linux那些事之 page table基本操作
通过遍历page table查找一个虚拟地址已经做了对应的物理内存映射是一个很常用的操作,因此整个walk遍历过程耗时要尽量小。
show_pte()
以show_pte()函数为例说明如何根据一个虚拟地址addr,获取到物理页转换过程(以x86 5级地址转换说明)
/** This is useful to dump out the page tables associated with* 'addr' in mm 'mm'.*/
static void show_pte(struct mm_struct *mm, unsigned long addr)
{pgd_t *pgd;if (mm) {pgd = mm->pgd;} else {pgd = get_TTB();if (unlikely(!pgd))pgd = swapper_pg_dir;}pr_alert("pgd = %p\n", pgd);pgd += pgd_index(addr);pr_alert("[%08lx] *pgd=%0*llx", addr, (u32)(sizeof(*pgd) * 2),(u64)pgd_val(*pgd));do {p4d_t *p4d;pud_t *pud;pmd_t *pmd;pte_t *pte;if (pgd_none(*pgd))break;if (pgd_bad(*pgd)) {pr_cont("(bad)");break;}p4d = p4d_offset(pgd, addr);if (PTRS_PER_P4D != 1)pr_cont(", *p4d=%0*Lx", (u32)(sizeof(*p4d) * 2),(u64)p4d_val(*p4d));if (p4d_none(*p4d))break;if (p4d_bad(*p4d)) {pr_cont("(bad)");break;}pud = pud_offset(p4d, addr);if (PTRS_PER_PUD != 1)pr_cont(", *pud=%0*llx", (u32)(sizeof(*pud) * 2),(u64)pud_val(*pud));if (pud_none(*pud))break;if (pud_bad(*pud)) {pr_cont("(bad)");break;}pmd = pmd_offset(pud, addr);if (PTRS_PER_PMD != 1)pr_cont(", *pmd=%0*llx", (u32)(sizeof(*pmd) * 2),(u64)pmd_val(*pmd));if (pmd_none(*pmd))break;if (pmd_bad(*pmd)) {pr_cont("(bad)");break;}/* We must not map this if we have highmem enabled */if (PageHighMem(pfn_to_page(pmd_val(*pmd) >> PAGE_SHIFT)))break;pte = pte_offset_kernel(pmd, addr);pr_cont(", *pte=%0*llx", (u32)(sizeof(*pte) * 2),(u64)pte_val(*pte));} while (0);pr_cont("\n");
}
show_pte() 参数addr虚拟地址,mm参数表明该虚拟地址所归属于哪个进程,主要操作步骤:
- pgd = mm->pgd:获取该进程的pgd表首地址
- pgd += pgd_index(addr):根据从虚拟地址中解析出pgd index索引,获取到该虚拟地址对应的pgd表
- p4d = p4d_offset(pgd, addr): 根据pgd表 对应entry(即p4d表首地址)以及地址addr的p4d index部分,获取到该虚拟地址对应的p4d
- pud = pud_offset(p4d, addr): 根据p4d表对应entry(即pud表首地址) 以及addr中的pud index部分获取到pud
- pmd = pmd_offset(pud, addr): 根据pud表对应entry(即pmd表首地址) 以及addr中的pmd index获取到pmd
- pte = pte_offset_kernel(pmd, addr):根据pmd表对应entry(即pte表首地址) 及addr中 pte index获取到pte。
内核为每个表的操作都提供了相应接口操作。
pgd 表操作
pgd_index
从addr 虚拟地址中获取到所对应的pgd 部分,其实就是一个典型位移操作:
#define pgd_index(a) (((a) >> PGDIR_SHIFT) & (PTRS_PER_PGD - 1))
