Linux之内存管理-malloc \kmalloc\vmalloc\dma

1、malloc 函数

1.1分配内存小于128k,调用brk

malloc是C库实现的函数，C库维护了一个缓存，当内存够用时，malloc直接从C库缓存分配，只有当C库缓存不够用；当申请的内存小于128K时，通过系统调用brk，向内核申请，从堆空间申请一个vma；当申请内存大于128K时，通过系统调用mmap申请内存。先分析brk系统调用

malloc实现流程图

下面来看brk系统调用

经过平台相关实现，malloc最终会调用SYSCALL_DEFINE1宏，扩展为__arm64_sys_brk函数.扩展的函数名和架构相关。SYSCALL_DEFINE1 的数字1 代表参数个数，1代表一个参数。SYSCALL_DEFINEx,x最大是6.

路径：mm/mmap.c

SYSCALL_DEFINE1(brk, unsigned long, brk)
{
unsigned long retval;
unsigned long newbrk, oldbrk, origbrk;
struct mm_struct *mm = current->mm;
struct vm_area_struct *next;
unsigned long min_brk;
bool populate;
bool downgraded = false;
LIST_HEAD(uf);

if (mmap_write_lock_killable(mm)) //申请写类型读写锁
return -EINTR;

origbrk = mm->brk; //origbrk记录动态分配区的当前底部

#ifdef CONFIG_COMPAT_BRK
/*
* CONFIG_COMPAT_BRK can still be overridden by setting
* randomize_va_space to 2, which will still cause mm->start_brk
* to be arbitrarily shifted
*/
if (current->brk_randomized)
min_brk = mm->start_brk;
else
min_brk = mm->end_data;
#else
min_brk = mm->start_brk;
#endif
if (brk < min_brk)
goto out;

/*
* Check against rlimit here. If this check is done later after the test
* of oldbrk with newbrk then it can escape the test and let the data
* segment grow beyond its set limit the in case where the limit is
* not page aligned -Ram Gupta
*/
if (check_data_rlimit(rlimit(RLIMIT_DATA), brk, mm->start_brk,
mm->end_data, mm->start_data))
goto out;

newbrk = PAGE_ALIGN(brk);
oldbrk = PAGE_ALIGN(mm->brk);
if (oldbrk == newbrk) {
mm->brk = brk;
goto success;
}

/*
* Always allow shrinking brk.
* __do_munmap() may downgrade mmap_lock to read.
*/
if (brk <= mm->brk) { //请求释放空间
int ret;

/*
* mm->brk must to be protected by write mmap_lock so update it
* before downgrading mmap_lock. When __do_munmap() fails,
* mm->brk will be restored from origbrk.
*/
mm->brk = brk;
ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true); //释放空间的真正函数
if (ret < 0) {
mm->brk = origbrk;
goto out;
} else if (ret == 1) {
downgraded = true;
}
goto success;
}

/* Check against existing mmap mappings. */
next = find_vma(mm, oldbrk);
if (next && newbrk + PAGE_SIZE > vm_start_gap(next))//发现有重叠地址空间，不在寻找VMA
goto out;

/* Ok, looks good - let it rip. */
if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0) //如果没有重叠，新分配一个VMA
goto out;
mm->brk = brk;

success:
populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
if (downgraded)
mmap_read_unlock(mm);
else
mmap_write_unlock(mm);
userfaultfd_unmap_complete(mm, &uf);
if (populate) //调用mlockall()系统调用，mm_populate 会立刻分配物理内存。
mm_populate(oldbrk, newbrk - oldbrk);
return brk;

out:
retval = origbrk;
mmap_write_unlock(mm);
return retval;
}

总结下__do_sys_brk()功能： (1)从旧的brk边界去查询，是否有可用vma，若发现有重叠，直接使用； (2)若无发现重叠，新分配一个vma； (3)应用程序若调用mlockall()，会锁住进程所有虚拟地址空间，防止内存被交换出去，且立刻分配物理内存；否则，物理页面会等到使用时，触发缺页异常分配；

do_brk_flags ()

(1)寻找一个可使用的线性地址； (2)查找最适合插入红黑树的节点； (3)寻到的线性地址是否可以合并现有vma，所不能，新建一个vma; (4)将新建vma插入mmap链表和红黑树中

