专题：Linux内存管理专题
关键词：分配掩码、伙伴系统、水位(watermark)、空闲伙伴块合并。
我们知道Linux内存管理是以页为单位进行的，对内存的管理是通过伙伴系统进行。
从Linux内存管理框架图可知，页面分配器是其他林林总总内存操作的基础。
这也是为什么在介绍了《Linux内存管理 (1)物理内存初始化》、《Linux内存管理 (2)页表的映射过程》、《Linux内存管理 (3)内核内存的布局图》之后，紧接着就要弄明白页面分配器的原因。
1. 重要数据结构
1.1 页面分配掩码
alloc_pages是内核中常用的分配物理内存页面的接口函数，他有两个参数，其中一个就是分配掩码。
include\linux\gfp.h存放了GFP(Get Free Page)分配掩码，分配掩码可以分为两类：以__GFP_开头的分配掩码；以GFP_开头的一般是__GFP_的组合。
__GFP_掩码分为两大类：zone modifiers和action modifiers。
zone modifiers是掩码的低4位，用来指定从那个zone分配页面。
action modifiers定义了分配页面的属性
/* Plain integer GFP bitmasks. Do not use this directly. */
#define ___GFP_DMA 0x01u
#define ___GFP_HIGHMEM 0x02u
#define ___GFP_DMA32 0x04u
#define ___GFP_MOVABLE 0x08u
#define ___GFP_WAIT 0x10u
#define ___GFP_HIGH 0x20u
#define ___GFP_IO 0x40u
#define ___GFP_FS 0x80u
#define ___GFP_COLD 0x100u
#define ___GFP_NOWARN 0x200u
#define ___GFP_REPEAT 0x400u
#define ___GFP_NOFAIL 0x800u
#define ___GFP_NORETRY 0x1000u
#define ___GFP_MEMALLOC 0x2000u
#define ___GFP_COMP 0x4000u
#define ___GFP_ZERO 0x8000u
#define ___GFP_NOMEMALLOC 0x10000u
#define ___GFP_HARDWALL 0x20000u
#define ___GFP_THISNODE 0x40000u
#define ___GFP_RECLAIMABLE 0x80000u
#define ___GFP_NOTRACK 0x200000u
#define ___GFP_NO_KSWAPD 0x400000u
#define ___GFP_OTHER_NODE 0x800000u
#define ___GFP_WRITE 0x1000000u
/* If the above are modified, __GFP_BITS_SHIFT may need updating */
在实际使用中多使用GFP_开头的掩码：
/* This equals 0, but use constants in case they ever change */
#define GFP_NOWAIT (GFP_ATOMIC & ~__GFP_HIGH)
/* GFP_ATOMIC means both !wait (__GFP_WAIT not set) and use emergency pool */
#define GFP_ATOMIC (__GFP_HIGH)
#define GFP_NOIO (__GFP_WAIT)
#define GFP_NOFS (__GFP_WAIT | __GFP_IO)
#define GFP_KERNEL (__GFP_WAIT | __GFP_IO | __GFP_FS)
#define GFP_TEMPORARY (__GFP_WAIT | __GFP_IO | __GFP_FS | \
__GFP_RECLAIMABLE)
#define GFP_USER (__GFP_WAIT | __GFP_IO | __GFP_FS | __GFP_HARDWALL)
#define GFP_HIGHUSER (GFP_USER | __GFP_HIGHMEM)
#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
#define GFP_IOFS (__GFP_IO | __GFP_FS)
#define GFP_TRANSHUGE (GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
__GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN | \
__GFP_NO_KSWAPD)/*
* GFP_THISNODE does not perform any reclaim, you most likely want to
* use __GFP_THISNODE to allocate from a given node without fallback!
*/
#ifdef CONFIG_NUMA
#define GFP_THISNODE (__GFP_THISNODE | __GFP_NOWARN | __GFP_NORETRY)
#else
#define GFP_THISNODE ((__force gfp_t)0)
#endif/* This mask makes up all the page movable related flags */
#define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)/* Control page allocator reclaim behavior */
#define GFP_RECLAIM_MASK (__GFP_WAIT|__GFP_HIGH|__GFP_IO|__GFP_FS|\
__GFP_NOWARN|__GFP_REPEAT|__GFP_NOFAIL|\
__GFP_NORETRY|__GFP_MEMALLOC|__GFP_NOMEMALLOC)/* Control slab gfp mask during early boot */
#define GFP_BOOT_MASK (__GFP_BITS_MASK & ~(__GFP_WAIT|__GFP_IO|__GFP_FS))/* Control allocation constraints */
#define GFP_CONSTRAINT_MASK (__GFP_HARDWALL|__GFP_THISNODE)/* Do not use these with a slab allocator */
#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)/* Flag - indicates that the buffer will be suitable for DMA. Ignored on some
platforms, used as appropriate on others */#define GFP_DMA __GFP_DMA/* 4GB DMA on some platforms */
