA结构下的`alloc_pages`是通过`alloc_pages_node`函数实现的, 下面我们看看`alloc_pages_node`函数的定义, 在[include/linux/gfp.h?v=4.7, line 448](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L448)
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所有体系结构都必须实现的标准函数clear_page，可帮助alloc_pages对页填充字节0, 实现如下表所示
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```cpp
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// http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L448
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/*
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* Allocate pages, preferring the node given as nid. When nid == NUMA_NO_NODE,
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* prefer the current CPU's closest node. Otherwise node must be valid and
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* online.
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*/
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static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
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unsigned int order)
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{
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if (nid == NUMA_NO_NODE)
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nid = numa_mem_id();
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| x86 | arm |
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|:----:|:-----:|
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| [arch/x86/include/asm/page_32.h?v=4.7, line 24](http://lxr.free-electrons.com/source/arch/x86/include/asm/page_32.h?v=4.7#L24) | [arch/arm/include/asm/page.h?v=4.7#L14](http://lxr.free-electrons.com/source/arch/arm/include/asm/page.h?v=4.7#L142)<br>[arch/arm/include/asm/page-nommu.h](http://lxr.free-electrons.com/source/arch/arm/include/asm/page-nommu.h?v=4.7#L20) |
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return __alloc_pages_node(nid, gfp_mask, order);
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}
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````
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它只是执行了一个简单的检查, 如果指定负的结点ID(不存在, 即[NUMA_NO_NODE = -1](http://lxr.free-electrons.com/source/include/linux/numa.h?v=4.7#L13)), 内核自动地使用当前执行CPU对应的结点nid = [numa_mem_id();](http://lxr.free-electrons.com/source/include/linux/topology.h?v=4.7#L137), 然后调用`__alloc_pages_node`函数进行了内存分配
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`__alloc_pages_node`函数定义在[include/linux/gfp.h?v=4.7, line 435)](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L435), 如下所示
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```cpp
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// http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L435
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/*
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* Allocate pages, preferring the node given as nid. The node must be valid and
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* online. For more general interface, see alloc_pages_node().
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*/
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static inline struct page *
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__alloc_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
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{
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VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
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VM_WARN_ON(!node_online(nid));
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return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
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}
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```
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内核假定传递给改alloc_pages_node函数的结点nid是被激活, 即online的.但是为了安全它还是检查并警告内存结点不存在的情况. 接下来的工作委托给__alloc_pages, 只需传递一组适当的参数, 其中包括节点nid的备用内存域列表zonelist.
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现在`__alloc_pages`函数没什么特别的, 它直接将自己的所有信息传递给`__alloc_pages_nodemask`来完成内存的分配
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```cpp
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// http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L428
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static inline struct page *
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__alloc_pages(gfp_t gfp_mask, unsigned int order,
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struct zonelist *zonelist)
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{
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return __alloc_pages_nodemask(gfp_mask, order, zonelist, NULL);
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}
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```
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###4.3.3 伙伴系统的心脏__alloc_pages_nodemask
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-------
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内核源代码将`__alloc_pages`称之为"伙伴系统的心脏"(`the 'heart' of the zoned buddy allocator``), 因为它处理的是实质性的内存分配.
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由于"心脏"的重要性, 我将在下文详细介绍该函数.
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`__alloc_pages`函数定义在[include/linux/gfp.h?v=4.7#L428](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L428)
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```cpp
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// http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3779
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/*
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* This is the 'heart' of the zoned buddy allocator.
