Linux内核最新的连续内存分配器(CMA)——避免预留大块内存【转】

在我们使用ARM等嵌入式Linux系统的时候,一个头疼的问题是GPU,Camera,HDMI等都需要预留大量连续内存,这部分内存平时不用,但是一般的做法又必须先预留着。目前,Marek Szyprowski和Michal Nazarewicz实现了一套全新的Contiguous Memory Allocator。通过这套机制,我们可以做到不预留内存,这些内存平时是可用的,只有当需要的时候才被分配给Camera,HDMI等设备。下面分析它的基本代码流程。

1. 声明连续内存

内核启动过程中arch/arm/mm/init.c中的arm_memblock_init()会调用dma_contiguous_reserve(min(arm_dma_limit, arm_lowmem_limit));

该函数位于:drivers/base/dma-contiguous.c

/**
* dma_contiguous_reserve() - reserve area for contiguous memory handling
* @limit: End address of the reserved memory (optional, 0 for any).
*
* This function reserves memory from early allocator. It should be
* called by arch specific code once the early allocator (memblock or bootmem)
* has been activated and all other subsystems have already allocated/reserved
* memory.
*/
void __init dma_contiguous_reserve(phys_addr_t limit)
{
unsigned long selected_size = 0; pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit); if (size_cmdline != -1) {
selected_size = size_cmdline;
} else {
#ifdef CONFIG_CMA_SIZE_SEL_MBYTES
selected_size = size_bytes;
#elif defined(CONFIG_CMA_SIZE_SEL_PERCENTAGE)
selected_size = cma_early_percent_memory();
#elif defined(CONFIG_CMA_SIZE_SEL_MIN)
selected_size = min(size_bytes, cma_early_percent_memory());
#elif defined(CONFIG_CMA_SIZE_SEL_MAX)
selected_size = max(size_bytes, cma_early_percent_memory());
#endif
} if (selected_size) {
pr_debug("%s: reserving %ld MiB for global area\n", __func__,
selected_size / SZ_1M); dma_declare_contiguous(NULL, selected_size, 0, limit);
}

其中的size_bytes定义为:

static const unsigned long size_bytes = CMA_SIZE_MBYTES * SZ_1M

默认情况下,CMA_SIZE_MBYTES会被定义为16MB,来源于CONFIG_CMA_SIZE_MBYTES=16

int __init dma_declare_contiguous(struct device *dev, unsigned long size,
phys_addr_t base, phys_addr_t limit)
{
...
/* Reserve memory */
if (base) {
if (memblock_is_region_reserved(base, size) ||
memblock_reserve(base, size) < 0) {
base = -EBUSY;
goto err;
}
} else {
/*
* Use __memblock_alloc_base() since
* memblock_alloc_base() panic()s.
*/
phys_addr_t addr = __memblock_alloc_base(size, alignment, limit);
if (!addr) {
base = -ENOMEM;
goto err;
} else if (addr + size > ~(unsigned long)0) {
memblock_free(addr, size);
base = -EINVAL;
                        base = -EINVAL;
                        goto err;
                } else {
                        base = addr;
                }
        }         /*
         * Each reserved area must be initialised later, when more kernel
         * subsystems (like slab allocator) are available.
         */
        r->start = base;
        r->size = size;
        r->dev = dev;
        cma_reserved_count++;
        pr_info("CMA: reserved %ld MiB at %08lx\n", size / SZ_1M,
                (unsigned long)base);         /* Architecture specific contiguous memory fixup. */
        dma_contiguous_early_fixup(base, size);
        return 0;
err:
        pr_err("CMA: failed to reserve %ld MiB\n", size / SZ_1M);
        return base;
}

由此可见,连续内存区域也是在内核启动的早期,通过__memblock_alloc_base()拿到的。

另外:

drivers/base/dma-contiguous.c里面的core_initcall()会导致cma_init_reserved_areas()被调用:

cma_create_area()会调用cma_activate_area(),cma_activate_area()函数则会针对每个page调用:

init_cma_reserved_pageblock(pfn_to_page(base_pfn));