#define GFP_DMA32 __GFP_DMA32
2. 伙伴系统分配内存
alloc_page-------------------------------分配单页
get_zeroed_page-->__get_free_pages
alloc_pages--------------------------分配2^odrder个页面
alloc_pages_node-----------------增加node id参数
__alloc_pages
__alloc_pages_node_mask--增加nodemaks参数
__alloc_pages_nodemask is the ‘heart’ of the zoned buddy allocator.
首先__alloc_pages_nodemask很重要，其次说明了这里的伙伴页面分配器是基于Zone的。
struct alloc_context是伙伴系统分配函数中用于保存相关参数的数据结构。
struct alloc_context {
struct zonelist *zonelist;
nodemask_t *nodemask;
struct zone *preferred_zone;
int classzone_idx;
int migratetype;
enum zone_type high_zoneidx;
};
这里的zonelist，已经通过node_zonelist(nid, gfp_mask)得到：zonelist=NODE_DATA(nid)->node_zonelists+gfp_zonelist(flags)
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
struct zonelist *zonelist, nodemask_t *nodemask)
{
struct zoneref *preferred_zoneref;
struct page *page = NULL;
unsigned int cpuset_mems_cookie;
int alloc_flags = ALLOC_WMARK_LOW|ALLOC_CPUSET|ALLOC_FAIR;
gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
struct alloc_context ac = {
.high_zoneidx = gfp_zone(gfp_mask),----------------------------------gfp_zone根据gfp_mask低4位，找到对应的zone_type。ZONE_NORMAL？ZONE_HIGHMEM？
.nodemask = nodemask,
.migratetype = gfpflags_to_migratetype(gfp_mask),--------------------根据gfp_mask得出页面migratetype，是MIGRATE_RECLAIMABLE？MIGRATE_MOVABLE？
}; gfp_mask &= gfp_allowed_mask; lockdep_trace_alloc(gfp_mask); might_sleep_if(gfp_mask & __GFP_WAIT); if (should_fail_alloc_page(gfp_mask, order))
return NULL; /*
* Check the zones suitable for the gfp_mask contain at least one
* valid zone. It's possible to have an empty zonelist as a result
* of GFP_THISNODE and a memoryless node
*/
if (unlikely(!zonelist->_zonerefs->zone))
return NULL; if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE)
alloc_flags |= ALLOC_CMA;retry_cpuset:
cpuset_mems_cookie = read_mems_allowed_begin(); /* We set it here, as __alloc_pages_slowpath might have changed it */
ac.zonelist = zonelist;
/* The preferred zone is used for statistics later */
preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
ac.nodemask ? : &cpuset_current_mems_allowed,
&ac.preferred_zone);
if (!ac.preferred_zone)
goto out;
ac.classzone_idx = zonelist_zone_idx(preferred_zoneref); /* First allocation attempt */
alloc_mask = gfp_mask|__GFP_HARDWALL;
page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);---------尝试分配物理页面
if (unlikely(!page)) {
/*
* Runtime PM, block IO and its error handling path
* can deadlock because I/O on the device might not
* complete.
*/
alloc_mask = memalloc_noio_flags(gfp_mask); page = __alloc_pages_slowpath(alloc_mask, order, &ac);-----------------如果分配失败，则在这里进行很多特殊场景的处理。
} if (kmemcheck_enabled && page)
kmemcheck_pagealloc_alloc(page, order, gfp_mask); trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);out:
/*
* When updating a task's mems_allowed, it is possible to race with
* parallel threads in such a way that an allocation can fail while
* the mask is being updated. If a page allocation is about to fail,
* check if the cpuset changed during allocation and if so, retry.
*/
if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
goto retry_cpuset;--------------------------------------------------重试页面分配 return page;
}
get_page_from_freelist遍历ac->zonelist中的zone，在里面寻找满足条件的zone，然后找到页面，返回。
static struct page *
get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
const struct alloc_context *ac)
{
struct zonelist *zonelist = ac->zonelist;
struct zoneref *z;
struct page *page = NULL;
struct zone *zone;
nodemask_t *allowednodes = NULL;/* zonelist_cache approximation */
int zlc_active = ; /* set if using zonelist_cache */
int did_zlc_setup = ; /* just call zlc_setup() one time */
bool consider_zone_dirty = (alloc_flags & ALLOC_WMARK_LOW) &&
(gfp_mask & __GFP_WRITE);
int nr_fair_skipped = ;
bool zonelist_rescan;zonelist_scan:-------------------------------------------------------------------开始检查ac->zonelist。
zonelist_rescan = false; /*
* Scan zonelist, looking for a zone with enough free.