/*
* this is really a simplified "do_mmap". it only handles
* anonymous maps. eventually we may be able to do some
* brk-specific accounting here.
*/
static int do_brk_flags(unsigned long addr, unsigned long len, unsigned long flags, struct list_head *uf)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma, *prev;
struct rb_node **rb_link, *rb_parent;
pgoff_t pgoff = addr >> PAGE_SHIFT;
int error;
unsigned long mapped_addr;

/* Until we need other flags, refuse anything except VM_EXEC. */
if ((flags & (~VM_EXEC)) != 0)
return -EINVAL;
flags |= VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;//默认属性，可读写

//返回未使用过的，未映射的线性地址空间的起始地址

mapped_addr = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
if (IS_ERR_VALUE(mapped_addr))
return mapped_addr;

error = mlock_future_check(mm, mm->def_flags, len);
if (error)
return error;

/* Clear old maps, set up prev, rb_link, rb_parent, and uf */

//寻找适合插入的红黑树节点
if (munmap_vma_range(mm, addr, len, &prev, &rb_link, &rb_parent, uf))
return -ENOMEM;

/* Check against address space limits *after* clearing old maps... */
if (!may_expand_vm(mm, flags, len >> PAGE_SHIFT))
return -ENOMEM;

if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;

if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
return -ENOMEM;

/* Can we just expand an old private anonymous mapping? */

//检查是否能合并到addr 到附近的vma,若不能，只能新建一个vma
vma = vma_merge(mm, prev, addr, addr + len, flags,
NULL, NULL, pgoff, NULL, NULL_VM_UFFD_CTX);
if (vma)
goto out;

/*
* create a vma struct for an anonymous mapping
*/
vma = vm_area_alloc(mm); // 新建一个vma
if (!vma) {
vm_unacct_memory(len >> PAGE_SHIFT);
return -ENOMEM;
}

vma_set_anonymous(vma);
vma->vm_start = addr;
vma->vm_end = addr + len;
vma->vm_pgoff = pgoff;
vma->vm_flags = flags;
vma->vm_page_prot = vm_get_page_prot(flags);
vma_link(mm, vma, prev, rb_link, rb_parent); // 新vma添加到mmap链表和红黑树中
out:
perf_event_mmap(vma);
mm->total_vm += len >> PAGE_SHIFT;
mm->data_vm += len >> PAGE_SHIFT;
if (flags & VM_LOCKED)
mm->locked_vm += (len >> PAGE_SHIFT);
vma->vm_flags |= VM_SOFTDIRTY;
return 0;
}

如果调用mlockall 系统调用，mm_populate 会立刻分配物理内存

mm_populate()

mm_populate

->_mm_populate

->populate_vma_page_range

->__get_user_pages

当设置VM_LOCKED标志时，表示要马上申请物理页面，并与vma建立映射；否则，这里不操作，直到访问该vma时，触发缺页异常，再分配物理页面，并建立映射；

__get_user_pages()

static long __get_user_pages(struct mm_struct *mm,
unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
struct vm_area_struct **vmas, int *locked)
{
long ret = 0, i = 0;
struct vm_area_struct *vma = NULL;
struct follow_page_context ctx = { NULL };

if (!nr_pages)
return 0;

start = untagged_addr(start);

VM_BUG_ON(!!pages != !!(gup_flags & (FOLL_GET | FOLL_PIN)));

/*
* If FOLL_FORCE is set then do not force a full fault as the hinting
* fault information is unrelated to the reference behaviour of a task
* using the address space
*/
if (!(gup_flags & FOLL_FORCE))
gup_flags |= FOLL_NUMA;

do { //依次处理每个页面
struct page *page;
unsigned int foll_flags = gup_flags;
unsigned int page_increm;

/* first iteration or cross vma bound */
if (!vma || start >= vma->vm_end) {
vma = find_extend_vma(mm, start); //检查是否可以扩展VMA
if (!vma && in_gate_area(mm, start)) {
ret = get_gate_page(mm, start & PAGE_MASK,
gup_flags, &vma,
pages ? &pages[i] : NULL);
if (ret)
goto out;
ctx.page_mask = 0;
goto next_page;
}

if (!vma || check_vma_flags(vma, gup_flags)) {
ret = -EFAULT;
goto out;
}
if (is_vm_hugetlb_page(vma)) { // 支持巨页
i = follow_hugetlb_page(mm, vma, pages, vmas,
&start, &nr_pages, i,
gup_flags, locked);
if (locked && *locked == 0) {
/*
* We've got a VM_FAULT_RETRY
* and we've lost mmap_lock.
* We must stop here.
*/
BUG_ON(gup_flags & FOLL_NOWAIT);
BUG_ON(ret != 0);
goto out;
}
continue;
}
}
retry:
/*
* If we have a pending SIGKILL, don't keep faulting pages and
* potentially allocating memory.
*/
if (fatal_signal_pending(current)) { //如果当前进程收到KILL 信号，直接退出
ret = -EINTR;
goto out;
}
cond_resched(); //判断是否需要调度，这个函数是为优化系统延迟