这个函数则会通过set_pageblock_migratetype(page, MIGRATE_CMA)将页设置为MIGRATE_CMA类型的:

#ifdef CONFIG_CMA
/* Free whole pageblock and set it's migration type to MIGRATE_CMA. */
void __init init_cma_reserved_pageblock(struct page *page)
{
unsigned i = pageblock_nr_pages;
struct page *p = page; do {
__ClearPageReserved(p);
set_page_count(p, 0);
} while (++p, --i); set_page_refcounted(page);
set_pageblock_migratetype(page, MIGRATE_CMA);
__free_pages(page, pageblock_order);
totalram_pages += pageblock_nr_pages;
}
#endif

同时其中调用的__free_pages(page, pageblock_order);最终会调用到__free_one_page(page, zone, order, migratetype);

相关的page会被加到MIGRATE_CMA的free_list上面去:

list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);

2. 申请连续内存

申请连续内存仍然使用标准的arch/arm/mm/dma-mapping.c中定义的dma_alloc_coherent()和dma_alloc_writecombine(),这二者会间接调用drivers/base/dma-contiguous.c中的

struct page *dma_alloc_from_contiguous(struct device *dev, int count,
unsigned int align)

->

struct page *dma_alloc_from_contiguous(struct device *dev, int count,
unsigned int align)
{
... for (;;) {
pageno = bitmap_find_next_zero_area(cma->bitmap, cma->count,
start, count, mask);
if (pageno >= cma->count) {
ret = -ENOMEM;
goto error;
} pfn = cma->base_pfn + pageno;
ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA);
if (ret == 0) {
bitmap_set(cma->bitmap, pageno, count);
break;
} else if (ret != -EBUSY) {
goto error;
}
pr_debug("%s(): memory range at %p is busy, retrying\n",
__func__, pfn_to_page(pfn));
/* try again with a bit different memory target */
start = pageno + mask + 1;
}
... }

--》

int alloc_contig_range(unsigned long start, unsigned long end,

                       unsigned migratetype)

需要隔离page,隔离page的作用通过代码的注释可以体现:

 /*
* What we do here is we mark all pageblocks in range as
* MIGRATE_ISOLATE. Because of the way page allocator work, we
* align the range to MAX_ORDER pages so that page allocator
* won't try to merge buddies from different pageblocks and
* change MIGRATE_ISOLATE to some other migration type.
*
* Once the pageblocks are marked as MIGRATE_ISOLATE, we
* migrate the pages from an unaligned range (ie. pages that
* we are interested in). This will put all the pages in
* range back to page allocator as MIGRATE_ISOLATE.
*
* When this is done, we take the pages in range from page
* allocator removing them from the buddy system. This way
* page allocator will never consider using them.
*
* This lets us mark the pageblocks back as
* MIGRATE_CMA/MIGRATE_MOVABLE so that free pages in the
* MAX_ORDER aligned range but not in the unaligned, original
* range are put back to page allocator so that buddy can use
* them.
*/ ret = start_isolate_page_range(pfn_align_to_maxpage_down(start),
pfn_align_to_maxpage_up(end),
migratetype);