将地址addr 偏移PGDIR_SHIFT,并取PGD 部分
pgd_offset
pgd_offset支持mm和addr 两个参数:
#define pgd_offset(mm, address) pgd_offset_pgd((mm)->pgd, (address))
pgd_t
pgd_t在x86结构中定义如下(/arch/x86/include/asm/pgtable_types.h):
typedef struct { pgdval_t pgd; } pgd_t;
pgd_none()
pgd_none()用于查询 当前虚拟地址对应的pgd表是否存在,x86 64位架构下 开启5级映射该函数实现位于(/arch/x86/include/asm/pgtable.h):
static inline int pgd_none(pgd_t pgd)
{if (!pgtable_l5_enabled())return 0;/** There is no need to do a workaround for the KNL stray* A/D bit erratum here. PGDs only point to page tables* except on 32-bit non-PAE which is not supported on* KNL.*/return !native_pgd_val(pgd);
}
如果没有开启五级映射 则返回0,否则就调用native_pgd_val()查看对应的ogd是否存在:
static inline pgdval_t native_pgd_val(pgd_t pgd)
{return pgd.pgd & PGD_ALLOWED_BITS;
}
pgd_bad
该pgd是否有效或者损坏:
static inline int pgd_bad(pgd_t pgd)
{unsigned long ignore_flags = _PAGE_USER;if (!pgtable_l5_enabled())return 0;if (IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION))ignore_flags |= _PAGE_NX;return (pgd_flags(pgd) & ~ignore_flags) != _KERNPG_TABLE;
}
通过查看pgd 表中标志位,该pdd是否有效, pgd中允许使用标记位(后面再详细描述):
#define PGD_ALLOWED_BITS (PGD_PAE_PHYS_MASK | _PAGE_PRESENT | \_PAGE_PWT | _PAGE_PCD | \_PAGE_SOFTW1 | _PAGE_SOFTW2 | _PAGE_SOFTW3)
pgd_page
获取p4d表所使用的物理页:
#define pgd_page(pgd) pfn_to_page(pgd_pfn(pgd))
pgd_pfn
pgd 表对应的物理页pfn:
static inline unsigned long pgd_pfn(pgd_t pgd)
{return (pgd_val(pgd) & PTE_PFN_MASK) >> PAGE_SHIFT;
}
pgd_val
pgd_val()用于获取pgd 表中一个entry对应的里面对应的值,即对应的为p4d表首地址,x86架构下定义:
#define pgd_val(x) native_pgd_val(x)
通用pgd 定义为:
#define pgd_val(x) ((x).pgd)
x86架构下native_pgd_val 定义如下:
static inline pgdval_t native_pgd_val(pgd_t pgd)
{return pgd.pgd & PGD_ALLOWED_BITS;
}
pgd 一个entry除了用于存放pd4 表首物理地址,物理地址是一个page对齐,因此可以将0~12位用作其他标志位, PGD_ALLOWED_BITS表明是使用的有效标记位。
p4d表
p4d_offset
根据该虚拟地址对应的pgd表对应entry 以及 addr中的p4d部分:
static inline p4d_t *p4d_offset(pgd_t *pgd, unsigned long address)
{if (!pgtable_l5_enabled())return (p4d_t *)pgd;return (p4d_t *)pgd_page_vaddr(*pgd) + p4d_index(address);
}
处理步骤分为两个部分:
- pgd entry对应的为p4d_t表首地址
- 根据address 获取到 p4d index.
