小星星镇楼
后端有时候rm，会出现一些问题。
这里作为一个子问题，讨论一下rm之后，发生的一些事。
打开rm源码：
[qianzichen@dev03v /src/app/coreutils/coreutils-8.21]$ vi src/rm.c
从main函数开始：
int
main (int argc, char **argv)
{
...
while ((c = getopt_long (argc, argv, "dfirvIR", long_opts, NULL)) != -1)
{
switch (c)
{
case 'f':
x.interactive = RMI_NEVER;
break;
...
}
}
...
enum RM_status status = rm (file, &x);
}
首先解析命令行参数，然后调用了rm：
enum RM_status status = rm (file, &x);
作者把rm函数的实现从rm.c中抽了出来，放在remove.c中：
/* Remove FILEs, honoring options specified via X.
Return RM_OK if successful. */
enum RM_status
rm (char *const *file, struct rm_options const *x)
{
enum RM_status rm_status = RM_OK;
if (*file)
{
FTS *fts = xfts_open (file, bit_flags, NULL);
while (1)
{
...
}
}
...
}
file参数是一个只读指针数组，代表要删除的文件名列表，x参数的结构定义如下，存储从命令行中解析后的rm的选项。
struct rm_options
{
/* If true, ignore nonexistent files. */
bool ignore_missing_files;
/* If true, query the user about whether to remove each file. */
enum rm_interactive interactive;
...
/* If true, recursively remove directories. */
bool recursive;
bool require_restore_cwd;
};
当file列表存在时，rm调用xfts_open：
FTS *
xfts_open (char * const *argv, int options,
int (*compar) (const FTSENT **, const FTSENT **))
{
FTS *fts = fts_open (argv, options | FTS_CWDFD, compar);
if (fts == NULL)
{
...
return fts;
}
xfts_open返回fts_open的有效返回值。fts_open的实现如下：
FTS *
fts_open (char * const *argv,
register int options,
int (*compar) (FTSENT const **, FTSENT const **))
{
register FTS *sp;
/* Options check. */
/* Allocate/initialize the stream */
/* Initialize fts_cwd_fd. */
sp->fts_cwd_fd = AT_FDCWD;
if ( ISSET(FTS_CWDFD) && ! HAVE_OPENAT_SUPPORT)
{
int fd = open (".",
O_SEARCH | (ISSET (FTS_NOATIME) ? O_NOATIME : 0));
/*
* Start out with 1K of file name space, and enough, in any case,
* to hold the user's file names.
*/
/* Allocate/initialize root's parent. */
if (*argv != NULL) {
if ((parent = fts_alloc(sp, "", 0)) == NULL)
goto mem2;
parent->fts_level = FTS_ROOTPARENTLEVEL;
}
/* Allocate/initialize root(s). */
for (root = NULL, nitems = 0; *argv != NULL; ++argv, ++nitems) {
/*
* If comparison routine supplied, traverse in sorted
* order; otherwise traverse in the order specified.
*/
if (compar) {
p->fts_link = root;
root = p;
} else {
p->fts_link = NULL;
if (root == NULL)
tmp = root = p;
else {
tmp->fts_link = p;
tmp = p;
}
}
}
if (compar && nitems > 1)
root = fts_sort(sp, root, nitems);
...
if (!ISSET(FTS_NOCHDIR) && !ISSET(FTS_CWDFD)
&& (sp->fts_rfd = diropen (sp, ".")) < 0)
SET(FTS_NOCHDIR);
i_ring_init (&sp->fts_fd_ring, -1);
return (sp);
mem3: fts_lfree(root);
...
return (NULL);
}
引用中已去除了一些Error handling，可以看出主要是获取文件系统的一些信息，保存在FTS结构中，FTS结构定义如下：
typedef struct {
struct _ftsent *fts_cur; /* current node */
int (*fts_compar) (struct _ftsent const **, struct _ftsent const **);
/* compare fn */
...
int fts_options; /* fts_open options, global flags */
struct hash_table *fts_leaf_optimization_works_ht;
union {
...
struct cycle_check_state *state;
} fts_cycle;
I_ring fts_fd_ring;
} FTS;
再回到rm函数，它将在一个loop中通过fts_read读取文件系统信息，并缓存在ent中：
rm (char *const *file, struct rm_options const *x)
{
enum RM_status rm_status = RM_OK;
if (*file)
{
FTS *fts = xfts_open (file, bit_flags, NULL);
while (1)
{
ent = fts_read (fts);
enum RM_status s = rm_fts (fts, ent, x);
}
}
...
