files_struct/fdtable解析
include/linux/fdtable.h
/*
* Open file table structure
*/
struct files_struct {
/*
* read mostly part
*/
atomic_t count;
bool resize_in_progress;
wait_queue_head_t resize_wait;
struct fdtable __rcu *fdt;
struct fdtable fdtab;
/*
* written part on a separate cache line in SMP
*/
spinlock_t file_lock ____cacheline_aligned_in_smp;
unsigned int next_fd;
unsigned long close_on_exec_init[1];
unsigned long open_fds_init[1];
unsigned long full_fds_bits_init[1];
struct file __rcu * fd_array[NR_OPEN_DEFAULT];
};
上述files_struct中最关键的成员是struct fdtable的fdt指针
对于小进程fork时父进程open的文件数小于NR_OPEN_DEFAULT，则fd table会直接使用files_struct里的；如果超过NR_OPEN_DEFAULT，则不会使用files_struct里的，会调用alloc_fdtable()进行分配fd table。
然后将父进程的fd table拷贝到新fork的进程的fd table。
NR_OPEN_DEFAULT定义为BITS_PER_LONG，一般为64。对于较大进程fork子进程时，父进程此时一般已经open了较多file，比如超过了64，此时就会alloc fd table，而不会使用files_struct里的default fd table；对于小进程fork子进程，此时一般就直接用了files_struct里default fd table。
fs/file.c
/*
* Allocate a new files structure and copy contents from the
* passed in files structure.
* errorp will be valid only when the returned files_struct is NULL.
*/
struct files_struct *dup_fd(struct files_struct *oldf, unsigned int max_fds, int *errorp)
{
struct files_struct *newf;
struct file **old_fds, **new_fds;
unsigned int open_files, i;
struct fdtable *old_fdt, *new_fdt;
*errorp = -ENOMEM;
newf = kmem_cache_alloc(files_cachep, GFP_KERNEL);
if (!newf)
goto out;
atomic_set(&newf->count, 1);
spin_lock_init(&newf->file_lock);
newf->resize_in_progress = false;
init_waitqueue_head(&newf->resize_wait);
newf->next_fd = 0;
new_fdt = &newf->fdtab;
new_fdt->max_fds = NR_OPEN_DEFAULT;
new_fdt->close_on_exec = newf->close_on_exec_init;
new_fdt->open_fds = newf->open_fds_init;
new_fdt->full_fds_bits = newf->full_fds_bits_init;
new_fdt->fd = &newf->fd_array[0];
spin_lock(&oldf->file_lock);
old_fdt = files_fdtable(oldf);
open_files = sane_fdtable_size(old_fdt, max_fds);
/*
* Check whether we need to allocate a larger fd array and fd set.
*/
while (unlikely(open_files > new_fdt->max_fds)) {
spin_unlock(&oldf->file_lock);
if (new_fdt != &newf->fdtab)
__free_fdtable(new_fdt);
new_fdt = alloc_fdtable(open_files - 1);
if (!new_fdt) {
*errorp = -ENOMEM;
goto out_release;
}
/* beyond sysctl_nr_open; nothing to do */
if (unlikely(new_fdt->max_fds < open_files)) {
__free_fdtable(new_fdt);
*errorp = -EMFILE;
goto out_release;
}
/*
* Reacquire the oldf lock and a pointer to its fd table
* who knows it may have a new bigger fd table. We need
* the latest pointer.
*/
spin_lock(&oldf->file_lock);
old_fdt = files_fdtable(oldf);
open_files = sane_fdtable_size(old_fdt, max_fds);
}
copy_fd_bitmaps(new_fdt, old_fdt, open_files);
old_fds = old_fdt->fd;
new_fds = new_fdt->fd;
for (i = open_files; i != 0; i--) {
struct file *f = *old_fds++;
if (f) {
get_file(f);
} else {
/*
* The fd may be claimed in the fd bitmap but not yet
* instantiated in the files array if a sibling thread
* is partway through open(). So make sure that this
* fd is available to the new process.