pgd_page_vaddr
pgd_page_vaddr将pgd 表物理地址中存储的内容即 entry转换称对应p4d_t 表首地址:
static inline unsigned long pgd_page_vaddr(pgd_t pgd)
{return (unsigned long)__va((unsigned long)pgd_val(pgd) & PTE_PFN_MASK);
}
p4d_index
获取到addr地址中p4d index:
static inline unsigned long p4d_index(unsigned long address)
{return (address >> P4D_SHIFT) & (PTRS_PER_P4D - 1);
}
p4d_none
p4d对应entry是否存在:
static inline int p4d_none(p4d_t p4d)
{return (native_p4d_val(p4d) & ~(_PAGE_KNL_ERRATUM_MASK)) == 0;
}
p4d_bad
p4d表是否有效
static inline int p4d_bad(p4d_t p4d)
{unsigned long ignore_flags = _KERNPG_TABLE | _PAGE_USER;if (IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION))ignore_flags |= _PAGE_NX;return (p4d_flags(p4d) & ~ignore_flags) != 0;
}
p4d_present
判断p4d entry对应PRESET 是否被设置,如果被设置则返回1说明存在下一级表,否则返回0:
static inline int p4d_present(p4d_t p4d)
{return p4d_flags(p4d) & _PAGE_PRESENT;
}
p4d_page_vaddr
根据 p4d表获取其中entry 中对应的下一级表首地址:
static inline unsigned long p4d_page_vaddr(p4d_t p4d)
{return (unsigned long)__va(p4d_val(p4d) & p4d_pfn_mask(p4d));
}
p4d_page
获取p4d表所使用的物理页:
#define p4d_page(p4d) pfn_to_page(p4d_pfn(p4d))
p4d_pfn
p4d表所使用的物理页帧号pfn:
static inline unsigned long p4d_pfn(p4d_t p4d)
{return (p4d_val(p4d) & p4d_pfn_mask(p4d)) >> PAGE_SHIFT;
}
p4d_val
用于获取p4d表中entry值,可以从entry中获取到pud表首地址:
#define p4d_val(x) native_p4d_val(x)
native_p4d_val 作用类似pgd,用于将entry中的标志位去除:
static inline p4dval_t native_p4d_val(p4d_t p4d)
{return p4d.p4d;
}
pud表操作
pud_offset
根据p4d 表中对应的entry 即pud表首地址,以及addr 中pud部分获取到pud:
static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address)
{return (pud_t *)p4d_page_vaddr(*p4d) + pud_index(address);
}
操作步骤和p4d类似,首先将p4d表对应的entry转换成pud表首地址,然后根据pud index获取到pub
p4d_page_vaddr
将p4d表对应的entry 转换成pud表首地址:
static inline unsigned long p4d_page_vaddr(p4d_t p4d)
{return (unsigned long)__va(p4d_val(p4d) & p4d_pfn_mask(p4d));
}
pud_index
从虚拟地址中获取到pud 部分:
static inline unsigned long pud_index(unsigned long address)
{return (address >> PUD_SHIFT) & (PTRS_PER_PUD - 1);
}
pud_none
pud表是否存在
static inline int pud_none(pud_t pud)
{return (native_pud_val(pud) & ~(_PAGE_KNL_ERRATUM_MASK)) == 0;
}
pud_bad
pud表是否有效 ,是否损坏:
static inline int pud_bad(pud_t pud)
{return (pud_flags(pud) & ~(_KERNPG_TABLE | _PAGE_USER)) != 0;
}
pud_present
同理p4d_present:
static inline int pud_present(pud_t pud)
{return pud_flags(pud) & _PAGE_PRESENT;
}
pud_pfn
pud表所使用的物理页帧号pfn:
static inline unsigned long pud_pfn(pud_t pud)
{phys_addr_t pfn = pud_val(pud);pfn ^= protnone_mask(pfn);return (pfn & pud_pfn_mask(pud)) >> PAGE_SHIFT;
}
pud_val
用于获取pud表entry,可以从entry获取到pmd表物理地址:
#define pud_val(x) native_pud_val(x)
native_pud_val定义如下:
static inline pudval_t native_pud_val(pud_t pud)
{return pud.pud;
}
pmd表操作
pmd_offset
根据pud 表中对应的entry 即pmd表首地址,以及addr 中pmd部分获取到pmd:
static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address)
{return pud_pgtable(*pud) + pmd_index(address);
}
操作步骤和pud类似,首先将pud表对应的entry转换成pmd表首地址,然后根据pmd index获取到pmb
pud_page_vaddr
将pud表对应的entry 转换成pmd表首地址:
static inline unsigned long pud_page_vaddr(pud_t pud)
