Linux内核最新的连续内存分配器(CMA)——避免预留大块内存 .

http://blog.csdn.net/21cnbao/article/details/7309757

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。

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

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/***dma_contiguous_reserve()-reserveareaforcontiguousmemoryhandling*@limit:Endaddressofthereservedmemory(optional,0forany).**Thisfunctionreservesmemoryfromearlyallocator.Itshouldbe*calledbyarchspecificcodeoncetheearlyallocator(memblockorbootmem)*hasbeenactivatedandallothersubsystemshavealreadyallocated/reserved*memory.*/void__initdma_contiguous_reserve(phys_addr_tlimit){unsignedlongselected_size=0;pr_debug("%s(limit%08lx)\n",__func__,(unsignedlong)limit);if(size_cmdline!=-1){selected_size=size_cmdline;}else{#ifdefCONFIG_CMA_SIZE_SEL_MBYTESselected_size=size_bytes;#elifdefined(CONFIG_CMA_SIZE_SEL_PERCENTAGE)selected_size=cma_early_percent_memory();#elifdefined(CONFIG_CMA_SIZE_SEL_MIN)selected_size=min(size_bytes,cma_early_percent_memory());#elifdefined(CONFIG_CMA_SIZE_SEL_MAX)selected_size=max(size_bytes,cma_early_percent_memory());#endif }if(selected_size){pr_debug("%s:reserving%ldMiBforglobalarea\n",__func__,selected_size/SZ_1M);dma_declare_contiguous(NULL,selected_size,0,limit);}};

/** * 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

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int__initdma_declare_contiguous(structdevice*dev,unsignedlongsize,phys_addr_tbase,phys_addr_tlimit){…/*Reservememory*/if(base){if(memblock_is_region_reserved(base,size)||memblock_reserve(base,size)<0){base=-EBUSY;gotoerr;}}else{/**Use__memblock_alloc_base()since*memblock_alloc_base()panic()s.*/phys_addr_taddr=__memblock_alloc_base(size,alignment,limit);if(!addr){base=-ENOMEM;gotoerr;}elseif(addr+size>~(unsignedlong)0){memblock_free(addr,size);base=-EINVAL;base=-EINVAL;gotoerr;}else{base=addr;}}/**Eachreservedareamustbeinitialisedlater,whenmorekernel*subsystems(likeslaballocator)areavailable.*/r->start=base;r->size=size;r->dev=dev;cma_reserved_count++;pr_info("CMA:reserved%ldMiBat%08lx\n",size/SZ_1M,(unsignedlong)base);/*Architecturespecificcontiguousmemoryfixup.*/dma_contiguous_early_fixup(base,size);return0;err:pr_err("CMA:failedtoreserve%ldMiB\n",size/SZ_1M);returnbase;}

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：

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static int __init cma_init_reserved_areas(void){ struct cma_reserved *r = cma_reserved; unsigned i = cma_reserved_count; pr_debug("%s()\n", __func__); for (; i; –i, ++r) { struct cma *cma; cma = cma_create_area(PFN_DOWN(r->start), r->size >> PAGE_SHIFT); if (!IS_ERR(cma)) dev_set_cma_area(r->dev, cma); } return 0;}core_initcall(cma_init_reserved_areas);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：

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#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]);

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

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structpage*dma_alloc_from_contiguous(structdevice*dev,intcount,unsignedintalign)

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

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structpage*dma_alloc_from_contiguous(structdevice*dev,intcount,unsignedintalign){…for(;;){pageno=bitmap_find_next_zero_area(cma->bitmap,cma->count,start,count,mask);if(pageno>=cma->count){ret=-ENOMEM;gotoerror;}pfn=cma->base_pfn+pageno;ret=alloc_contig_range(pfn,pfn+count,MIGRATE_CMA);if(ret==0){bitmap_set(cma->bitmap,pageno,count);break;}elseif(ret!=-EBUSY){gotoerror;}pr_debug("%s():memoryrangeat%pisbusy,retrying\n",__func__,pfn_to_page(pfn));/*tryagainwithabitdifferentmemorytarget*/start=pageno+mask+1;}…}

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：

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/**Whatwedohereiswemarkallpageblocksinrangeas*MIGRATE_ISOLATE.Becauseofthewaypageallocatorwork,we*aligntherangetoMAX_ORDERpagessothatpageallocator*won’ttrytomergebuddiesfromdifferentpageblocksand*changeMIGRATE_ISOLATEtosomeothermigrationtype.**OncethepageblocksaremarkedasMIGRATE_ISOLATE,we*migratethepagesfromanunalignedrange(ie.pagesthat*weareinterestedin).Thiswillputallthepagesin*rangebacktopageallocatorasMIGRATE_ISOLATE.**Whenthisisdone,wetakethepagesinrangefrompage*allocatorremovingthemfromthebuddysystem.Thisway*pageallocatorwillneverconsiderusingthem.**Thisletsusmarkthepageblocksbackas*MIGRATE_CMA/MIGRATE_MOVABLEsothatfreepagesinthe*MAX_ORDERalignedrangebutnotintheunaligned,original*rangeareputbacktopageallocatorsothatbuddycanuse*them.*/ret=start_isolate_page_range(pfn_align_to_maxpage_down(start),pfn_align_to_maxpage_up(end),migratetype);