简单地说,就是把相关的page标记为MIGRATE_ISOLATE,这样buddy系统就不会再使用他们。

/*
* start_isolate_page_range() -- make page-allocation-type of range of pages
* to be MIGRATE_ISOLATE.
* @start_pfn: The lower PFN of the range to be isolated.
* @end_pfn: The upper PFN of the range to be isolated.
* @migratetype: migrate type to set in error recovery.
*
* Making page-allocation-type to be MIGRATE_ISOLATE means free pages in
* the range will never be allocated. Any free pages and pages freed in the
* future will not be allocated again.
*
* start_pfn/end_pfn must be aligned to pageblock_order.
* Returns 0 on success and -EBUSY if any part of range cannot be isolated.
*/
int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
unsigned migratetype)
{
unsigned long pfn;
unsigned long undo_pfn;
struct page *page; BUG_ON((start_pfn) & (pageblock_nr_pages - 1));
BUG_ON((end_pfn) & (pageblock_nr_pages - 1)); for (pfn = start_pfn;
pfn < end_pfn;
pfn += pageblock_nr_pages) {
page = __first_valid_page(pfn, pageblock_nr_pages);
if (page && set_migratetype_isolate(page)) {
undo_pfn = pfn;
goto undo;
}
}
return 0;
undo:
for (pfn = start_pfn;
pfn < undo_pfn;
pfn += pageblock_nr_pages)
unset_migratetype_isolate(pfn_to_page(pfn), migratetype); return -EBUSY;
}

接下来调用__alloc_contig_migrate_range()进行页面隔离和迁移:

static int __alloc_contig_migrate_range(unsigned long start, unsigned long end)
{
/* This function is based on compact_zone() from compaction.c. */ unsigned long pfn = start;
unsigned int tries = 0;
int ret = 0; struct compact_control cc = {
.nr_migratepages = 0,
.order = -1,
.zone = page_zone(pfn_to_page(start)),
.sync = true,
};
INIT_LIST_HEAD(&cc.migratepages); migrate_prep_local(); while (pfn < end || !list_empty(&cc.migratepages)) {
if (fatal_signal_pending(current)) {
ret = -EINTR;
break;
} if (list_empty(&cc.migratepages)) {
cc.nr_migratepages = 0;
pfn = isolate_migratepages_range(cc.zone, &cc,
pfn, end);
if (!pfn) {
ret = -EINTR;
break;
}
tries = 0;
} else if (++tries == 5) {
ret = ret < 0 ? ret : -EBUSY;
break;
} ret = migrate_pages(&cc.migratepages,
__alloc_contig_migrate_alloc,
0, false, true);
} putback_lru_pages(&cc.migratepages);
return ret > 0 ? 0 : ret;
}

其中的函数migrate_pages()会完成页面的迁移,迁移过程中通过传入的__alloc_contig_migrate_alloc()申请新的page,并将老的page付给新的page:

int migrate_pages(struct list_head *from,
new_page_t get_new_page, unsigned long private, bool offlining,
bool sync)
{
int retry = 1;
int nr_failed = 0;
int pass = 0;
struct page *page;
struct page *page2;
int swapwrite = current->flags & PF_SWAPWRITE;
int rc; if (!swapwrite)
current->flags |= PF_SWAPWRITE; for(pass = 0; pass < 10 && retry; pass++) {
retry = 0; list_for_each_entry_safe(page, page2, from, lru) {
cond_resched(); rc = unmap_and_move(get_new_page, private,
page, pass > 2, offlining,
sync); switch(rc) {
case -ENOMEM:
goto out;
case -EAGAIN:
retry++;
break;
case 0:
break;
default:
/* Permanent failure */
nr_failed++;
break;
}
}
}
rc = 0;
...
}