{return (unsigned long)__va(pud_val(pud) & pud_pfn_mask(pud));
}
pmd_index
从虚拟地址中获取到pmd 部分:
static inline unsigned long pmd_index(unsigned long address)
{return (address >> PMD_SHIFT) & (PTRS_PER_PMD - 1);
}
pmd_none
pmd表是否存在
static inline int pmd_none(pmd_t pmd)
{/* Only check low word on 32-bit platforms, since it might beout of sync with upper half. */unsigned long val = native_pmd_val(pmd);return (val & ~_PAGE_KNL_ERRATUM_MASK) == 0;
}
pmd_bad
pmd表是否有效 ,是否损坏:
static inline int pmd_bad(pmd_t pmd)
{return (pmd_flags(pmd) & ~_PAGE_USER) != _KERNPG_TABLE;
}
pmd_present
同理p4d_present:
static inline int pmd_present(pmd_t pmd)
{/** Checking for _PAGE_PSE is needed too because* split_huge_page will temporarily clear the present bit (but* the _PAGE_PSE flag will remain set at all times while the* _PAGE_PRESENT bit is clear).*/return pmd_flags(pmd) & (_PAGE_PRESENT | _PAGE_PROTNONE | _PAGE_PSE);
}
pmd_pfn
pmd表所使用的物理页帧号pfn:
static inline unsigned long pmd_pfn(pmd_t pmd)
{phys_addr_t pfn = pmd_val(pmd);pfn ^= protnone_mask(pfn);return (pfn & pmd_pfn_mask(pmd)) >> PAGE_SHIFT;
}
pmd_page
pmd表所使用的物理页:
#define pmd_page(pmd) pfn_to_page(pmd_pfn(pmd))
pmd_val
pmd表中entry,可以从entry中获取到pte表物理地址:
#define pmd_val(x) native_pmd_val(x)
native_pmd_val定义如下:
static inline pmdval_t native_pmd_val(pmd_t pmd)
{return pmd.pmd;
}
pte表操作
pte_offset_kernel
根据pmd 表中对应的entry 即pte表首地址,以及addr 中pte部分获取到pte:
static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
{return (pte_t *)pmd_page_vaddr(*pmd) + pte_index(address);
}
操作步骤和pmd类似,首先将pmd表对应的entry转换成pte表首地址,然后根据pte index获取到pte
pmd_page_vaddr
将pud表对应的entry 转换成pmd表首地址:
static inline unsigned long pmd_page_vaddr(pmd_t pmd)
{return (unsigned long)__va(pmd_val(pmd) & pmd_pfn_mask(pmd));
}
pte_index
从虚拟地址中获取到pmd 部分:
static inline unsigned long pte_index(unsigned long address)
{return (address >> PAGE_SHIFT) & (PTRS_PER_PTE - 1);
}
pte_none
pte表是否存在
static inline int pte_none(pte_t pte)
{return !(pte.pte & ~(_PAGE_KNL_ERRATUM_MASK));
}
pte_present
同理p4d_present:
static inline int pte_present(pte_t a)
{return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE);
}
pte_pfn
pte表所占用的物理页帧号,由于pte属于最低一级,下一级就是页内偏移,因此实际上获取的就是该虚拟地址对应的物理页帧号:
static inline unsigned long pte_pfn(pte_t pte)
{phys_addr_t pfn = pte_val(pte);pfn ^= protnone_mask(pfn);return (pfn & PTE_PFN_MASK) >> PAGE_SHIFT;
}
pte_page
pte表的物理页:
#define pte_page(pte) pfn_to_page(pte_pfn(pte))
pte_val
pte表entry,可以从entry中获取到虚拟地址对应申请到的物理地址:
#define pte_val(x) native_pte_val(x)
native_pte_va定义如下l:
static inline pteval_t native_pte_val(pte_t pte)
{return pte.pte;
}
各级页表申请
各级页表的建立也是有专门的函数进行处理,本质都是从slab或者直接从buddy中申请相应的物理内存用于保存各级页表信息。
pgd_alloc
pgd_alloc用于申请pgd 表,一般在fork或vfork等创建线程时会首先将该进程pgd表建好.x86平台下该函数实现位于(/arch/x86/mm/pgtable.c):主要实现如下:
pgd_t *pgd_alloc(struct mm_struct *mm)