/* * 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。

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/**start_isolate_page_range()–makepage-allocation-typeofrangeofpages*tobeMIGRATE_ISOLATE.*@start_pfn:ThelowerPFNoftherangetobeisolated.*@end_pfn:TheupperPFNoftherangetobeisolated.*@migratetype:migratetypetosetinerrorrecovery.**Makingpage-allocation-typetobeMIGRATE_ISOLATEmeansfreepagesin*therangewillneverbeallocated.Anyfreepagesandpagesfreedinthe*futurewillnotbeallocatedagain.**start_pfn/end_pfnmustbealignedtopageblock_order.*Returns0onsuccessand-EBUSYifanypartofrangecannotbeisolated.*/intstart_isolate_page_range(unsignedlongstart_pfn,unsignedlongend_pfn,unsignedmigratetype){unsignedlongpfn;unsignedlongundo_pfn;structpage*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;gotoundo;}}return0;undo:for(pfn=start_pfn;pfn<undo_pfn;pfn+=pageblock_nr_pages)unset_migratetype_isolate(pfn_to_page(pfn),migratetype);return-EBUSY;}

/* * 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:

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staticint__alloc_contig_migrate_range(unsignedlongstart,unsignedlongend){/*Thisfunctionisbasedoncompact_zone()fromcompaction.c.*/unsignedlongpfn=start;unsignedinttries=0;intret=0;structcompact_controlcc={.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;}elseif(++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);returnret>0?0:ret;}

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：

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intmigrate_pages(structlist_head*from,new_page_tget_new_page,unsignedlongprivate,boolofflining,boolsync){intretry=1;intnr_failed=0;intpass=0;structpage*page;structpage*page2;intswapwrite=current->flags&PF_SWAPWRITE;intrc;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:gotoout;case-EAGAIN:retry++;break;case0:break;default:/*Permanentfailure*/nr_failed++;break;}}}rc=0;…}

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

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/**Obtainthelockonpage,removeallptesandmigratethepage*tothenewlyallocatedpageinnewpage.*/staticintunmap_and_move(new_page_tget_new_page,unsignedlongprivate,structpage*page,intforce,boolofflining,boolsync){intrc=0;int*result=NULL;structpage*newpage=get_new_page(page,private,&result);intremap_swapcache=1;intcharge=0;structmem_cgroup*mem=NULL;structanon_vma*anon_vma=NULL;…/*chargeagainstnewpage*/charge=mem_cgroup_prepare_migration(page,newpage,&mem);…if(PageWriteback(page)){if(!force||!sync)gotouncharge;wait_on_page_writeback(page);}/**Bytry_to_unmap(),page->mapcountgoesdownto0here.Inthiscase,*wecannotnoticethatanon_vmaisfreedwhilewemigratesapage.*Thisget_anon_vma()delaysfreeinganon_vmapointeruntiltheend*ofmigration.Filecachepagesarenoproblembecauseofpage_lock()*FileCachesmayusewrite_page()orlock_page()inmigration,then,*justcareAnonpagehere.*/if(PageAnon(page)){/**Onlypage_lock_anon_vma()understandsthesubtletiesof*gettingaholdonananon_vmafromoutsideoneofitsmms.*/anon_vma=page_lock_anon_vma(page);if(anon_vma){/**Takeareferencecountontheanon_vmaifthe*pageismappedsothatitisguaranteedto*existwhenthepageisremappedlater*/get_anon_vma(anon_vma);page_unlock_anon_vma(anon_vma);}elseif(PageSwapCache(page)){/**Wecannotbesurethattheanon_vmaofanunmapped*swapcachepageissafetousebecausewedon’t*knowinadvanceiftheVMAthatthispagebelonged*tostillexists.IftheVMAandotherssharingthe*datahavebeenfreed,thentheanon_vmacould*alreadybeinvalid.**Toavoidthispossibility,swapcachepagesget*migratedbutarenotremappedwhenmigration*completes*/remap_swapcache=0;}else{gotouncharge;}}…/*Establishmigrationptesorremoveptes*/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);/*Dropananon_vmareferenceifwetookone*/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:…}

/* * 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：

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/**Reclaimenoughpagestomakesurethatcontiguousallocation*willnotstarvethesystem.*/__reclaim_pages(zone,GFP_HIGHUSER_MOVABLE,end-start);