其中的unmap_and_move()函数较为关键,它定义在mm/migrate.c中

/*
* Obtain the lock on page, remove all ptes and migrate the page
* to the newly allocated page in newpage.
*/
static int unmap_and_move(new_page_t get_new_page, unsigned long private,
struct page *page, int force, bool offlining, bool sync)
{
int rc = 0;
int *result = NULL;
struct page *newpage = get_new_page(page, private, &result);
int remap_swapcache = 1;
int charge = 0;
struct mem_cgroup *mem = NULL;
struct anon_vma *anon_vma = NULL; ... /* charge against new page */
charge = mem_cgroup_prepare_migration(page, newpage, &mem);
... if (PageWriteback(page)) {
if (!force || !sync)
goto uncharge;
wait_on_page_writeback(page);
}
/*
* By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
* we cannot notice that anon_vma is freed while we migrates a page.
* This get_anon_vma() delays freeing anon_vma pointer until the end
* of migration. File cache pages are no problem because of page_lock()
* File Caches may use write_page() or lock_page() in migration, then,
* just care Anon page here.
*/
if (PageAnon(page)) {
/*
* Only page_lock_anon_vma() understands the subtleties of
* getting a hold on an anon_vma from outside one of its mms.
*/
anon_vma = page_lock_anon_vma(page);
if (anon_vma) {
/*
* Take a reference count on the anon_vma if the
* page is mapped so that it is guaranteed to
* exist when the page is remapped later
*/
get_anon_vma(anon_vma);
page_unlock_anon_vma(anon_vma);
} else if (PageSwapCache(page)) {
/*
* We cannot be sure that the anon_vma of an unmapped
* swapcache page is safe to use because we don't
* know in advance if the VMA that this page belonged
* to still exists. If the VMA and others sharing the
* data have been freed, then the anon_vma could
* already be invalid.
*
* To avoid this possibility, swapcache pages get
* migrated but are not remapped when migration
* completes
*/
remap_swapcache = 0;
} else {
goto uncharge;
}
} ...
/* Establish migration ptes or remove ptes */
try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS); skip_unmap:
if (!page_mapped(page))
rc = move_to_new_page(newpage, page, remap_swapcache); if (rc && remap_swapcache)
remove_migration_ptes(page, page); /* Drop an anon_vma reference if we took one */
if (anon_vma)
drop_anon_vma(anon_vma); uncharge:
if (!charge)
mem_cgroup_end_migration(mem, page, newpage, rc == 0);
unlock:
unlock_page(page); move_newpage:
...
}

通过unmap_and_move(),老的page就被迁移过去新的page。

接下来要回收page,回收page的作用是,不至于因为拿了连续的内存后,系统变得内存饥饿:

->

/*
* Reclaim enough pages to make sure that contiguous allocation
* will not starve the system.
*/
__reclaim_pages(zone, GFP_HIGHUSER_MOVABLE, end-start);

->

/*
* Trigger memory pressure bump to reclaim some pages in order to be able to
* allocate 'count' pages in single page units. Does similar work as
*__alloc_pages_slowpath() function.
*/
static int __reclaim_pages(struct zone *zone, gfp_t gfp_mask, int count)
{
enum zone_type high_zoneidx = gfp_zone(gfp_mask);
struct zonelist *zonelist = node_zonelist(0, gfp_mask);
int did_some_progress = 0;
int order = 1;
unsigned long watermark; /*
* Increase level of watermarks to force kswapd do his job
* to stabilise at new watermark level.
*/
__update_cma_watermarks(zone, count); /* Obey watermarks as if the page was being allocated */
watermark = low_wmark_pages(zone) + count;
while (!zone_watermark_ok(zone, 0, watermark, 0, 0)) {
wake_all_kswapd(order, zonelist, high_zoneidx, zone_idx(zone)); did_some_progress = __perform_reclaim(gfp_mask, order, zonelist,
NULL);
if (!did_some_progress) {
/* Exhausted what can be done so it's blamo time */
out_of_memory(zonelist, gfp_mask, order, NULL);
}
} /* Restore original watermark levels. */
__update_cma_watermarks(zone, -count); return count;
}

3. 释放连续内存

内存释放的时候也比较简单,直接就是:

arch/arm/mm/dma-mapping.c

void dma_free_coherent(struct device *dev, size_t size, void *cpu_addr, dma_addr_t handle)

->

arch/arm/mm/dma-mapping.c:

static void __free_from_contiguous(struct device *dev, struct page *page,
size_t size)
{
__dma_remap(page, size, pgprot_kernel);
dma_release_from_contiguous(dev, page, size >> PAGE_SHIFT);
}

->

bool dma_release_from_contiguous(struct device *dev, struct page *pages,
int count)
{
...
free_contig_range(pfn, count);
.. }