//查看VMA的虚拟页面是否已经分配物理页面内存，返回已经映射的页面的page.

page = follow_page_mask(vma, start, foll_flags, &ctx);
if (!page) {

/若无映射，主动触发虚拟页面到物理页面的映射
ret = faultin_page(vma, start, &foll_flags, locked);
switch (ret) {
case 0:
goto retry;
case -EBUSY:
ret = 0;
fallthrough;
case -EFAULT:
case -ENOMEM:
case -EHWPOISON:
goto out;
case -ENOENT:
goto next_page;
}
BUG();
} else if (PTR_ERR(page) == -EEXIST) {
/*
* Proper page table entry exists, but no corresponding
* struct page.
*/
goto next_page;
} else if (IS_ERR(page)) {
ret = PTR_ERR(page);
goto out;
}
if (pages) {
pages[i] = page;
flush_anon_page(vma, page, start);//分配完物理页面，刷新缓存
flush_dcache_page(page);
ctx.page_mask = 0;
}
next_page:
if (vmas) {
vmas[i] = vma;
ctx.page_mask = 0;
}
page_increm = 1 + (~(start >> PAGE_SHIFT) & ctx.page_mask);
if (page_increm > nr_pages)
page_increm = nr_pages;
i += page_increm;
start += page_increm * PAGE_SIZE;
nr_pages -= page_increm;
} while (nr_pages);
out:
if (ctx.pgmap)
put_dev_pagemap(ctx.pgmap);
return i ? i : ret;
}

follow_page_mask

->follow_p4d_mask

->follow_pud_mask

->follow_pmd_mask

->follow_page_pte

follow_page_pte

static struct page *follow_page_pte(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmd, unsigned int flags,
struct dev_pagemap **pgmap)
{
struct mm_struct *mm = vma->vm_mm;
struct page *page;
spinlock_t *ptl;
pte_t *ptep, pte;
int ret;

/* FOLL_GET and FOLL_PIN are mutually exclusive. */
if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
(FOLL_PIN | FOLL_GET)))
return ERR_PTR(-EINVAL);
retry:
if (unlikely(pmd_bad(*pmd)))
return no_page_table(vma, flags);

ptep = pte_offset_map_lock(mm, pmd, address, &ptl); //获得pte 和一个锁
pte = *ptep;
if (!pte_present(pte)) { //若此pte 不在内存中，作下面的处理
swp_entry_t entry;
/*
* KSM's break_ksm() relies upon recognizing a ksm page
* even while it is being migrated, so for that case we
* need migration_entry_wait().
*/
if (likely(!(flags & FOLL_MIGRATION)))
goto no_page;
if (pte_none(pte))
goto no_page;
entry = pte_to_swp_entry(pte);
if (!is_migration_entry(entry))
goto no_page;
pte_unmap_unlock(ptep, ptl);
migration_entry_wait(mm, pmd, address); //等待页面合并完成后再尝试
goto retry;
}
if ((flags & FOLL_NUMA) && pte_protnone(pte))
goto no_page;
if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
pte_unmap_unlock(ptep, ptl);
return NULL;
}

//根据pte,返回物理页面page,只返回普通页面，特殊页面不参与内存管理

page = vm_normal_page(vma, address, pte);
if (!page && pte_devmap(pte) && (flags & (FOLL_GET | FOLL_PIN))) {
/*
* Only return device mapping pages in the FOLL_GET or FOLL_PIN
* case since they are only valid while holding the pgmap
* reference.
*/
*pgmap = get_dev_pagemap(pte_pfn(pte), *pgmap);//处理设备映射文件
if (*pgmap)
page = pte_page(pte);
else
goto no_page;
} else if (unlikely(!page)) {//处理vm_normal_page()没有返回有效页面情况
if (flags & FOLL_DUMP) {
/* Avoid special (like zero) pages in core dumps */
page = ERR_PTR(-EFAULT);
goto out;
}

if (is_zero_pfn(pte_pfn(pte))) {//系统零页，不会返回错误
page = pte_page(pte);
} else {
ret = follow_pfn_pte(vma, address, ptep, flags);
page = ERR_PTR(ret);
goto out;
}
}

if (flags & FOLL_SPLIT && PageTransCompound(page)) {
get_page(page);
pte_unmap_unlock(ptep, ptl);
lock_page(page);
ret = split_huge_page(page);
unlock_page(page);
put_page(page);
if (ret)
return ERR_PTR(ret);
goto retry;
}

/* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
if (unlikely(!try_grab_page(page, flags))) {
page = ERR_PTR(-ENOMEM);
goto out;
}
/*
* We need to make the page accessible if and only if we are going
* to access its content (the FOLL_PIN case). Please see
* Documentation/core-api/pin_user_pages.rst for details.
*/
if (flags & FOLL_PIN) {
ret = arch_make_page_accessible(page);
if (ret) {
unpin_user_page(page);
page = ERR_PTR(ret);
goto out;
}
}
if (flags & FOLL_TOUCH) { //标记页面可访问
if ((flags & FOLL_WRITE) &&
!pte_dirty(pte) && !PageDirty(page))
set_page_dirty(page);
/*
* pte_mkyoung() would be more correct here, but atomic care
* is needed to avoid losing the dirty bit: it is easier to use
* mark_page_accessed().
*/
mark_page_accessed(page);
}
if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {

/* Do not mlock pte-mapped THP */
if (PageTransCompound(page))
goto out;

/*
* The preliminary mapping check is mainly to avoid the
* pointless overhead of lock_page on the ZERO_PAGE
* which might bounce very badly if there is contention.
*
* If the page is already locked, we don't need to
* handle it now - vmscan will handle it later if and
* when it attempts to reclaim the page.
*/
if (page->mapping && trylock_page(page)) {
lru_add_drain(); /* push cached pages to LRU */
/*
* Because we lock page here, and migration is
* blocked by the pte's page reference, and we
* know the page is still mapped, we don't even
* need to check for file-cache page truncation.
*/
mlock_vma_page(page);
unlock_page(page);
}
}
out:
pte_unmap_unlock(ptep, ptl);
return page;
no_page:
pte_unmap_unlock(ptep, ptl);
if (!pte_none(pte))
return NULL;
return no_page_table(vma, flags);
}

总结：

(1)malloc函数，从C库缓存分配内存，其分配或释放内存，未必马上会执行；

(2)malloc实际分配内存动作，要么主动设置mlockall()，人为触发缺页异常，分配物理页面；或者在访问内存时触发缺页异常，分配物理页面；

(3)malloc分配虚拟内存，有三种情况： a.malloc()分配内存后，直接读，linux内核进入缺页异常，调用do_anonymous_page函数使用零页映射，此时PTE属性只读； b.malloc()分配内存后，先读后写，linux内核第一次触发缺页异常，映射零页；第二次触发异常，触发写时复制； c.malloc()分配内存后, 直接写，linux内核进入匿名页面的缺页异常，调用alloc_zeroed_user_highpage_movable分配一个新页面，这个PTE是可写的；

1.2 分配内存大于128K，调用mmap函数

mmap一般用于用户程序分配内存，读写大文件，链接动态库，多进程内存共享等；实现过程流程图：

mmap根据文件关联性和映射区域是否共享等属性，其映射分为4类：

1.私有匿名映射 fd=-1,且flags=MAP_ANONYMOUS|MAP_PRIVATE,创建的mmap映射是私有匿名映射；用途是在glibc分配大内存时，如果需分配内存大于MMAP_THREASHOLD（128KB），glibc默认用mmap代替brk分配内存；

2.共享匿名映射 fd=-1,且flags=MAP_ANONYMOUS|MAP_SHARED; 常用于父子进程的通信，共享一块内存区域； do_mmap_pgoff()->mmap_region(),最终调用shmem_zero_setup打开/dev/zero设备文件；

另外直接打开/dev/zero设备文件，然后使用这个句柄创建mmap，也是最终调用shmem模块创建共享匿名映射；

3.私有文件映射 flags=MAP_PRIVATE；常用场景是，加载动态共享库；

4.共享文件映射 flags=MAP_SHARED；有两个应用场景； (1)读写文件：内核的会写机制会将内存数据同步到磁盘； (2)进程间通信：多个独立进程，打开同一个文件，互相都可以观察到，可是实现多进程通信；