}
ent的结构比较大，这里不展开了。
再通过rm_fts对某一个ent进行操作，这里我们rm的是一个regular file，所以控制结构会执行到FTS_F分支下，最终调用execise。
static enum RM_status
rm_fts (FTS *fts, FTSENT *ent, struct rm_options const *x)
{
switch (ent->fts_info)
{
case FTS_D: /* preorder directory */
if (s == RM_OK && is_empty_directory == T_YES)
{
/* When we know (from prompt when in interactive mode)
that this is an empty directory, don't prompt twice. */
s = excise (fts, ent, x, true);
fts_skip_tree (fts, ent);
}
...
}
case FTS_F: /* regular file */
{
bool is_dir = ent->fts_info == FTS_DP || ent->fts_info == FTS_DNR;
enum RM_status s = prompt (fts, ent, is_dir, x, PA_REMOVE_DIR, NULL);
if (s != RM_OK)
return s;
return excise (fts, ent, x, is_dir);
}
...
}
}
这里再次忽略一些容错和优化，execise最终调用了unlinkat
static enum RM_status
excise (FTS *fts, FTSENT *ent, struct rm_options const *x, bool is_dir)
{
int flag = is_dir ? AT_REMOVEDIR : 0;
if (unlinkat (fts->fts_cwd_fd, ent->fts_accpath, flag) == 0)
{
if (x->verbose)
{
printf ((is_dir
? _("removed directory: %s\n")
...
}
return RM_OK;
}
...
}
如上我们看出，rm最终调用了unlinkat这一核心函数，比如，删除a.txt：
unlinkat(AT_FDCWD, "a.txt", 0)
用户态rm调用了C库中的unlinkat，经查找，其声明是在<unistd.h>中
#ifdef __USE_ATFILE
/* Remove the link NAME relative to FD. */
extern int unlinkat (int __fd, const char *__name, int __flag)
__THROW __nonnull ((2));
#endif
/* Remove the directory PATH. */
extern int rmdir (const char *__path) __THROW __nonnull ((1));
用户态进程只要调用unlink函数就可以了，具体unlinkat函数的实现是由glibc
提供的，其定义在io/unlink.c中：
* Remove the link named NAME. */
int
__unlink (name)
const char *name;
{
if (name == NULL)
{
__set_errno (EINVAL);
return -1;
}
__set_errno (ENOSYS);
return -1;
}
stub_warning (unlink)
weak_alias (__unlink, unlink)
额好吧，这儿是个弱符号，真正的实现在./sysdeps/unix/sysv/linux/unlinkat.c
...
/* Remove the link named NAME. */
int
unlinkat (fd, file, flag)
int fd;
const char *file;
int flag;
{
int result;
#ifdef __NR_unlinkat
# ifndef __ASSUME_ATFCTS
if (__have_atfcts >= 0)
# endif
{
result = INLINE_SYSCALL (unlinkat, 3, fd, file, flag);
# ifndef __ASSUME_ATFCTS
if (result == -1 && errno == ENOSYS)
__have_atfcts = -1;
else
# endif
return result;
}
char *buf = NULL;
}
...
INTERNAL_SYSCALL_DECL (err);
if (flag & AT_REMOVEDIR)
result = INTERNAL_SYSCALL (rmdir, err, 1, file);
else
result = INTERNAL_SYSCALL (unlink, err, 1, file);
...
}
syscall的name为_NR##name，通过宏中字符串粘合而得本例中的__NR_unlinkat。其定义在/usr/include/asm/unistd_64.h中。
#ifndef _ASM_X86_UNISTD_64_H
#define _ASM_X86_UNISTD_64_H 1
#define __NR_read 0
#define __NR_write 1
...
#define __NR_newfstatat 262
#define __NR_unlinkat 263
...