->

void free_contig_range(unsigned long pfn, unsigned nr_pages)
{
for (; nr_pages--; ++pfn)
__free_page(pfn_to_page(pfn));
}

将page交还给buddy。

4. 内核内存分配的migratetype

内核内存分配的时候,带的标志是GFP_,但是GFP_可以转化为migratetype:

static inline int allocflags_to_migratetype(gfp_t gfp_flags)
{
WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK); if (unlikely(page_group_by_mobility_disabled))
return MIGRATE_UNMOVABLE; /* Group based on mobility */
return (((gfp_flags & __GFP_MOVABLE) != 0) << 1) |
((gfp_flags & __GFP_RECLAIMABLE) != 0);
}

之后申请内存的时候,会对比迁移类型匹配的free_list:

        page = get_page_from_freelist(gfp_mask|__GFP_HARDWALL, nodemask, order,
zonelist, high_zoneidx, ALLOC_WMARK_LOW|ALLOC_CPUSET,
preferred_zone, migratetype);

另外,笔者也编写了一个测试程序,透过它随时测试CMA的功能:

/*
* kernel module helper for testing CMA
*
* Licensed under GPLv2 or later.
*/ #include <linux/module.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/dma-mapping.h> #define CMA_NUM 10
static struct device *cma_dev;
static dma_addr_t dma_phys[CMA_NUM];
static void *dma_virt[CMA_NUM]; /* any read request will free coherent memory, eg.
* cat /dev/cma_test
*/
static ssize_t
cma_test_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
int i; for (i = 0; i < CMA_NUM; i++) {
if (dma_virt[i]) {
dma_free_coherent(cma_dev, (i + 1) * SZ_1M, dma_virt[i], dma_phys[i]);
_dev_info(cma_dev, "free virt: %p phys: %p\n", dma_virt[i], (void *)dma_phys[i]);
dma_virt[i] = NULL;
break;
}
}
return 0;
} /*
* any write request will alloc coherent memory, eg.
* echo 0 > /dev/cma_test
*/
static ssize_t
cma_test_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
{
int i;
int ret; for (i = 0; i < CMA_NUM; i++) {
if (!dma_virt[i]) {
dma_virt[i] = dma_alloc_coherent(cma_dev, (i + 1) * SZ_1M, &dma_phys[i], GFP_KERNEL); if (dma_virt[i]) {
void *p;
/* touch every page in the allocated memory */
for (p = dma_virt[i]; p < dma_virt[i] + (i + 1) * SZ_1M; p += PAGE_SIZE)
*(u32 *)p = 0; _dev_info(cma_dev, "alloc virt: %p phys: %p\n", dma_virt[i], (void *)dma_phys[i]);
} else {
dev_err(cma_dev, "no mem in CMA area\n");
ret = -ENOMEM;
}
break;
}
} return count;
} static const struct file_operations cma_test_fops = {
.owner = THIS_MODULE,
.read = cma_test_read,
.write = cma_test_write,
}; static struct miscdevice cma_test_misc = {
.name = "cma_test",
.fops = &cma_test_fops,
}; static int __init cma_test_init(void)
{
int ret = 0; ret = misc_register(&cma_test_misc);
if (unlikely(ret)) {
pr_err("failed to register cma test misc device!\n");
return ret;
}
cma_dev = cma_test_misc.this_device;
cma_dev->coherent_dma_mask = ~0;
_dev_info(cma_dev, "registered.\n"); return ret;
}
module_init(cma_test_init); static void __exit cma_test_exit(void)
{
misc_deregister(&cma_test_misc);
}
module_exit(cma_test_exit); MODULE_LICENSE("GPL");
MODULE_AUTHOR("Barry Song <21cnbao@gmail.com>");
MODULE_DESCRIPTION("kernel module to help the test of CMA");
MODULE_ALIAS("CMA test");

申请内存:

# echo 0 > /dev/cma_test

释放内存:

# cat /dev/cma_test
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