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linux内核中的文件描述符(六)--fd的分配--expand_files
Kernel version:2.6.14
CPU architecture:ARM920T
Author:ce123(http://blog.csdn.net/ce123)
我们先贴出expand_files函数的源码:
int expand_files(struct files_struct *files, int nr)
{
int err, expand = 0;
struct fdtable *fdt;
fdt = files_fdtable(files);
if (nr >= fdt->max_fdset || nr >= fdt->max_fds) { //我们在前面的文章中已经分析过,初始时max_fdset = 1024,max_fds = 32
if (fdt->max_fdset >= NR_OPEN || //#define NR_OPEN (1024*1024) /* Absolute upper limit on fd num */
fdt->max_fds >= NR_OPEN || nr >= NR_OPEN) {
err = -EMFILE; //max_fdset和max_fds都不能大于 NR_OPEN,否则返回 -EMFILE,即打开太多的文件
goto out;
}
expand = 1;
if ((err = expand_fdtable(files, nr)))//真正进行扩展
goto out;
}
err = expand;
out:
return err;
}
expand_files函数进行一些检查后调用expand_fdtable进行文件描述符表的扩展,下面分析expand_fdtable函数。
static int expand_fdtable(struct files_struct *files, int nr)
__releases(files->file_lock)
__acquires(files->file_lock)
{
int error = 0;
struct fdtable *fdt;
struct fdtable *nfdt = NULL;
spin_unlock(&files->file_lock);
nfdt = alloc_fdtable(nr);//根据nr重新创建一个新的fdtable
if (!nfdt) {
error = -ENOMEM;
spin_lock(&files->file_lock);
goto out;
}
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
/*
* Check again since another task may have expanded the
* fd table while we dropped the lock
*/
if (nr >= fdt->max_fds || nr >= fdt->max_fdset) { //nr值必须大于max_fds和max_fdset值,这里再次进行检查是防止另一个进程进行了expand
copy_fdtable(nfdt, fdt); //将旧的fdtable中的内容拷贝至新的fdtable
} else {
/* Somebody expanded while we dropped file_lock */
spin_unlock(&files->file_lock);
__free_fdtable(nfdt);
spin_lock(&files->file_lock);
goto out;
}
rcu_assign_pointer(files->fdt, nfdt);//用新的fdtable替换旧的fdtable
free_fdtable(fdt);//释放旧的fdtable
out:
return error;
}
我们再来看一下扩展文件描述符表的关键函数alloc_fdtable,其定义如下:
static struct fdtable *alloc_fdtable(int nr)
{
struct fdtable *fdt = NULL;
int nfds = 0;
fd_set *new_openset = NULL, *new_execset = NULL;
struct file **new_fds;
fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);
if (!fdt)
goto out;
memset(fdt, 0, sizeof(*fdt));
nfds = __FD_SETSIZE; //#define __FD_SETSIZE 1024
// #define PAGE_SHIFT 12
// #define PAGE_SIZE (1UL << PAGE_SHIFT)
/* Expand to the max in easy steps */
do {
if (nfds < (PAGE_SIZE * 8))//dfds = 1024
nfds = PAGE_SIZE * 8;
else {
nfds = nfds * 2;
if (nfds > NR_OPEN)
nfds = NR_OPEN;
}
} while (nfds <= nr);//第一次expand时,nr应该等于32
new_openset = alloc_fdset(nfds);//分配打开文件位图
new_execset = alloc_fdset(nfds);
if (!new_openset || !new_execset)
goto out;
fdt->open_fds = new_openset;
fdt->close_on_exec = new_execset;
fdt->max_fdset = nfds;//更新max_fdset值,此时这个值为32k
nfds = NR_OPEN_DEFAULT;//nfds = 32
/*
* Expand to the max in easy steps, and keep expanding it until
* we have enough for the requested fd array size.
*/
do {
#if NR_OPEN_DEFAULT < 256
if (nfds < 256)
nfds = 256;//nfds = 256(32->256->1024)
//无法超过1024,因为在最开始的就进行了检查,一定要小于current->signal->rlim[RLIMIT_NOFILE].rlim_cur)
else
#endif
if (nfds < (PAGE_SIZE / sizeof(struct file *)))
nfds = PAGE_SIZE / sizeof(struct file *);
else {
nfds = nfds * 2;
if (nfds > NR_OPEN)
nfds = NR_OPEN;
}
} while (nfds <= nr);
new_fds = alloc_fd_array(nfds);//分配文件描述符数组
if (!new_fds)
goto out;
fdt->fd = new_fds;
fdt->max_fds = nfds;//更新max_fds
fdt->free_files = NULL;
return fdt;
out:
if (new_openset)
free_fdset(new_openset, nfds);
if (new_execset)
free_fdset(new_execset, nfds);
kfree(fdt);
return NULL;
}
alloc_fd_array和alloc_fdset采用kmalloc或者vmalloc进行内存分配。
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