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*/
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struct page *
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__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
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struct zonelist *zonelist, nodemask_t *nodemask)
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{
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struct page *page;
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unsigned int cpuset_mems_cookie;
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unsigned int alloc_flags = ALLOC_WMARK_LOW|ALLOC_FAIR;
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gfp_t alloc_mask = gfp_mask; /* The gfp_t that was actually used for allocation */
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struct alloc_context ac = {
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.high_zoneidx = gfp_zone(gfp_mask),
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.zonelist = zonelist,
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.nodemask = nodemask,
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.migratetype = gfpflags_to_migratetype(gfp_mask),
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};
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if (cpusets_enabled()) {
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alloc_mask |= __GFP_HARDWALL;
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alloc_flags |= ALLOC_CPUSET;
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if (!ac.nodemask)
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ac.nodemask = &cpuset_current_mems_allowed;
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}
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gfp_mask &= gfp_allowed_mask;
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lockdep_trace_alloc(gfp_mask);
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might_sleep_if(gfp_mask & __GFP_DIRECT_RECLAIM);
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if (should_fail_alloc_page(gfp_mask, order))
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return NULL;
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/*
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* Check the zones suitable for the gfp_mask contain at least one
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* valid zone. It's possible to have an empty zonelist as a result
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* of __GFP_THISNODE and a memoryless node
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*/
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if (unlikely(!zonelist->_zonerefs->zone))
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return NULL;
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if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE)
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alloc_flags |= ALLOC_CMA;
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retry_cpuset:
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cpuset_mems_cookie = read_mems_allowed_begin();
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/* Dirty zone balancing only done in the fast path */
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ac.spread_dirty_pages = (gfp_mask & __GFP_WRITE);
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/*
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* The preferred zone is used for statistics but crucially it is
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* also used as the starting point for the zonelist iterator. It
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* may get reset for allocations that ignore memory policies.
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*/
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ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
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ac.high_zoneidx, ac.nodemask);
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if (!ac.preferred_zoneref) {
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page = NULL;
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goto no_zone;
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}
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/* First allocation attempt */
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page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
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if (likely(page))
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goto out;
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/*
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* Runtime PM, block IO and its error handling path can deadlock
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* because I/O on the device might not complete.
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*/
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alloc_mask = memalloc_noio_flags(gfp_mask);
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ac.spread_dirty_pages = false;
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/*
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* Restore the original nodemask if it was potentially replaced with
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* &cpuset_current_mems_allowed to optimize the fast-path attempt.
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*/
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if (cpusets_enabled())
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ac.nodemask = nodemask;
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page = __alloc_pages_slowpath(alloc_mask, order, &ac);
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no_zone:
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/*
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* When updating a task's mems_allowed, it is possible to race with
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* parallel threads in such a way that an allocation can fail while
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* the mask is being updated. If a page allocation is about to fail,
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* check if the cpuset changed during allocation and if so, retry.
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*/
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if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie))) {
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alloc_mask = gfp_mask;
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goto retry_cpuset;
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}
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out:
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if (kmemcheck_enabled && page)
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kmemcheck_pagealloc_alloc(page, order, gfp_mask);
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trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
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return page;
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}
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EXPORT_SYMBOL(__alloc_pages_nodemask);
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```
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##4.4 __free_pages
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类似地，内存释放函数也可以归约到一个主要的函数（__free_pages），只是用不同的参数调
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用而已：
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类似地，内存释放函数也可以归约到一个主要的函数(\__free_pages), 只是用不同的参数调用而已
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前面我们讲过内核释放的两个主要函数有\__free_page和free_page, 它们的定义在[include/linux/gfp.h?v=4.7#L519](http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L519)
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```cpp
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// http://lxr.free-electrons.com/source/include/linux/gfp.h?v=4.7#L519
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#define __free_page(page) __free_pages((page), 0)
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#define free_page(addr) free_pages((addr), 0)
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```
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而free_pages是通过__free_pages来完成内存释放的, 参见[mm/page_alloc.c?v=4.7#L3918](http://lxr.free-electrons.com/source/mm/page_alloc.c?v=4.7#L3918)
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```cpp
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void free_pages(unsigned long addr, unsigned int order)
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{
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if (addr != 0) {
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VM_BUG_ON(!virt_addr_valid((void *)addr));
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__free_pages(virt_to_page((void *)addr), order);
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}
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}
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```
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`free_pages`和`__free_pages`之间的关系通过函数而不是宏建立, 因为首先必须将虚拟地址转换为指向`struct page`的指针
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`virt_to_page`将虚拟内存地址转换为指向page实例的指针. 基本上, 这是讲解内存分配函数时介绍的page_address辅助函数的逆过程.
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下图以图形化方式综述了各个内存释放函数之间的关系
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