#define __NR_kexec_file_load 320
#define __NR_userfaultfd 323
#endif /* _ASM_X86_UNISTD_64_H */
所以该宏被启用。
/* The *at syscalls were introduced just after 2.6.16-rc1. Due to the way the
kernel versions are advertised we can only rely on 2.6.17 to have
the code. On PPC they were introduced in 2.6.17-rc1,
on SH in 2.6.19-rc1. */
#if __LINUX_KERNEL_VERSION >= 0x020611 \
&& (!defined __sh__ || __LINUX_KERNEL_VERSION >= 0x020613)
# define __ASSUME_ATFCTS 1
#endif
显然可以看出，若kernel版本在2.6.17之后，__ASSUME_ATFCTS宏被启用。无需校验__have_atfcts >= 0，直接调用INLINE_SYSCALL (unlinkat, 3, fd, file, flag)。
这里直接看底层实现吧（./sysdeps/unix/sysv/linux/x86_64/sysdep.h），是一段内联汇编：
# undef INLINE_SYSCALL_TYPES
# define INLINE_SYSCALL_TYPES(name, nr, args...) \
({ \
unsigned long int resultvar = INTERNAL_SYSCALL_TYPES (name, , nr, args); \
if (__builtin_expect (INTERNAL_SYSCALL_ERROR_P (resultvar, ), 0)) \
{ \
__set_errno (INTERNAL_SYSCALL_ERRNO (resultvar, )); \
resultvar = (unsigned long int) -1; \
} \
(long int) resultvar; })
# undef INTERNAL_SYSCALL_DECL
# define INTERNAL_SYSCALL_DECL(err) do { } while (0)
# define INTERNAL_SYSCALL_NCS(name, err, nr, args...) \
({ \
unsigned long int resultvar; \
LOAD_ARGS_##nr (args) \
LOAD_REGS_##nr \
asm volatile ( \
"syscall\n\t" \
: "=a" (resultvar) \
: "0" (name) ASM_ARGS_##nr : "memory", "cc", "r11", "cx"); \
(long int) resultvar; })
# undef INTERNAL_SYSCALL
# define INTERNAL_SYSCALL(name, err, nr, args...) \
INTERNAL_SYSCALL_NCS (__NR_##name, err, nr, ##args)
# define INTERNAL_SYSCALL_NCS_TYPES(name, err, nr, args...) \
在syscall之前先将参数传入寄存器。返回值在eax寄存器中，通常0表示成功。
从C库代码上来看，就是这么实现了的，rm实用程序调用glibc，然后再到汇编syscall -> kernel
但是当前机器安装的不一定是upstream的C库。
我们还是来亲眼看一下最终机器码是如何实现的吧，我这里直接反汇编一下：
[qianzichen@dev03v /usr/lib64]$ objdump -D -S libc.so.6 > /tmp/libc.txt
[qianzichen@dev03v /usr/lib64]$ cd /tmp
[qianzichen@dev03v /tmp]$ grep -A12 'unlinkat' libc.txt
00000000000e9c00 <unlinkat>:
e9c00: 48 63 d2 movslq %edx,%rdx
e9c03: 48 63 ff movslq %edi,%rdi
e9c06: b8 07 01 00 00 mov $0x107,%eax
e9c0b: 0f 05 syscall
e9c0d: 48 3d 00 f0 ff ff cmp $0xfffffffffffff000,%rax
e9c13: 77 02 ja e9c17 <unlinkat+0x17>
e9c15: f3 c3 repz retq
e9c17: 48 8b 15 4a 12 2d 00 mov 0x2d124a(%rip),%rdx # 3bae68 <_DYNAMIC+0x2e8>
e9c1e: f7 d8 neg %eax
e9c20: 64 89 02 mov %eax,%fs:(%rdx)
e9c23: 48 83 c8 ff or $0xffffffffffffffff,%rax
e9c27: c3 retq
e9c28: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
e9c2f: 00
00000000000e9c30 <rmdir>:
e9c30: b8 54 00 00 00 mov $0x54,%eax
e9c35: 0f 05 syscall
[qianzichen@dev03v /tmp]$
这里可以看到glibc-2.17最终使用了一些AT&T syntax Assembly language。
先用一个比较新的指令movslq，把第一个寄存器扩展到64位并复制到第二个寄存器中，不填充符号位。
下一步，将0x107这个值载入eax寄存器
随后，调用syscall指令。
打开Intel的相关芯片手册，搜索“syscall”，找到相关描述如下图。
datasheet 中关于 syscall 的描述
从这段描述中看出，syscall是Intel对64位处理器做的优化，被设计用来为操作系统提供一个平面内存模式，我的当前64位机器，syscall/sysret就和32位体系上的sysenter/sysexit的作用相似，可能和旧平台的int 80中断类似，主要是将CPU运行级别从level 3升级为level 0，操作一些应用层无法访问的资源。
