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