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

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/**Triggermemorypressurebumptoreclaimsomepagesinordertobeableto*allocate’count’pagesinsinglepageunits.Doessimilarworkas*__alloc_pages_slowpath()function.*/staticint__reclaim_pages(structzone*zone,gfp_tgfp_mask,intcount){enumzone_typehigh_zoneidx=gfp_zone(gfp_mask);structzonelist*zonelist=node_zonelist(0,gfp_mask);intdid_some_progress=0;intorder=1;unsignedlongwatermark;/**Increaselevelofwatermarkstoforcekswapddohisjob*tostabiliseatnewwatermarklevel.*/__update_cma_watermarks(zone,count);/*Obeywatermarksasifthepagewasbeingallocated*/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){/*Exhaustedwhatcanbedonesoit’sblamotime*/out_of_memory(zonelist,gfp_mask,order,NULL);}}/*Restoreoriginalwatermarklevels.*/__update_cma_watermarks(zone,-count);returncount;}

/* * 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;}

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：

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voiddma_free_coherent(structdevice*dev,size_tsize,void*cpu_addr,dma_addr_thandle)

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

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

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staticvoid__free_from_contiguous(structdevice*dev,structpage*page,size_tsize){__dma_remap(page,size,pgprot_kernel);dma_release_from_contiguous(dev,page,size>>PAGE_SHIFT);}

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);}->

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booldma_release_from_contiguous(structdevice*dev,structpage*pages,intcount){…free_contig_range(pfn,count);..}

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

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voidfree_contig_range(unsignedlongpfn,unsignednr_pages){for(;nr_pages–;++pfn)__free_page(pfn_to_page(pfn));}

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。

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：

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staticinlineintallocflags_to_migratetype(gfp_tgfp_flags){WARN_ON((gfp_flags&GFP_MOVABLE_MASK)==GFP_MOVABLE_MASK);if(unlikely(page_group_by_mobility_disabled))returnMIGRATE_UNMOVABLE;/*Groupbasedonmobility*/return(((gfp_flags&__GFP_MOVABLE)!=0)<<1)|((gfp_flags&__GFP_RECLAIMABLE)!=0);}

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：

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

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：

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/**kernelmodulehelperfortestingCMA**LicensedunderGPLv2orlater.*/#include<linux/module.h>#include<linux/device.h> #include<linux/fs.h> #include<linux/miscdevice.h> #include<linux/dma-mapping.h>#defineCMA_NUM10 staticstructdevice*cma_dev;staticdma_addr_tdma_phys[CMA_NUM];staticvoid*dma_virt[CMA_NUM];/*anyreadrequestwillfreecoherentmemory,eg.*cat/dev/cma_test*/staticssize_tcma_test_read(structfile*file,char__user*buf,size_tcount,loff_t*ppos){inti;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,"freevirt:%pphys:%p\n",dma_virt[i],(void*)dma_phys[i]);dma_virt[i]=NULL;break;}}return0;}/**anywriterequestwillalloccoherentmemory,eg.*echo0>/dev/cma_test*/staticssize_tcma_test_write(structfile*file,constchar__user*buf,size_tcount,loff_t*ppos){inti;intret;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;/*toucheverypageintheallocatedmemory*/for(p=dma_virt[i];p<dma_virt[i]+(i+1)*SZ_1M;p+=PAGE_SIZE)*(u32*)p=0;_dev_info(cma_dev,"allocvirt:%pphys:%p\n",dma_virt[i],(void*)dma_phys[i]);}else{dev_err(cma_dev,"nomeminCMAarea\n");ret=-ENOMEM;}break;}}returncount;}staticconststructfile_operationscma_test_fops={.owner=THIS_MODULE,.read=cma_test_read,.write=cma_test_write,};staticstructmiscdevicecma_test_misc={.name="cma_test",.fops=&cma_test_fops,};staticint__initcma_test_init(void){intret=0;ret=misc_register(&cma_test_misc);if(unlikely(ret)){pr_err("failedtoregistercmatestmiscdevice!\n");returnret;}cma_dev=cma_test_misc.this_device;cma_dev->coherent_dma_mask=~0;_dev_info(cma_dev,"registered.\n");returnret;}module_init(cma_test_init);staticvoid__exitcma_test_exit(void){misc_deregister(&cma_test_misc);}module_exit(cma_test_exit);MODULE_LICENSE("GPL");MODULE_AUTHOR("BarrySong<21cnbao@gmail.com>");MODULE_DESCRIPTION("kernelmoduletohelpthetestofCMA");MODULE_ALIAS("CMAtest");

/* * 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 10static 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_tcma_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_tcma_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：

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#echo0>/dev/cma_test

# echo 0 > /dev/cma_test释放内存：

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#cat/dev/cma_test

# cat /dev/cma_test参考链接：

[1] http://www.spinics.net/lists/arm-kernel/msg160854.html

[2] http://www.spinics.net/lists/arm-kernel/msg162063.html

[3] http://lwn.net/Articles/447405/

接受我们不能改变的一切，改变我们能改变的一切。

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