{pgd_t *pgd;pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];pmd_t *pmds[MAX_PREALLOCATED_PMDS];pgd = _pgd_alloc();if (pgd == NULL)goto out;mm->pgd = pgd;if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)goto out_free_pgd;if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)goto out_free_pmds;if (paravirt_pgd_alloc(mm) != 0)goto out_free_user_pmds;/** Make sure that pre-populating the pmds is atomic with* respect to anything walking the pgd_list, so that they* never see a partially populated pgd.*/spin_lock(&pgd_lock);pgd_ctor(mm, pgd);pgd_prepopulate_pmd(mm, pgd, pmds);pgd_prepopulate_user_pmd(mm, pgd, u_pmds);spin_unlock(&pgd_lock);return pgd;out_free_user_pmds:free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
out_free_pmds:free_pmds(mm, pmds, PREALLOCATED_PMDS);
out_free_pgd:_pgd_free(pgd);
out:return NULL;
}
主要处理过程:
- 调用_pgd_alloc申请pgd表物理内存
- 将该申请的pgd表赋保存到相应的进程空间中mm->pgd = pgd;
- 预申请一定数量的pmd表,以下次申请使用pdm表时可以加速过程,该pmd主要用于对内核态页表转换
- 预申请一定数量的i_pmd表,主要用于用户态内存空间页表转换使用
- 将预申请到的pmd 设置到页表中
_pgd_alloc
_pgd_alloc为pgd 表申请物理内存:
static inline pgd_t *_pgd_alloc(void)
{/** If no SHARED_KERNEL_PMD, PAE kernel is running as a Xen domain.* We allocate one page for pgd.*/if (!SHARED_KERNEL_PMD)return (pgd_t *)__get_free_pages(GFP_PGTABLE_USER,PGD_ALLOCATION_ORDER);/** Now PAE kernel is not running as a Xen domain. We can allocate* a 32-byte slab for pgd to save memory space.*/return kmem_cache_alloc(pgd_cache, GFP_PGTABLE_USER);
}
如果SHARED_KERNEL_PMD 没有设置,直接从buddy中申请一个物理页用作pgd表,否则从slab中申请物理内存
p4d_alloc
为p4d表申请物理内存:
static inline p4d_t *p4d_alloc(struct mm_struct *mm, pgd_t *pgd,unsigned long address)
{return (unlikely(pgd_none(*pgd)) && __p4d_alloc(mm, pgd, address)) ?NULL : p4d_offset(pgd, address);
}
- 如果pgd对应的entry不存在,则调用__p4d_alloc,申请一个p4d表,利用p4d_offset根据addr计算出p4d
- pgd对应的entry存在,则没有必要新申请一个p4d表,利用p4d_offset根据addr计算出p4d。
- unlikely意味着基本上不会出现entry不存在,且申请p4d表失败场景。
该函数在page fault中处理需要用到
__p4d_alloc
/** Allocate p4d page table.* We've already handled the fast-path in-line.*/
int __p4d_alloc(struct mm_struct *mm, pgd_t *pgd, unsigned long address)
{p4d_t *new = p4d_alloc_one(mm, address);if (!new)return -ENOMEM;smp_wmb(); /* See comment in __pte_alloc */spin_lock(&mm->page_table_lock);if (pgd_present(*pgd)) /* Another has populated it */p4d_free(mm, new);elsepgd_populate(mm, pgd, new);spin_unlock(&mm->page_table_lock);return 0;
}
- 调用p4d_alloc_one 申请p4d表
- smp_wmb:考虑smp系统,需要保证一致性
- pgd_populate:将申请的p4d表,设置到对应的pgd 表中
pud_alloc
申请pud表:
static inline pud_t *pud_alloc(struct mm_struct *mm, p4d_t *p4d,unsigned long address)
{return (unlikely(p4d_none(*p4d)) && __pud_alloc(mm, p4d, address)) ?NULL : pud_offset(p4d, address);
}
__pud_alloc
pud表申请物理内存:
/** Allocate page upper directory.* We've already handled the fast-path in-line.*/
int __pud_alloc(struct mm_struct *mm, p4d_t *p4d, unsigned long address)
{pud_t *new = pud_alloc_one(mm, address);if (!new)return -ENOMEM;smp_wmb(); /* See comment in __pte_alloc */spin_lock(&mm->page_table_lock);if (!p4d_present(*p4d)) {mm_inc_nr_puds(mm);p4d_populate(mm, p4d, new);} else /* Another has populated it */pud_free(mm, new);spin_unlock(&mm->page_table_lock);return 0;
}
- pud_alloc_one: pud表申请物理内存
- p4d_populate: 将申请的pud表设置到对应的p4d中
pmd_alloc