*/
__clear_open_fd(open_files - i, new_fdt);
}
rcu_assign_pointer(*new_fds++, f);
}
spin_unlock(&oldf->file_lock);
/* clear the remainder */
memset(new_fds, 0, (new_fdt->max_fds - open_files) * sizeof(struct file *));
rcu_assign_pointer(newf->fdt, new_fdt);
return newf;
out_release:
kmem_cache_free(files_cachep, newf);
out:
return NULL;
}
fd table里的内容
include/linux/fdtable.h
struct fdtable {
unsigned int max_fds;
struct file __rcu **fd; /* current fd array */
unsigned long *close_on_exec;
unsigned long *open_fds;
unsigned long *full_fds_bits;
struct rcu_head rcu;
};
max_fds，表示此fd table最多能容纳多少个fd；
struct file的二重指针fd，这个是指向一个数组，这个数组里的元素是struct file *指针；
open_fds，long型指针，指向一个long型数组，数组中的每个元素的每一个bit代表一个文件，如果这个bit为1，表示此文件已open；
close_on_exec，和open_fds功能一样，含义不同；
full_fds_bits，long型数组，每一个bit代表open_fds里每个元素所有bit是否都为1，如果都为1，这个bit置上；只要有一个不为1，这个bit将被clear；
根据allc_fdtable()，可以看到open_fds/close_on_exec/full_fds_bits数组是一块申请分配的，内存layout顺序依次是open_fds/close_on_exec/full_fds_bits
alloc_fdtable()
fs/file.c
static struct fdtable * alloc_fdtable(unsigned int nr)
{
struct fdtable *fdt;
void *data;
/*
* Figure out how many fds we actually want to support in this fdtable.
* Allocation steps are keyed to the size of the fdarray, since it
* grows far faster than any of the other dynamic data. We try to fit
* the fdarray into comfortable page-tuned chunks: starting at 1024B
* and growing in powers of two from there on.
*/
nr /= (1024 / sizeof(struct file *));
nr = roundup_pow_of_two(nr + 1);
nr *= (1024 / sizeof(struct file *));
/*
* Note that this can drive nr *below* what we had passed if sysctl_nr_open
* had been set lower between the check in expand_files() and here. Deal
* with that in caller, it's cheaper that way.
*
* We make sure that nr remains a multiple of BITS_PER_LONG - otherwise
* bitmaps handling below becomes unpleasant, to put it mildly...
*/
if (unlikely(nr > sysctl_nr_open))
nr = ((sysctl_nr_open - 1) | (BITS_PER_LONG - 1)) + 1;
fdt = kmalloc(sizeof(struct fdtable), GFP_KERNEL_ACCOUNT);
if (!fdt)
goto out;
fdt->max_fds = nr;
data = kvmalloc_array(nr, sizeof(struct file *), GFP_KERNEL_ACCOUNT);
if (!data)
goto out_fdt;
fdt->fd = data;
data = kvmalloc(max_t(size_t,
2 * nr / BITS_PER_BYTE + BITBIT_SIZE(nr), L1_CACHE_BYTES),
GFP_KERNEL_ACCOUNT);
if (!data)
goto out_arr;
fdt->open_fds = data;
data += nr / BITS_PER_BYTE;
fdt->close_on_exec = data;
data += nr / BITS_PER_BYTE;
fdt->full_fds_bits = data;
return fdt;
out_arr:
kvfree(fdt->fd);
out_fdt:
kfree(fdt);
out:
return NULL;
}
重点看下上述kvmalloc()，这个分配的大小是2 * nr / BITS_PER_BYTE + BITBIT_SIZE(nr)，nr表示此fd table要容纳多少个fd，nr/BITS_PER_BYTE，一个bit表示一个fd，所以这个表示容纳nr个fd需要多少个byte；
*2是因为有open_fds和close_on_exec两个大小一样的数组；
BITBIT_SIZE(nr)，这个宏定义如下。假设nr为65*64，则BITBIT_SIZE(65*64)的结果为2*8，即为两个long型，这两个long型的每个bit为1表示open_fds里一个元素（long型）所有bit均为1
#define BITBIT_NR(nr) BITS_TO_LONGS(BITS_TO_LONGS(nr)) #define BITBIT_SIZE(nr) (BITBIT_NR(nr) * sizeof(long))