路径： mm/mmap.c

mmap_pgoff 宏定义

SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
unsigned long, prot, unsigned long, flags,
unsigned long, fd, unsigned long, pgoff)
{
return ksys_mmap_pgoff(addr, len, prot, flags, fd, pgoff);
}

ksys_mmap_pgoff

->vm_mmap_pgoff

->do_mmap

->mmap_region

核心函数：

mmap_region

unsigned long mmap_region(struct file *file, unsigned long addr,
unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
struct list_head *uf)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma, *prev, *merge;
int error;
struct rb_node **rb_link, *rb_parent;
unsigned long charged = 0;

/* Check against address space limit. */
if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) {
unsigned long nr_pages;

/*
* MAP_FIXED may remove pages of mappings that intersects with
* requested mapping. Account for the pages it would unmap.
*/
nr_pages = count_vma_pages_range(mm, addr, addr + len);

if (!may_expand_vm(mm, vm_flags,
(len >> PAGE_SHIFT) - nr_pages))
return -ENOMEM;
}

/* Clear old maps, set up prev, rb_link, rb_parent, and uf */
if (munmap_vma_range(mm, addr, len, &prev, &rb_link, &rb_parent, uf))
return -ENOMEM;
/*
* Private writable mapping: check memory availability
*/
if (accountable_mapping(file, vm_flags)) {
charged = len >> PAGE_SHIFT;
if (security_vm_enough_memory_mm(mm, charged))
return -ENOMEM;
vm_flags |= VM_ACCOUNT;
}

/*
* Can we just expand an old mapping?
*/
vma = vma_merge(mm, prev, addr, addr + len, vm_flags,
NULL, file, pgoff, NULL, NULL_VM_UFFD_CTX); //尝试合并VMA
if (vma)
goto out;

/*
* Determine the object being mapped and call the appropriate
* specific mapper. the address has already been validated, but
* not unmapped, but the maps are removed from the list.
*/
vma = vm_area_alloc(mm);// 分配VMA
if (!vma) {
error = -ENOMEM;
goto unacct_error;
}

vma->vm_start = addr;
vma->vm_end = addr + len;
vma->vm_flags = vm_flags;
vma->vm_page_prot = vm_get_page_prot(vm_flags);
vma->vm_pgoff = pgoff;

if (file) { -----------------------------------------------文件映射
if (vm_flags & VM_DENYWRITE) {
error = deny_write_access(file);
if (error)
goto free_vma;
}
if (vm_flags & VM_SHARED) {
error = mapping_map_writable(file->f_mapping);
if (error)
goto allow_write_and_free_vma;
}

/* ->mmap() can change vma->vm_file, but must guarantee that
* vma_link() below can deny write-access if VM_DENYWRITE is set
* and map writably if VM_SHARED is set. This usually means the
* new file must not have been exposed to user-space, yet.
*/
vma->vm_file = get_file(file);
error = call_mmap(file, vma);
if (error)
goto unmap_and_free_vma;

/* Can addr have changed??
*
* Answer: Yes, several device drivers can do it in their
* f_op->mmap method. -DaveM
* Bug: If addr is changed, prev, rb_link, rb_parent should
* be updated for vma_link()
*/
WARN_ON_ONCE(addr != vma->vm_start);

addr = vma->vm_start;

/* If vm_flags changed after call_mmap(), we should try merge vma again
* as we may succeed this time.
*/
if (unlikely(vm_flags != vma->vm_flags && prev)) {
merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
if (merge) {
/* ->mmap() can change vma->vm_file and fput the original file. So
* fput the vma->vm_file here or we would add an extra fput for file
* and cause general protection fault ultimately.

*/
fput(vma->vm_file);
vm_area_free(vma);
vma = merge;
/* Update vm_flags to pick up the change. */
vm_flags = vma->vm_flags;
goto unmap_writable;
}
}

vm_flags = vma->vm_flags;
} else if (vm_flags & VM_SHARED) { -------------共享映射
error = shmem_zero_setup(vma); -----------共享匿名映射
if (error)
goto free_vma;
} else {
vma_set_anonymous(vma); --------------匿名映射
}