申请pmd表:
static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
{return (unlikely(pud_none(*pud)) && __pmd_alloc(mm, pud, address))?NULL: pmd_offset(pud, address);
}
原理同p4d_malloc,只有pud对应的entry不存在 才会申请新的pmd表。
__pmd_alloc
/** Allocate page middle directory.* We've already handled the fast-path in-line.*/
int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address)
{spinlock_t *ptl;pmd_t *new = pmd_alloc_one(mm, address);if (!new)return -ENOMEM;smp_wmb(); /* See comment in __pte_alloc */ptl = pud_lock(mm, pud);if (!pud_present(*pud)) {mm_inc_nr_pmds(mm);pud_populate(mm, pud, new);} else /* Another has populated it */pmd_free(mm, new);spin_unlock(ptl);return 0;
}
- pmd_alloc_one:申请pmd表
- pud_populate: 将申请的pmd表设置到pud中。
pte_alloc
申请pte表:
#define pte_alloc(mm, pmd) (unlikely(pmd_none(*(pmd))) && __pte_alloc(mm, pmd))
__pte_alloc
int __pte_alloc(struct mm_struct *mm, pmd_t *pmd)
{spinlock_t *ptl;pgtable_t new = pte_alloc_one(mm);if (!new)return -ENOMEM;/** Ensure all pte setup (eg. pte page lock and page clearing) are* visible before the pte is made visible to other CPUs by being* put into page tables.** The other side of the story is the pointer chasing in the page* table walking code (when walking the page table without locking;* ie. most of the time). Fortunately, these data accesses consist* of a chain of data-dependent loads, meaning most CPUs (alpha* being the notable exception) will already guarantee loads are* seen in-order. See the alpha page table accessors for the* smp_read_barrier_depends() barriers in page table walking code.*/smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */ptl = pmd_lock(mm, pmd);if (likely(pmd_none(*pmd))) { /* Has another populated it ? */mm_inc_nr_ptes(mm);pmd_populate(mm, pmd, new);new = NULL;}spin_unlock(ptl);if (new)pte_free(mm, new);return 0;
}
- pte_alloc_one 申请pte
- pmd_populate:将申请到的pte设置到pmd中。
各级页表释放
pgd_free
释放pgd表:
void pgd_free(struct mm_struct *mm, pgd_t *pgd)
{pgd_mop_up_pmds(mm, pgd);pgd_dtor(pgd);paravirt_pgd_free(mm, pgd);_pgd_free(pgd);
}
最终调用_pgd_free 释放pgd表
_pgd_free
static inline void _pgd_free(pgd_t *pgd)
{if (!SHARED_KERNEL_PMD)free_pages((unsigned long)pgd, PGD_ALLOCATION_ORDER);elsekmem_cache_free(pgd_cache, pgd);
}
调用free_page 或者通过slab将物理内存释放
p4d_free
p4d表释放:
static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
{if (!pgtable_l5_enabled())return;BUG_ON((unsigned long)p4d & (PAGE_SIZE-1));free_page((unsigned long)p4d);
}
通过free_page释放p4d表占用的物理内存页
pud_free
释放pud表:
static inline void pud_free(struct mm_struct *mm, pud_t *pud)
{BUG_ON((unsigned long)pud & (PAGE_SIZE-1));free_page((unsigned long)pud);
}
pmd_free
释放pmd表:
static inline void pmd_free(struct mm_struct *mm, pmd_t *pmd)
{BUG_ON((unsigned long)pmd & (PAGE_SIZE-1));pgtable_pmd_page_dtor(virt_to_page(pmd));free_page((unsigned long)pmd);
}
pte_free
释放pte表:
static inline void pte_free(struct mm_struct *mm, struct page *pte_page)
{pgtable_pte_page_dtor(pte_page);__free_page(pte_page);
}
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