翻译文献-linux 采用普通的内存分页模式同时适合32位和64位结构

外文原文25PAGINGINLINUXLINUXADOPTSACOMMONPAGINGMODELTHATFITSBOTH32BITAND64BITARCHITECTURESASEXPLAINEDINTHEEARLIERSECTION“PAGINGFOR64BITARCHITECTURES“TWOPAGINGLEVELSARESUFFICIENTFOR32BITARCHITECTURES,WHILE64BITARCHITECTURESREQUIREAHIGHERNUMBEROFPAGINGLEVELSUPTOVERSION2610,THELINUXPAGINGMODELCONSISTEDOFTHREEPAGINGLEVELSSTARTINGWITHVERSION2611,AFOURLEVELPAGINGMODELHASBEENADOPTEDTHEFOURTYPESOFPAGETABLESILLUSTRATEDINFIGURE212ARECALLEDTHISCHANGEHASBEENMADETOFULLYSUPPORTTHELINEARADDRESSBITSPLITTINGUSEDBYTHEX86_64PLATFORMSEETABLE24PAGEGLOBALDIRECTORYPAGEUPPERDIRECTORYPAGEMIDDLEDIRECTORYPAGETABLETHEPAGEGLOBALDIRECTORYINCLUDESTHEADDRESSESOFSEVERALPAGEUPPERDIRECTORIES,WHICHINTURNINCLUDETHEADDRESSESOFSEVERALPAGEMIDDLEDIRECTORIES,WHICHINTURNINCLUDETHEADDRESSESOFSEVERALPAGETABLESEACHPAGETABLEENTRYPOINTSTOAPAGEFRAMETHUSTHELINEARADDRESSCANBESPLITINTOUPTOFIVEPARTSFIGURE212DOESNOTSHOWTHEBITNUMBERS,BECAUSETHESIZEOFEACHPARTDEPENDSONTHECOMPUTERARCHITECTUREFOR32BITARCHITECTURESWITHNOPHYSICALADDRESSEXTENSION,TWOPAGINGLEVELSARESUFFICIENTLINUXESSENTIALLYELIMINATESTHEPAGEUPPERDIRECTORYANDTHEPAGEMIDDLEDIRECTORYFIELDSBYSAYINGTHATTHEYCONTAINZEROBITSHOWEVER,THEPOSITIONSOFTHEPAGEUPPERDIRECTORYANDTHEPAGEMIDDLEDIRECTORYINTHESEQUENCEOFPOINTERSAREKEPTSOTHATSOTHATTHESAMECODECANWORKON32BITAND64BITARCHITECTURESTHEKERNELKEEPSAPOSITIONFORTHEPAGEUPPERDIRECTORYANDTHEPAGEMIDDLEDIRECTORYBYSETTINGTHENUMBEROFENTRIESINTHEMTO1ANDMAPPINGTHESETWOENTRIESINTOTHEPROPERENTRYOFTHEPAGEGLOBALDIRECTORYFOR32BITARCHITECTURESWITHTHEPHYSICALADDRESSEXTENSIONENABLED,THREEPAGINGLEVELSAREUSEDTHELINUXSPAGEGLOBALDIRECTORYCORRESPONDSTOTHE8086SPAGEDIRECTORYPOINTERTABLE,THEPAGEUPPERDIRECTORYISELIMINATED,THEPAGEMIDDLEDIRECTORYCORRESPONDSTOTHE8086SPAGEDIRECTORY,ANDTHELINUXSPAGETABLECORRESPONDSTOTHE8086SPAGETABLEFINALLY,FOR64BITARCHITECTURESTHREEORFOURLEVELSOFPAGINGAREUSEDDEPENDINGONTHELINEARADDRESSBITSPLITTINGPERFORMEDBYTHEHARDWARESEETABLE22LINUXSHANDLINGOFPROCESSESRELIESHEAVILYONPAGINGINFACT,THEAUTOMATICTRANSLATIONOFLINEARADDRESSESINTOPHYSICALONESMAKESTHEFOLLOWINGDESIGNOBJECTIVESFEASIBLEASSIGNADIFFERENTPHYSICALADDRESSSPACETOEACHPROCESS,ENSURINGANEFFICIENTPROTECTIONAGAINSTADDRESSINGERRORSDISTINGUISHPAGESGROUPSOFDATAFROMPAGEFRAMESPHYSICALADDRESSESINMAINMEMORYTHISALLOWSTHESAMEPAGETOBESTOREDINAPAGEFRAME,THENSAVEDTODISKANDLATERRELOADEDINADIFFERENTPAGEFRAMETHISISTHEBASICINGREDIENTOFTHEVIRTUALMEMORYMECHANISMSEECHAPTER17INTHEREMAININGPARTOFTHISCHAPTER,WEWILLREFERFORTHESAKEOFCONCRETENESSTOTHEPAGINGCIRCULTRYUSEDBYTHE8086PROCESSORSASWEWILLSEEINCHAPTER9,EACHPROCESSHASITSOWNPAGEGLOBALDIRECTORYANDITSOWNSETOFPAGETABLESWHENAPROCESSSWITCHOCCURSSEETHESECTION“PROCESSSWITCH“INCHAPTER3,LINUXSAVESTHECR3CONTROLREGISTERINTHEDESCRIPTOROFTHEPROCESSPREVIOUSLYINEXECUTIONANDTHENLOADSCR3WITHTHEVALUESTOREDINTHEDESCRIPTOROFTHEPROCESSTOBEEXECUTEDNEXTTHUS,WHENTHENEWPROCESSRESUMESITSEXECUTIONONTHECPU,THEPAGINGUNITREFERSTOTHECORRECTSETOFPAGETABLESMAPPINGLINEARTOPHYSICALADDRESSESNOWBECOMESAMECHANICALTASK,ALTHOUGHITISSTILLSOMEWHATCOMPLEXTHENEXTFEWSECTIONSOFTHISCHAPTERAREARATHERTEDIOUSLISTOFFUNCTIONSANDMACROSTHATRETRIEVEINFORMATIONTHEKERNELNEEDSTOFINDADDRESSESANDMANANGETHETABLESMOSTOFTHEFUNCTIONSAREONEORTWOLINESLONGYOUMAYWANTTOONLYSKIMTHESESECTIONSNOW,BUTITISUSEFULTOKNOWTHEROLEOFTHESEFUNCTIONSANDMACROS,BECAUSEYOULLSEETHEMOFTENINDISCUSSIONSTHROUGHOUTTHISBOOK251THELINEARADDRESSFIELDSTHEFOLLOWINGMACROSSIMPLIFYPAGETABLEHANDLINGPAGE_SHIFTSPECIFIESTHELENGTHINBITSOFTHEOFFSETFIELDWHENAPPLIEDTO8086PROCESSORS,ITYIELDSTHEVALUE12BECAUSEALLTHEADDRESSESINAPAGEMUSTFITINTHEOFFSETFIELD,THESIZEOFAPAGEON8086SYSTEMSIS212ORTHEFAMILIAR4,096BYTESTHEPAGE_SHIFTOF12PAGE_SIZETORETURNTHESIZEOFTHEPAGEFINALLY,THEPAGE_MASKMACROYIELDSTHEVALUE0XFFFFF000ANDISUSEDTOMASKALLTHEBITSOFTHEOFFSETFIELDPMD_SHIFTTHETOTALLENGTHINBITSOFTHEOFFSETANDTABLEFIELDSOFALINEARADDRESSINOTHERWORDS,THELOGARITHMOFTHESIZEOFTHEAREAAPAGEMIDDLEDIRECTORYENTRYCANMAPTHEPMD_SIZEMACROCOMPUTESTHESIZEOFTHEAREAMAPPEDBYASINGLEENTRYOFTHEPAGEMIDDLEDIRECTORYTHATIS,OFAPAGETABLETHEPMD_MASKMACROISUSEDTOMASKALLTHEBITSOFTHEOFFSETANDTABLEFIELDSWHEPAEISDISABLED,PMD_SHIFTYIELDSTHEVALUE2212FROMOFFSETPLUS10FROMTABLE,PMD_SIZEYIELDS222OR4MB,ANDPMD_MASKYIELDS0XFFC00000CONVERSELY,WHENPAEISENABLED,PMD_SHIFTYIELDSTHEVALUE2112FROMOFFSETPLUS9FROMTABLE,PMD_SIZEYIELDS221OR2MB,ANDPMD_MASKYIELDS0XFFE00000LARGEPAGESDOMOTMAKEUSEOFTHELASTLEVELOFPAGETABLES,THUSLARGE_PAGE_SIZE,WHICHYIELDSTHESIZEOFALARGEPAGE,ISEQUALTOPMD_SIZE2PMD_SHIFTWHILELARGE_PAGE_MASK,WHICHISUSEDTOMASKALLTHEBITSOFTHEOFFSETANDTABLEFIELDSINALARGEPAGEADDRESS,ISEQUALTOPMD_MASKPUD_SHIFTDETERMINESTHELOGARITHMOFTHESIZEOFTHEAREAAPAGEUPPERDIRECTORYENTRYCANMAPTHEPUD_SIZEMACROCOMPUTESTHESIZEOFTHEAREAMAPPEDBYASINGLEENTRYOFTHEPAGEGLOBALDIRECTORYTHEPUD_MASKMACROISUSEDTOMASKALLTHEBITSOFTHEOFFSET,TABLE,MIDDLEAIR,ANDUPPERAIRFIELDSONTHE8086PROCESSORS,PUD_SHIFTISALWAYSEQUALTOPMD_SHIFTANDPUD_SIZEISEQUALTO4MBOR2MBPGDIR_SHIFTDETERMINESTHELOGARITHMOFTHESIZEOFTHEAREATHATAPAGEGLOBALDIRECTORYENTRYCANMAPTHEPGDIR_SIZEMACROCOMPUTESTHESIZEOFTHEAREAMAPPEDBYASINGLEENTRYOFTHEPAGEGLOBALDIRECTORYTHEPGDIR_MASKMACROISUSEDTOMASKALLTHEBITSOFTHEOFFSET,TABLE,MIDDLEAIR,ANDUPPERAIRFIELDSWHENPAEISDISABLED,PGDIR_SHIFTYIELDSTHEVALUE22THESAMEVALUEYIELDEDBYPMD_SHIFTANDBYPUD_SHIFT,PGDIR_SIZEYIELDS222OR4MB,ANDPGDIR_MASKYIELDS0XFFC00000CONVERSELY,WHENPAEISENABLED,PGDIR_SHIFTYIELDSTHEVALUE3012FROMOFFSETPLUS9FROMTABLEPLUS9FROMMIDDLEAIR,PGDIR_SIZEYIELDS230OR1GB,ANDPGDIR_MASKYIELDS0XC0000000PTRS_PER_PTE,PTRS_PER_PMD,PTRS_PER_PUD,ANDPTRS_PER_PGDCOMPUTETHENUMBEROFENTRIESINTHEPAGETABLE,PAGEMIDDLEDIRECTORY,PAGEUPPERDIRECTORY,ANDPAGEGLOBALDIRECTORYTHEYYIELDTHEVALUES1,024,1,1,AND1,024,RESPECTIVELY,WHENPAEISDISABLEDANDTHEVALUES512,512,1,AND4,RESPECTIVELY,WHENPAEISENABLED252PAGETABLEHANDINGPTE_T,PMD_T,PUD_T,ANDPGD_TDESCRIBETHEFORMATOF,RESPECTIVELY,APAGETABLE,APAGEMIDDLEDIRECTORY,APAGEUPPERDIRECTORY,ANDAPAGEGLOBALDIRECTORYENTRY,THEYARE64BITDATATYPESWHENPAEISENABLEDAND32BITDATATYPESOTHERWISE,PGPROT_TISANOTHER64BITPAEENABLEDOR32BITPAEENABLEDOR32BITPAEDISABLEDDATATYPETHATREPRESENTSTHEPROTECTIONFLAGSASSOCIATEDWITHASINGLEENTRYFIVETYPECONVERSIONMACROS_PTE,_PMD,_PGD,AND_PGPROTCASTANUNSIGNEDINTEGERINTOTHEREQUIREDTYPEFIVEOTHERTYPECONVERSIONMACROSPTE_VAL,PMD_VAL,PUD_VAL,PGD_VAL,ANDPGPROT_VALPERFORMTHEREVERSECASTINGFROMONEOFTHEFOURPREVIOUSLYMENTIONEDSPECIALIZEDTYPESINTOANUNSIGNEDINTEGERTHEKERNELALSOPROVIDESSEVERALMACROSANDFUNCTIONSTOREADORMODIFYPAGETABLEENTRIESPTE_NONE,PMD_NONE,PUD_NONE,ANDPGD_NONEYIELDTHEVALUE1IFTHECORRESPONDINGENTRYHASTHEVALUE0OTHERWISE,THEYYIELDTHEVALUE0PTE_CLEAR,PMD_CLEAR,PUD_CLEAR,ANDPGD_CLEARCLEARANENTRYOFTHECORRESPONDINGPAGETABLE,THUSFORBIDDINGAPROCESSTOUSETHELINEARADDRESSESMAPPEDBYTHEPAGETABLEENTRYTHEPTEP_GET_AND_CLEARFUNCTIONCLEARSAPAGETABLEENTRYANDRETURNSTHEPREVIOUSVALUESET_PTE,SET_PMD,SET_PUD,ANDSET_PGDWRITEAGIVENVALUEINTOAPAGETABLEENTRYSET_PTE_ATOMICISIDENTICALTOSET_PTE,BUTWHENPAEISENABLEDITALSOENSURESTHATTHE64BITVALUEISWRITTENATOMICALLYPTE_SAMEA,BRETURNS1IFTWOPAGETABLEENTRIESAANDBREFERTOTHESAMEPAGEANDSPECIFYTHESAMEACCESSPRIVILEGES,0OTHERWISEPME_LARGEERETURNS1IFTHEPAGEMIDDLEDIRECTORYENTRYREFERSTOALARGEPAGE2MBOR4MB,0OTHERWISETHEPMD_BADMACROISUSEDBYFUNCTIONSTOCHECKPAGEMIDDLEDIRECTORYENTRIESPASSEDASINPUTPARAMETERSITYIELDSTHEVALUE1IFTHEENTRYPOINTSTOABADPAGETABLETHATIS,IFATLEASTONEOFTHEFOLLOWINGCONDITIONSAPPLIESTHEPAGEISNOTINMAINMEMORYPRESENTFLAGCLEAREDTHEPAGEALLOWSONLYREADACCESSREAD/WRITEFLAGCLEAREDEITHERACCESSEDORDIRTYISCLEAREDLINUXALWAYSFORCESTHESEFLAGSTOBESETFOREVERYEXISTINGPAGETABLETHEPUD_BADANDPGD_BADMACROSALWAYSYIELD0NOPTE_BADMACROISDEFINED,BECAUSEITISLEGALFORAPAGETABLEENTRYTOREFERTOAPAGETHATISNOTPRESENTINMAINMEMORY,NOTWRITABLE,ORNOTACCESSIBLEATALLTHEPTE_PRESENTMACROYIELDSTHEVALUE1IFEITHERTHEPRESENTFLAGORTHEPAGESIZEFLAGOFAPAGETABLEENTRYISEQUALTO1,THEVALUE0OTHERWISERECALLTHATTHEPAGESIZEFLAGINPAGETABLEENTRIESHASNOMEANINGFORTHEPAGINGUNITOFTHEMICROPROCESSORTHEKERNEL,HOWEVER,MARKSPRESENTEQUALTO0ANDPAGESIZEEQUALTO1FORTHEPAGESPRESENTINMAINMEMORYBUTWITHOUTREAD,WRITE,OREXECUTEPRIVILEGESINTHISWAY,ANYACCESSTOSUCHPAGESTRIGGERSAPAGEFAULTEXCEPTIONBECAUSEPRESENTISCLEARED,ANDTHEKERNELCANDETECTTHATTHEFAULTISNOTDUETOAMISSINGPAGEBYCHECKINGTHEVALUEOFPAGESIZETHEPMD_PRESENTMACROYIELDSTHEVALUE1IFTHEPRESENTFLAGOFTHECORRESPONDINGENTRYISEQUALTO1THATIS,IFTHECORRESPONDINGPAGEORPAGETABLEISLOADEDINMAINMEMORYTHEPUD_PRESENTANDPGD_PRESENTMACROSALWAYSYIELDTHEVALUE1THEFUNCTIONSLISTEDINTABLE25QUERYTHECURRENTVALUEOFANYOFTHEFLAGSINCLUDEDINAPAGETABLEENTRYWITHTHEEXCEPTIONOFPTE_FILE,THESEFUNCTIONSWORKPROPERLYONLYONPAGETABLEENTRIESFORWHICHPTE_PRESENTRETURNS1TABLE25PAGEFLAGREADINGFUNCTIONSFUNCTIONNAMEDESCRIPTIONPTE_USERREADSTHEUSER/SUPERVISORFLAGPTE_READREADSTHEUSER/SUPERVISORFLAGPAGESONTHE8086PROCESSORCANNOTBEPROTECTEDAGAINSTREADINGPTE_WRITEREADSTHEREAD/WRITEFLAGPTE_EXECREADSTHEUSER/SUPERVISORFLAGPAGESONTHE8086PROCESSORCANNOTBEPROTECTEDAGAINSTCODEEXECUTIONPTE_DIRTYREADSTHEDIRTYFLAGPTE_YOUNGREADSTHEACCESSEDFLAGPTE_FILEREADSTHEDIRTYFLAGWHENTHEPRESENTFLAGISCLEAREDANDTHEDIRTYFLAGISSET,THEPAGEBELONGSTOANONLINEARDISKFILEMAPPINGSEECHAPTER16ANOTHERGROUPOFFUNCTIONSLISTEDINTABLE26SETSTHEVALUEOFTHEFLAGSINAPAGETABLEENTRYTABLE26,PAGEFLAGSETTINGFUNCTIONSFUNCTIONNAMEDESCRIPTIONMK_PTE_HUGESETSTHEPAGESIZEANDPRESENTFLAGSOFAPAGETABLEENTRYPTE_WRPROTECTCLEARSTHEREAD/WRITEFLAGPTE_RDPROTECTCLEARSTHEUSER/SUPERVISORFLAGPTE_EXPROTECTCLEARSTHEUSER/SUPERVISORFLAGPTE_MKWRITESETSTHEREAD/WRITEFLAGPTE_MKREADSETSTHEUSER/SUPERVISORFLAGPTE_MKEXECSETSTHEUSER/SUPERVISORFLAGPTE_MKCLEANCLEARSTHEDIRTYFLAGPTE_MKDIRTYSETSTHEDIRTYFLAGPTE_MKOLDCLEARSTHEACCESSEDFLAGMAKESTHEPAGEOLDPTE_MKYOUNGSETSTHEACCESSEDFLAGMAKESTHEPAGEYOUNGPTE_MODIFYP,VSETSALLACCESSRIGHTSINAPAGETABLEENTRYPTOASPECIFIEDVALUEVPTEP_SET_WRPROTECTLIKEPTE_WRPROTECT,BUTACTSONAPOINTERTOAPAGETABLEENTRYPTEP_SET_ACCESS_FLAGSIFTHEDIRTYFLAGISSET,SETSTHEPAGESACCESSRIGHTSTOASPECIFIEDVALUEANDINVOKESFLUSH_TLB_PAGESEETHESECTION“TRANSLATIONLOOKASIDEBUFFERSTLB“LATERINTHISCHAPTERPTEP_MKDIRTYLIKEPTE_MKDIRTYBUTACTSONAPOINTERTOAPAGETABLEENTRYPTEP_TEST_AND_CLEAR_DIRTYLIKEPTE_MKCLEANBUTACTSONAPOINTERTOAPAGETABLEENTRYANDRETURNSTHEOLDVALUEOFTHEFLAGPTEP_TEST_AND_CLEAR_YOUNGLIKEPTE_MKOLDBUTACTSONAPOINTERTOAPAGETABLEENTRYANDRETURNSTHEOLDVALUEOFTHEFLAGNOW,LETSDISCUSSTHEMACROSLISTEDINTABLE27THATCOMBINEAPAGEADDRESSANDAGROUPOFPROTECTIONFLAGSINTOAPAGETABLEENTRYORPERFORMTHEREVERSEOPERATIONOFEXTRACTINGTHEPAGEADDRESSFROMAPAGETABLEENTRYNOTICETHATSOMEOFTHESEMACROSREFERTOAPAGETHROUGHTHELINEARADDRESSOFITS“PAGEDESCRIPTOR“SEETHESECTION“PAGEDESCRIPTORS“INCHAPTER8RATHERTHANTHELINEARADDRESSOFTHEPAGEITSELFTABLE27MACROSACTINGONPAGETABLEENTRIESMACRONAMEDESCRIPTIONPGD_INDEXADDRYIELDSTHEINDEXRELATIVEPOSITIONOFTHEENTRYINTHEPAGEGLOBALDIRECTORYTHATMAPSTHELINEARADDRESSADDRPGD_OFFSETMM,ADDRRECEIVESASPARAMETERSTHEADDRESSOFAMEMORYDESCRIPTORCWSEECHAPTER9ANDALINEARADDRESSADDRTHEMACROYIELDSTHELINEARADDRESSOFTHEENTRYINAPAGEGLOBALDIRECTORYTHATCORRESPONDSTOTHEADDRESSADDRTHEPAGEGLOBALDIRECTORYISFOUNDTHROUGHAPOINTERWITHINTHEMEMORYDESCRIPTORPGD_OFFSET_KADDRYIELDSTHELINEARADDRESSOFTHEENTRYINTHEMASTERKERNELPAGEGLOBALDIRECTORYTHATCORRESPONDSTOTHEADDRESSADDRSEETHELATERSECTION“KERNELPAGETABLES“PGD_PAGEPGDYIELDSTHEPAGEDESCRIPTORADDRESSOFTHEPAGEFRAMECONTAININGTHEPAGEUPPERDIRECTORYREFERREDTOBYTHEPAGEGLOBALDIRECTORYENTRYPGDINATWOORTHREELEVELPAGINGSYSTEM,THISMACROISEQUIVALENTTOPUD_PAGEAPPLIEDTOTHEFOLDEDPAGEUPPERDIRECTORYENTRYPUD_OFFSETPGD,ADDRRECEIVESASPARAMETERSAPOINTERPGDTOAPAGEGLOBALDIRECTORYENTRYANDALINEARADDRESSADDRTHEMACROYIELDSTHELINEARADDRESSOFTHEENTRYINAPAGEUPPERDIRECTORYTHATCORRESPONDSTOADDRINATWOORTHREELEVELPAGINGSYSTEM,THISMACROYIELDSPGD,THEADDRESSOFAPAGEGLOBALDIRECTORYENTRYPUD_PAGEPUDYIELDSTHELINEARADDRESSOFTHEPAGEMIDDLEDIRECTORYREFERREDTOBYTHEPAGEUPPERDIRECTORYENTRYPUDINATWOLEVELPAGINGSYSTEM,THISMACROISEQUIVALENTTOPMD_PAGEAPPLIEDTOTHEFOLDEDPAGEMIDDLEDIRECTORYENTRYPMD_INDEXADDRYIELDSTHEINDEXRELATIVEPOSITIONOFTHEENTRYINTHEPAGEMIDDLEDIRECTORYTHATMAPSTHELINEARADDRESSADDRPMD_OFFSETPUD,ADDRRECEIVESASPARAMETERSAPOINTERPUDTOAPAGEUPPERDIRECTORYENTRYANDALINEARADDRESSADDRTHEMACROYIELDSTHEADDRESSOFTHEENTRYINAPAGEMIDDLEDIRECTORYTHATCORRESPONDSTOADDRINATWOLEVELPAGINGSYSTEM,ITYIELDSPUD,THEADDRESSOFAPAGEGLOBALDIRECTORYENTRYPMD_PAGEPMDYIELDSTHEPAGEDESCRIPTORADDRESSOFTHEPAGETABLEREFERREDTOBYTHEPAGEMIDDLEDIRECTORYENTRYPMDINATWOLEVELPAGINGSYSTEM,PMDISACTRALLYANENTRYOFAPAGEGLOBALDIRECTORYMK_PTEP,PROTRECEIVESASPARAMETERSTHEADDRESSOFAPAGEDESCRIPTORPANDAGROUPOFACCESSRIGHTSPROT,ANDBUILDSTHECORRESPONDINGPAGETABLEENTRYPTE_INDEXADDRYIELDSTHEINDEXRELATIVEPOSITIONOFTHEENTRYINTHEPAGETABLETHATMAPSTHELINEARADDRESSADDRPTE_OFFSET_KERNELDIR,ADDRYIELDSTHELINEARADDRESSOFTHEPAGETABLETHATCORRESPONDSTOTHELINEARADDRESSADDRMAPPEDBYTHEPAGEMIDDLEDIRECTORYDIRUSEDONLYONTHEMASTERKERNELPAGETABLESSEETHELATERSECTION“KERNELPAGETABLES“PTE_OFFSET_MAPDIF,ADDRRECEIVESASPARAMETERSAPOINTERDIRTOAPAGEMIDDLEDIRECTORYENTRYANDALINEARADDRESSADDRITYIELDSTHELINEARADDRESSOFTHEENTRYINTHEPAGETABLETHATCORRESPONDSTOTHELINEARADDRESSADDRIFTHEPAGETABLEISKEPTINHIGHMEMORYTHEKERNELESTABLISHESATEMPORARYKERNELMAPPINGSEETHESECTION“KERNELMAPPINGSOFHIGHMEMORYPAGEFRAMES“INCHAPTER8,TOBERELEASEDBYMEANSOFPTE_UNMAPTHEMACROSPTE_OFFSET_MAP_NESTEDANDPTE_UNMAP_NESTEDAREIDENTICAL,BUTTHEYUSEADIFFERENTTEMPORARYKERNELMAPPINGPTE_PAGEXRETURNSTHEPAGEDESCRIPTORADDRESSOFTHEPAGEREFERECEDBYTHEPAGETABLEENTRYXPTE_TO_PGOFFPTEEXTRACTSFROMTHECONTENTPTEOFAPAGETABLEENTRYTHEFILEOFFSETCORRESPONDINGTOAPAGEBELONGINGTOANONLINEARFILEMEMORYMAPPINGSEETHESECTION“NONLINEARMEMORYMAPPINGS“INCHAPTER16PGOFF_TO_PTEOFFSETSETSUPTHECONTENTOFAPAGETABLEENTRYFORAPAGEBELONGINGTOANONLINEARFILEMEMORYMAPPINGTHELASTGROUPOFFUNCTIONSOFTHISLONGLISTWASINTRODUCEDTOSIMPLIFYTHECREATIONANDDELETIONOFPAGETABLEENTRIESWHENTWOLEVELPAGINGISUSED,CREATINGORDELETINGAPAGEMIDDLEDIRECTORYENTRYISTRIVIALASWEEXPLAINEDEARLIERINTHISSECTION,THEPAGEMIDDLEDIRECTORYCONTAINSASINGLEENTRYTHATPOINTSTOTHESUBORDINATEPAGETABLE,THUS,THEPAGEMIDDLEDIRECTORYENTRYISTHEENTRYWITHINTHEPAGEGLOBALDIRECTORY,TOOWHENDEALINGWITHPAGETABLES,HOWEVER,CREATINGANENTRYMAYBEMORECOMPLEX,BECAUSETHEPAGETABLETHATISSUPPOSEDTOCONTAINITMIGHTNOTEXISTINSUCHCASES,ITISNECESSARYTOALLOCATEANEWPAGEFRAME,FILLITWITHZEROS,ANDADDTHEENTRYIFPAEISENABLED,THEKERNELUSESTREELEVELPAGINGWHENTHEKERNELCREATESANEWPAGEGLOBALDIRECTORY,ITALSOALLOCATESTHEFOURCORRESPONDINGPAGEMIDDLEDIRECTORIESTHESEAREFREEDONLYWHENTHEPARENTPAGEGLOBALDIRECTORYISRELEASEDWHENTWOORTHREELEVELPAGINGISUSED,THEPAGEUPPERDIRECTORYENTRYISALWAYSMAPPEDASASINGLEENTRYWITHINTHEPAGEGLOBALDIRECTORYASUSUAL,THEDESCRIPTIONOFTHEFUNCTIONSLISTEDINTABLE28REFERSTOTHE8086ARCHITECTURETABLE28PAGEALLOCATIONFUNCTIONSFUNCTIONNAMEDESCRIPTIONPGD_ALLOCMMALLOCATESANEWPAGEGLOBALDIRECTORYIFPAEISENABLED,ITALSOALLOCATESTHETHREECHILDRENPAGEMIDDLEDIRECTORIESTHATMAPTHEUSERMODELINEARADDRESSESTHEARGUMENTMMTHEADDRESSOFAMEMORYDESCRIPTORISIGNOREDONTHE8086ARCHITECTUREPGD_FREEPGDRELEASESTHEPAGEGLOBALDIRECTORYATADDRESSPGDIFPAEISENABLED,ITALSORELEASESTHETHREEPAGEMIDDLEDIRECTORIESTHATMAPTHEUSERMODELINEARADDRESSESPUD_ALLOCMM,PGD,ADDRINATWOORTHREELEVELPAGINGSYSTEM,THISFUNCTIONDOESNOTHINGITSIMPLYRETURNSTHELINEARADDRESSOFTHEPAGEGLOBALDIRECTORYENTRYPGDPUD_FREEXINATWOORTHREELEVELPAGINGSYSTEM,THISMACRRODOESNOTHINGPMD_ALLOCMM,PUD,ADDRDEFINEDSOGENERICTHREELEVELPAGINGSYSTEMSCANALLOCATEANEWPAGEMIDDLEDIRECTORYFORTHELINEARADDRESSADDRIFPAEISNOTENABLED,THEFUNCTIONSIMPLYRETURNSTHEINPUTPARAMETERPUDTHATIS,THEADDRESSOFTHEENTRYINTHEPAGEGLOBALDIRECTORYIFPAEISENABLED,THEFUNCTIONRETURNSTHELINEARADDRESSOFTHEPAGEMIDDLEDIRECTORYENTRYTHATMAPSTHELINEARADDRESSADDRTHEARGUMENTCWISIGNOREDPMD_FREEXDOESNOTHING,BECAUSEPAGEMIDDLEDIRECTORIESAREALLOCATEDANDDEALLOCATEDTOGETHERWITHTHEIRPARENTPAGEGLOBALDIRECTORYPTE_ALLOC_MAPMM,PMD,ADDRRECEIVESASPARAMETERSTHEADDRESSOFAPAGEMIDDLEDIRECTORYENTRYPMDANDALINEARADDRESSADDR,ANDRETURNSTHEADDRESSOFTHEPAGETABLEENTRYCORRESPONDINGTOADDRIFTHEPAGEMIDDLEDIRECTORYENTRYISNULL,THEFUNCTIONALLOCATESANEWPAGETABLEBYINVOKINGPTE_ALLOC_ONEIFANEWPAGETABLEISALLOCATED,THEENTRYCORRESPONDINGTOADDRISINITIALIZEDANDTHEUSER/SUPERVISORFLAGISSETIFTHEPAGETABLEISKEPTINHIGHMEMORY,THEKERNELESTABLISHESATEMPORARYKERNELMAPPINGSEETHESECTION“KERNELMAPPINGSOFHIGHMEMORYPAGEFRAMES“INCHAPTER8,TOBERELEASEDBYPTE_UNMAPPTE_ALLOC_KERNELMM,PMD,ADDRIFTHEPAGEMIDDLEDIRECTORYENTRYPMDASSOCIATEDWITHTHEADDRESSADDRISNULL,THEFUNCTIONALLOCATESANEWPAGETABLEITTHENRETURNSTHELINEARADDRESSOFTHEPAGETABLEENTRYASSOCIATEDWITHADDRUSEDONLYFORMASTERKERNELPAGETABLESSEETHELATERSECTION“KERNELPAGETABLES“PTE_FREEPTERELEASESTHEPAGETABLEASSOCIATEDWITHTHEPTEPAGEDESCRIPTORPOINTERPTE_FREE_KERNELPTEEQUIVALENTTOPTE_FREE,BUTUSEDFORMASTERKERNELPAGETABLESCLEAR_PAGE_RANGEMMU,START,ENDCLEARSTHECONTENTSOFTHEPAGETABLESOFAPROCESSFROMLINEARADDRESSSTARTTOENDBYITERATIVELYRELEASINGITSPAGETABLESANDCLEARINGTHEPAGEMIDDLEDIRECTORYENTRIES中文翻译LINUX采用普通的内存分页模式同时适合32位和64位结构就像在早期的版本“64位内存分页结构“中所期待的那样对于32位结构来说,2级内存分页就足够了,但是64位结构要求一个更高级别的内存分页技术一直到2610版本出现,LINUX内存分页技术达到了3级分页从2611版本开始,4级内存分页技术就被采用了在图212中列举了四种页表,它们被叫做整体目录页面上层目录页面中间目录页面表页面整体目录页面包括若干上层目录页面的地址,它依次包括若干上层目录页面上层目录页面依次又包括若干中间目录页面中间目录页面依次又包括若干表页面每一个表页面入口均指向一个页帧,这样线性地址可以分解为5部分212图未表示出位数,因为每一部分的容量决定于计算机体系结构对于不带有实在地址的延长的32位数据结构,2个分页层次就够用了,LINUX本质上消除了上层目录页面机制中间目录层面领域,就说他们含有内核然而,上层目录页面和中间目录页面的位置在指示器的顺序中保持着在32位和64位数据结构上所工作的同样码。其内核保持上层目录页面和中间目录页面的一个位置。这是靠在它们中间设置输入数到1的及绘制此两个输入到整体目录页面的固有输入完成的LINUX总体目录页面是和8086的表页面指示相一致的。对于带有实在地址这样的32位数据结构条件页面是能完成的，删去了上层目录页面。中层目录页面是和8086的表页面相一致的，而LINUX表页面是和8086表页面相一致的。最终，采用3式4层次页面对于64位数据结构是受由硬件实行线形地址分解来决定的。（见表22）。LINUX操作过程主要是依靠页面进行。实际上，线形地址到实在地址的自动传输使下述硬件客观上是可行的把不同的实在地址空间分配到每一个的过程，保证了有效的寻码误差出现。从页桢（在主存储器中的实际地址）用区别页面（数据组）。这就允许同样页面存储于页桢中，再存盘和接着在不同的页面桢中再重装。这是实际存储机制中的基本元素。（见17章）在本章剩余部分，我们将关系到为调用8086处理器而具体存储到页面的线路。在第9章可见，每一过程都有其自己的总体目录以及表页面的置位。当一个过程开关发生时（见第3章过程开关一节），LINUX在每一次过程描述符中存储CX3控制寄存由于执行中之后将执行存储具有数值的负载于过程描述符中。这样，当在CPU上重新开始执行新过程时，页面单元就和表页面的正确置位有关。线形绘图对实在地址就变成一个机制工作，导致它们仍有某些复杂关系存在。本章以下少数几节是颇冗长的功能表。取回纹息的宏功能（内核）需要去寻址，并操作表页；大多数的功能是1到2线长。现在您可以只浏览这些节段，但是晓得这些功能和宏功能的角色是有用的。因为在整个这本书的研讨中您会经常见到。251线形地址域当应用到8086处理器时，详述在补偿域位数的长度；它应给出12数值。因为所有在页面中的地址必须适合于补偿域中。页面的大小在8086系统中是2的12次方或所熟悉的4。096位；12的PAGESHIFT可以考虑为整个页面大小的以2为底的对数。宏功能是靠PAGESIZE回到页面的容量来达到。最后，PAGESIZE宏功能给出0XFFFFF000数值和用它来掩盖补偿域的所有的位数。PMDSIFT在补偿的位数中总长以及线性地址的表域，换句话说，一个面积大小的对数，中层目