/* Allow architectures to sanity-check the vm_flags */
if (!arch_validate_flags(vma->vm_flags)) {
error = -EINVAL;
if (file)
goto unmap_and_free_vma;
else
goto free_vma;
}

vma_link(mm, vma, prev, rb_link, rb_parent); ---------vma 加入mm系统
/* Once vma denies write, undo our temporary denial count */
if (file) {
unmap_writable:
if (vm_flags & VM_SHARED)
mapping_unmap_writable(file->f_mapping);
if (vm_flags & VM_DENYWRITE)
allow_write_access(file);
}
file = vma->vm_file;
out:
perf_event_mmap(vma);

vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
if (vm_flags & VM_LOCKED) {
if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
is_vm_hugetlb_page(vma) ||
vma == get_gate_vma(current->mm))
vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
else
mm->locked_vm += (len >> PAGE_SHIFT);
}

if (file)
uprobe_mmap(vma);

/*
* New (or expanded) vma always get soft dirty status.
* Otherwise user-space soft-dirty page tracker won't
* be able to distinguish situation when vma area unmapped,
* then new mapped in-place (which must be aimed as
* a completely new data area).
*/
vma->vm_flags |= VM_SOFTDIRTY;

vma_set_page_prot(vma);

return addr;

unmap_and_free_vma:
vma->vm_file = NULL;
fput(file);

/* Undo any partial mapping done by a device driver. */
unmap_region(mm, vma, prev, vma->vm_start, vma->vm_end);
charged = 0;
if (vm_flags & VM_SHARED)
mapping_unmap_writable(file->f_mapping);
allow_write_and_free_vma:
if (vm_flags & VM_DENYWRITE)
allow_write_access(file);
free_vma:
vm_area_free(vma);
unacct_error:
if (charged)
vm_unacct_memory(charged);
return error;
}

总结：以上的malloc,mmap函数，若无特别设定，默认都是指建立虚拟地址空间，但没有建立虚拟地址空间到物理地址空间的映射；当访问未映射的虚拟空间时，触发缺页异常，linxu内核会处理缺页异常，缺页异常服务程序中，会分配物理页，并建立虚拟地址到物理页的映射；

补充两个问题：

1.当mmap重复申请相同地址，为什么不会失败？ find_vma_links()函数便利该进程所有的vma，当检查到当前要映射区域和已有vma重叠时，先销毁旧映射区，重新映射，所以第二次申请，不会报错。

2.mmap打开多个文件时，比如播放视频时，为什么会卡顿？ mmap只是建立vma，并未实际分配物理页面读取文件内存，当播放器真正读取文件时，会频繁触发缺页异常，再从磁盘读取文件到页面高速缓存中，会导致磁盘读性能较差；

madvise(add,len,MADV_WILLNEED|MADV_SEQUENTIAL)对文件内容进行预读和顺序读；

但是内核默认的预读功能就可以实现；且madvise不适合流媒体，只适合随机读取场景；

能够有效提高流媒体服务I/O性能的方法是增大内核默认预读窗口；内核默认是128K，可以通过“blockdev --setra”命令修改；

3、mmap 访问文件为什么快？

mmap()系统调用用于在进程的虚拟地址空间中创建新的内存映射。内存分配器通常使用这个系统调用来创建私有匿名映射，以分配内存。内核会按照页面大小的倍数（通常为4096字节）来分配内存，函数原型如下：

#include <unistd\.h>

void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);

一旦建立了这种映射关系，进程就可以通过指针的方式来读写这段内存，而系统会自动将脏页（被修改的页）回写到相应的磁盘文件上。这意味着进程可以通过直接访问内存来完成对文件的操作，而无需再调用read、write等系统调用函数。

除了减少read、write等系统调用外，mmap()还可以减少内存拷贝的次数。例如，在使用read调用时，典型的流程是操作系统首先将磁盘文件内容读入页缓存，然后再将数据从页缓存复制到read传递的缓冲区中。然而，使用mmap()后，操作系统只需将磁盘数据读入页缓存，然后进程可以直接通过指针方式操作mmap映射的内存，从而减少了从内核态到用户态的数据拷贝。

2、kmalloc 函数

路径：include/linux/slab.h

static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
if (__builtin_constant_p(size)) {
#ifndef CONFIG_SLOB
unsigned int index;
#endif
if (size > KMALLOC_MAX_CACHE_SIZE)
return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
index = kmalloc_index(size);

if (!index)
return ZERO_SIZE_PTR;

return kmem_cache_alloc_trace(
kmalloc_caches[kmalloc_type(flags)][index],
flags, size);
#endif
}
return __kmalloc(size, flags);
}