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gatieme
@@ -225,124 +225,16 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
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##2.3 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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内核源代码将`__alloc_pages_nodemask`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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`__alloc_pages_nodemask`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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#3 选择页
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-------
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@@ -617,8 +509,19 @@ struct alloc_context {
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| preferred_zone | 表示从high_zoneidx后找到的合适的zone，一般会从该zone分配；分配失败的话，就会在zonelist再找一个preferred_zone = 合适的zone |
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| migratetype | 迁移类型，在zone->free_area.free_list[XXX] 作为分配下标使用，这个是用来反碎片化的，修改了以前的free_area结构体，在该结构体中再添加了一个数组，该数组以迁移类型为下标，每个数组元素都挂了对应迁移类型的页链表 |
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| high_zoneidx | 是表示该分配时，所能分配的最高zone，一般从high-->normal-->dma 内存越来越昂贵，所以一般从high到dma分配依次分配 |
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| spread_dirty_pages | |
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zonelist是指向备用列表的指针. 在预期内存域没有空闲空间的情况下, 该列表确定了扫描系统其他内存域(和结点)的顺序.
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随后的for循环所作的基本上与直觉一致, 遍历备用列表的所有内存域，用最简单的方式查找一个适当的空闲内存块
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* 首先，解释ALLOC_*标志(\__cpuset_zone_allowed_softwall是另一个辅助函数, 用于检查给定内存域是否属于该进程允许运行的CPU).
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* zone_watermark_ok接下来检查所遍历到的内存域是否有足够的空闲页，并试图分配一个连续内存块。如果两个条件之一不能满足，即或者没有足够的空闲页，或者没有连续内存块可满足分配请求，则循环进行到备用列表中的下一个内存域，作同样的检查. 直到找到一个合适的页面, 在进行try_this_node进行内存分配
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* 如果内存域适用于当前的分配请求, 那么buffered_rmqueue试图从中分配所需数目的页
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```cpp
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/*
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* get_page_from_freelist goes through the zonelist trying to allocate
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@@ -760,4 +663,131 @@ reset_fair:
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return NULL;
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}
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```
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```
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#4 分配控制
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-------
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如前所述, `__alloc_pages_nodemask`是伙伴系统的心脏. 我们已经处理了所有的准备工作并描述了所有可能的标志, 现在我们把注意力转向相对复杂的部分 : 函数`__alloc_pages_nodemask`的实现, 这也是内核中比较冗长的部分
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之一. 特别是在可用内存太少或逐渐用完时, 函数就会比较复杂. 如果可用内存足够，则必要的工作会很快完成，就像下述代码
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##4.1 函数源代码注释
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-------
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`__alloc_pages_nodemask`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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最简单的情形中, 分配空闲内存区只涉及调用一次`get_page_from_freelist`, 然后返回所需数目的页(由标号got_pg处的代码处理).
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第一次内存分配尝试不会特别积极. 如果在某个内存域中无法找到空闲内存, 则意味着内存没剩下多少了, 内核需要增加较多的工作量才能找到更多内存("重型武器"稍后才会出现).
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