* See also __cpuset_node_allowed() comment in kernel/cpuset.c.
*/
for_each_zone_zonelist_nodemask(zone, z, zonelist, ac->high_zoneidx,--------从zonelist给定的ac->high_zoneidx开始查找，返回的是zone。
ac->nodemask) {...-----------------------------------------------------------------------------一系列检查条件，不满足跳出当前for循环，进入下一个zone。满足的进入水位检查。
mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];-----------------这里的alloc_flags包含ALLOC_WMARK_LOW
if (!zone_watermark_ok(zone, order, mark,-------------------------------所以此处会检查zone的低水位，不满足则进行检查，或者尝试zone_reclaim。
ac->classzone_idx, alloc_flags)) {
int ret; /* Checked here to keep the fast path fast */
BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
if (alloc_flags & ALLOC_NO_WATERMARKS)
goto try_this_zone;
...
ret = zone_reclaim(zone, gfp_mask, order);-------------------------通过zone_reclaim进行一些页面回收
switch (ret) {
...
default:
/* did we reclaim enough */
if (zone_watermark_ok(zone, order, mark,
ac->classzone_idx, alloc_flags))---------------------再次检查水位是否满足
goto try_this_zone; /*
* Failed to reclaim enough to meet watermark.
* Only mark the zone full if checking the min
* watermark or if we failed to reclaim just
* 1<<order pages or else the page allocator
* fastpath will prematurely mark zones full
* when the watermark is between the low and
* min watermarks.
*/
if (((alloc_flags & ALLOC_WMARK_MASK) == ALLOC_WMARK_MIN) ||
ret == ZONE_RECLAIM_SOME)
goto this_zone_full; continue;
}
}try_this_zone:---------------------------------------------------------------包括水位各种条件都满足之后，可以在此zone进行页面分配工作。
page = buffered_rmqueue(ac->preferred_zone, zone, order,-------------从zone中进行页面分配工作
gfp_mask, ac->migratetype);
if (page) {
if (prep_new_page(page, order, gfp_mask, alloc_flags))
goto try_this_zone;
return page;
}
this_zone_full:
if (IS_ENABLED(CONFIG_NUMA) && zlc_active)
zlc_mark_zone_full(zonelist, z);
} /*
* The first pass makes sure allocations are spread fairly within the
* local node. However, the local node might have free pages left
* after the fairness batches are exhausted, and remote zones haven't
* even been considered yet. Try once more without fairness, and
* include remote zones now, before entering the slowpath and waking
* kswapd: prefer spilling to a remote zone over swapping locally.
*/
if (alloc_flags & ALLOC_FAIR) {
alloc_flags &= ~ALLOC_FAIR;
if (nr_fair_skipped) {
zonelist_rescan = true;
reset_alloc_batches(ac->preferred_zone);
}
if (nr_online_nodes > )
zonelist_rescan = true;
} if (unlikely(IS_ENABLED(CONFIG_NUMA) && zlc_active)) {
/* Disable zlc cache for second zonelist scan */
zlc_active = ;
zonelist_rescan = true;
} if (zonelist_rescan)
goto zonelist_scan; return NULL;
}
关于水位的计算在watermark中有详细介绍。
下面看看判断当前zone空闲页面是否满足alloc_flags指定水位的函数__zone_watermark_ok。
z-zone结构体，order待分配页面的阶数，mark水位数值，classzone_idx是zone序号，alloc_flags分配掩码，free_pages当前空闲页面数。
static bool __zone_watermark_ok(struct zone *z, unsigned int order,
unsigned long mark, int classzone_idx, int alloc_flags,
long free_pages)
{
/* free_pages may go negative - that's OK */
long min = mark;
int o;
long free_cma = ; free_pages -= ( << order) - ;---------------------------------------------减去待分配页面后剩余页面数，-1？？
if (alloc_flags & ALLOC_HIGH)
min -= min / ;
if (alloc_flags & ALLOC_HARDER)
min -= min / ;
...
if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])--------空闲页面数要保证大于min值和lowmem_resreve保留值之和
return false;
for (o = ; o < order; o++) {-----------------------------------------------遍历buddy中比当前请求分配order小的所有order，依次检查free pages是否满足watermark需求
/* At the next order, this order's pages become unavailable */
free_pages -= z->free_area[o].nr_free << o;-----------------------------从总free_pages种减去当前order的free pages /* Require fewer higher order pages to be free */
min >>= ;--------------------------------------------------------------水位值缩半 if (free_pages <= min)--------------------------------------------------在比较是否满足水位需求
return false;
}
return true;----------------------------------------------------------------以上所有条件都满足，返回True
}
函数中循环的目的可归结为：
依次循环，检查内存中是否有足够多的大块（即order比较高）空闲内存。
每次循环处理中，先把当前order的free page从总free pages中减掉，因为我们是看是否有足够多的大块内存。
当然，既然已经把free pages中的一部分已经划掉了，比较标准也应该相应放宽。
放宽多少，就是前面说的对min的右移处理。
zone_reclaim:
int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
{
int node_id;
int ret; /*
* Zone reclaim reclaims unmapped file backed pages and
* slab pages if we are over the defined limits.
*
* A small portion of unmapped file backed pages is needed for
* file I/O otherwise pages read by file I/O will be immediately
* thrown out if the zone is overallocated. So we do not reclaim
* if less than a specified percentage of the zone is used by
* unmapped file backed pages.
*/
if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
return ZONE_RECLAIM_FULL; if (!zone_reclaimable(zone))
return ZONE_RECLAIM_FULL; /*
* Do not scan if the allocation should not be delayed.
*/
if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
return ZONE_RECLAIM_NOSCAN; /*
* Only run zone reclaim on the local zone or on zones that do not
* have associated processors. This will favor the local processor
* over remote processors and spread off node memory allocations
* as wide as possible.
*/
node_id = zone_to_nid(zone);
if (node_state(node_id, N_CPU) && node_id != numa_node_id())
return ZONE_RECLAIM_NOSCAN; if (test_and_set_bit(ZONE_RECLAIM_LOCKED, &zone->flags))
return ZONE_RECLAIM_NOSCAN; ret = __zone_reclaim(zone, gfp_mask, order);
clear_bit(ZONE_RECLAIM_LOCKED, &zone->flags); if (!ret)
count_vm_event(PGSCAN_ZONE_RECLAIM_FAILED); return ret;
}
buffered_rmqueue:
/*
* Allocate a page from the given zone. Use pcplists for order-0 allocations.
*/
static inline
struct page *buffered_rmqueue(struct zone *preferred_zone,
struct zone *zone, unsigned int order,
gfp_t gfp_flags, int migratetype)
{
unsigned long flags;
struct page *page;
bool cold = ((gfp_flags & __GFP_COLD) != ); if (likely(order == )) {
struct per_cpu_pages *pcp;
struct list_head *list; local_irq_save(flags);
pcp = &this_cpu_ptr(zone->pageset)->pcp;
list = &pcp->lists[migratetype];
if (list_empty(list)) {
pcp->count += rmqueue_bulk(zone, ,
pcp->batch, list,
migratetype, cold);
if (unlikely(list_empty(list)))
goto failed;
} if (cold)
page = list_entry(list->prev, struct page, lru);
else
page = list_entry(list->next, struct page, lru); list_del(&page->lru);
pcp->count--;
} else {
if (unlikely(gfp_flags & __GFP_NOFAIL)) {
/*
* __GFP_NOFAIL is not to be used in new code.
*
* All __GFP_NOFAIL callers should be fixed so that they
* properly detect and handle allocation failures.
*
* We most definitely don't want callers attempting to
* allocate greater than order-1 page units with
* __GFP_NOFAIL.
*/
WARN_ON_ONCE(order > );
}
spin_lock_irqsave(&zone->lock, flags);
page = __rmqueue(zone, order, migratetype);
spin_unlock(&zone->lock);
if (!page)
goto failed;
__mod_zone_freepage_state(zone, -( << order),
get_freepage_migratetype(page));
} __mod_zone_page_state(zone, NR_ALLOC_BATCH, -( << order));
if (atomic_long_read(&zone->vm_stat[NR_ALLOC_BATCH]) <= &&
!test_bit(ZONE_FAIR_DEPLETED, &zone->flags))
set_bit(ZONE_FAIR_DEPLETED, &zone->flags); __count_zone_vm_events(PGALLOC, zone, << order);
zone_statistics(preferred_zone, zone, gfp_flags);
local_irq_restore(flags); VM_BUG_ON_PAGE(bad_range(zone, page), page);
return page;failed:
local_irq_restore(flags);
return NULL;
}
3. 释放页面
__free_page
free_page-->free_pages
__free_pages
free_hot_cold_page
__free_pages_ok
4. 伙伴系统相关节点
4.1 /proc/pagetypeinfo
Page block order:
Pages per block: Free pages count per migrate type at order
Node , zone Normal, type Unmovable
Node , zone Normal, type Reclaimable
Node , zone Normal, type Movable
Node , zone Normal, type Reserve
Node , zone Normal, type CMA
Node , zone Normal, type Isolate
Node , zone HighMem, type Unmovable
Node , zone HighMem, type Reclaimable
Node , zone HighMem, type Movable
Node , zone HighMem, type Reserve
Node , zone HighMem, type CMA
Node , zone HighMem, type Isolate Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
Node , zone Normal
Node , zone HighMem