从"Use CPUID to check if SYSCALL and SYSRET are available (CPUID.80000001H.EDX[bit 11] = 1)"这一句可以看出，在调用前需要置edx寄存器中的11位来使能64位平台的syscall/sysret，好的我们找出edx寄存器相关。
edx寄存器相关
之前操作edx寄存器，就是“使能bit 11位和bit 29”这种准备工作。
我们确定了，unlinkat是一个system call，rm实用程序将删除文件的任务交给操作系统，至此程序陷入内核态。
好的，我们现在到kernel下，直接搜索unlinkat：
[qianzichen@dev03v /src/linux/linux]$ grep unlinkat ./ -rn
./arch/parisc/include/uapi/asm/unistd.h:297:#define __NR_unlinkat (__NR_Linux + 281)
./arch/parisc/kernel/syscall_table.S:379: ENTRY_SAME(unlinkat)
./arch/m32r/include/uapi/asm/unistd.h:309:#define __NR_unlinkat 301
./arch/m32r/kernel/syscall_table.S:303: .long sys_unlinkat
./arch/sparc/include/uapi/asm/unistd.h:358:#define __NR_unlinkat 290
./arch/sparc/kernel/systbls_32.S:78:/*290*/ .long sys_unlinkat,
./arch/ia64/include/uapi/asm/unistd.h:279:#define __NR_unlinkat 1287
./arch/ia64/kernel/entry.S:1695: data8 sys_unlinkat
./arch/ia64/kernel/fsys.S:815: data8 0 // unlinkat
./arch/alpha/include/uapi/asm/unistd.h:420:#define __NR_unlinkat 456
./arch/alpha/kernel/systbls.S:477: .quad sys_unlinkat
...
./arch/x86/entry/syscalls/syscall_32.tbl:310:301 i386 unlinkat sys_unlinkat
./arch/x86/entry/syscalls/syscall_64.tbl:272:263 common unlinkat sys_unlinkat
...
[qianzichen@dev03v /src/linux/linux]$
直接看x86体系下的源码：
[qianzichen@dev03v /src/linux/linux]$ vi arch/x86/entry/syscalls/syscall_64.tbl
这是一个列表文件，
#
# 64-bit system call numbers and entry vectors
#
# The format is:
# <number> <abi> <name> <entry point>
#
# The abi is "common", "64" or "x32" for this file.
#
0 common read sys_read
...
261 common futimesat sys_futimesat
262 common newfstatat sys_newfstatat
263 common unlinkat sys_unlinkat
264 common renameat sys_renameat
265 common linkat sys_linkat
...
#
# x32-specific system call numbers start at 512 to avoid cache impact
# for native 64-bit operation.
#
512 x32 rt_sigaction compat_sys_rt_sigaction
...
这里看出，unlinkat对应的number是263
还记得写入eax寄存器中的值吗，是0x107。
很显然，0x107 = 1 * 16 ^ 2 + 0 * 16 ^ 1 + 7 * 16 ^ 0 = 263
common代表32/64位平台通用
user space 和 kernel space 的 system call 映射建立。
其实kernel space对编号的映射不是这么简单，这里不再展开。
我们大概知道 user space 的 unlinkat 最终在 kernel space 的 entry point 是 sys_unlinkat 就好了。
还是直接查看汇编代码吧：
[qianzichen@dev03v /src/linux/linux]$ vi arch/x86/entry/entry_64.S
...
ENTRY(entry_SYSCALL_64)
/*
* Interrupts are off on entry.
* We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
* it is too small to ever cause noticeable irq latency.
*/
SWAPGS_UNSAFE_STACK
movq %rsp, PER_CPU_VAR(rsp_scratch)
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
TRACE_IRQS_OFF
/* Construct struct pt_regs on stack */
pushq $__USER_DS
...
ja 1f /* return -ENOSYS (already in pt_regs->ax) */
movq %r10, %rcx
/*
* This call instruction is handled specially in stub_ptregs_64.
* It might end up jumping to the slow path. If it jumps, RAX
* and all argument registers are clobbered.
*/
call *sys_call_table(, %rax, 8)
...
END(entry_SYSCALL_64)
rax中存的就是这次syscall的num，即__NR_unlinkat。
ENTRY(entry_SYSCALL_64)是64位的 syscall 汇编入口点，在准备一系列寄存器之后，call *sys_call_table(, %rax, 8)将跳转到系统调用表中的偏移地址，也就是sys_call_table数组中下标为syscall num对应的函数。
sys_call_table在另一个文件中定义，这里用到了一点编译器扩展和预编译技术的一种高效用法，这里也不再展开。
/* System call table for x86-64. */
...
#define __SYSCALL_64_QUAL_(sym) sym
#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_##sym
#define __SYSCALL_64(nr, sym, qual) extern asmlinkage long __SYSCALL_64_QUAL_##qual(sym)(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
#include <asm/syscalls_64.h>
#undef __SYSCALL_64
#define __SYSCALL_64(nr, sym, qual) [nr] = __SYSCALL_64_QUAL_##qual(sym),
extern long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);
asmlinkage const sys_call_ptr_t sys_call_table[__NR_syscall_max+1] = {
/*
* Smells like a compiler bug -- it doesn't work
* when the & below is removed.
*/
[0 ... __NR_syscall_max] = &sys_ni_syscall,
#include <asm/syscalls_64.h>
};
什么时候建立syscall number和sys_unlinkat的映射呢？这要看<asm/syscalls_64.h>，这个头文件是一个过程文件，在编译时生成。原映射信息就是从上文提到的./arch/x86/entry/syscalls/syscall_64.tbl中获得。
编译出来的syscalls_64.h结果为：
__SYSCALL_COMMON(49, sys_bind, sys_bind)
__SYSCALL_COMMON(50, sys_listen, sys_listen)
...
__SYSCALL_COMMON(263, sys_unlinkat, sys_unlinkat)
__SYSCALL_COMMON就是__SYSCALL_64，如上文述sys_call_table的定义，第一个__SYSCALL_64的定义是为了将syscalls_64.h展开为函数声明，之后将__SYSCALL_64重新定义后，是为了将syscalls_64.h展开为数组成员的定义。
所以最终内核得到的，是一个只读的sys_call_table数组，下标为syscall number，指向的是内核的sys_call_ptr_t。syscall num从0开始，所以直接根据263就可以找到sys_unlinkat。
现在内核已经确定了要调用的是sys_unlinkat，那么这个函数在哪里定义的呢？经过我的一番尝试，4.9中直接找sys_unlinkat是找不到实现的，因为这个字符串可能经过预编译粘合。
我最终找到的宏是这样定义的：
...
#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)
#define SYSCALL_DEFINEx(x, sname, ...) \
SYSCALL_METADATA(sname, x, __VA_ARGS__) \
__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
#define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)
#define __SYSCALL_DEFINEx(x, name, ...) \
asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__)) \
__attribute__((alias(__stringify(SyS##name)))); \
static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__)); \
asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \
asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \
{ \
long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \
__MAP(x,__SC_TEST,__VA_ARGS__); \
__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \
return ret; \
} \
static inline long SYSC##name(__MAP(x,__SC_DECL,__VA_ARGS__))
asmlinkage long sys32_quotactl(unsigned int cmd, const char __user *special,
...
然后找到，sys_unlinkat的代码在fs/namei.c中：
4078 SYSCALL_DEFINE3(unlinkat, int, dfd, const char __user *, pathname, int, flag)
4079 {
4080 if ((flag & ~AT_REMOVEDIR) != 0)
4081 return -EINVAL;
4082
4083 if (flag & AT_REMOVEDIR)
4084 return do_rmdir(dfd, pathname);
4085
4086 return do_unlinkat(dfd, pathname);
4087 }
然后调用do_unlinkat：
3999 /*
4000 * Make sure that the actual truncation of the file will occur outside its
4001 * directory's i_mutex. Truncate can take a long time if there is a lot of
4002 * writeout happening, and we don't want to prevent access to the directory
4003 * while waiting on the I/O.
4004 */
4005 static long do_unlinkat(int dfd, const char __user *pathname)
4006 {
4007 int error;
4008 struct filename *name;
4009 struct dentry *dentry;
4010 struct path path;
4011 struct qstr last;
4012 int type;
4013 struct inode *inode = NULL;
4014 struct inode *delegated_inode = NULL;
4015 unsigned int lookup_flags = 0;
4016 retry:
4017 name = filename_parentat(dfd, getname(pathname), lookup_flags,
4018 &path, &last, &type);
4019 if (IS_ERR(name))
4020 return PTR_ERR(name);
4021
4022 error = -EISDIR;
4023 if (type != LAST_NORM)
4024 goto exit1;
4025
4026 error = mnt_want_write(path.mnt);
4027 if (error)
4028 goto exit1;
4029 retry_deleg:
4030 inode_lock_nested(path.dentry->d_inode, I_MUTEX_PARENT);
4031 dentry = __lookup_hash(&last, path.dentry, lookup_flags);
4032 error = PTR_ERR(dentry);
4033 if (!IS_ERR(dentry)) {
4034 /* Why not before? Because we want correct error value */
4035 if (last.name[last.len])
4036 goto slashes;
inode = dentry->d_inode;
4038 if (d_is_negative(dentry))
4039 goto slashes;
4040 ihold(inode);
4041 error = security_path_unlink(&path, dentry);
4042 if (error)
4043 goto exit2;
4044 error = vfs_unlink(path.dentry->d_inode, dentry, &delegated_inode);
4045 exit2:
4046 dput(dentry);
4047 }
4048 inode_unlock(path.dentry->d_inode);
4049 if (inode)
4050 iput(inode); /* truncate the inode here */
4051 inode = NULL;
4052 if (delegated_inode) {
4053 error = break_deleg_wait(&delegated_inode);
4054 if (!error)
4055 goto retry_deleg;
4056 }
4057 mnt_drop_write(path.mnt);
4058 exit1:
4059 path_put(&path);
4060 putname(name);
4061 if (retry_estale(error, lookup_flags)) {
4062 lookup_flags |= LOOKUP_REVAL;
4063 inode = NULL;
4064 goto retry;
4065 }
4066 return error;
4067
4068 slashes:
4069 if (d_is_negative(dentry))
4070 error = -ENOENT;
4071 else if (d_is_dir(dentry))
4072 error = -EISDIR;
4073 else
4074 error = -ENOTDIR;
4075 goto exit2;
4076 }
好了，读者随着我到这一步，已经看到了软件工程中比较具有美感的一个地方：4044行，调用了vfs_unlink。从user space到system call再至此，sys_unlinkat将unlinkat的任务，dispatch给操作系统的虚拟文件系统。
我们看一下vfs_unlink的实现：
3941 /**
3942 * vfs_unlink - unlink a filesystem object
3943 * @dir: parent directory
3944 * @dentry: victim
3945 * @delegated_inode: returns victim inode, if the inode is delegated.
3946 *
3947 * The caller must hold dir->i_mutex.
3948 *
3949 * If vfs_unlink discovers a delegation, it will return -EWOULDBLOCK and
3950 * return a reference to the inode in delegated_inode. The caller
3951 * should then break the delegation on that inode and retry. Because
3952 * breaking a delegation may take a long time, the caller should drop
3953 * dir->i_mutex before doing so.
3954 *
3955 * Alternatively, a caller may pass NULL for delegated_inode. This may
3956 * be appropriate for callers that expect the underlying filesystem not
3957 * to be NFS exported.
3958 */
3959 int vfs_unlink(struct inode *dir, struct dentry *dentry, struct inode **delegated_inode)
3960 {
3961 struct inode *target = dentry->d_inode;
3962 int error = may_delete(dir, dentry, 0);
3963
3964 if (error)
3965 return error;
3966
3967 if (!dir->i_op->unlink)
3968 return -EPERM;
3969
3970 inode_lock(target);
3971 if (is_local_mountpoint(dentry))
3972 error = -EBUSY;
3973 else {
3974 error = security_inode_unlink(dir, dentry);
3975 if (!error) {
3976 error = try_break_deleg(target, delegated_inode);
3977 if (error)
3978 goto out;
3979 error = dir->i_op->unlink(dir, dentry);
3980 if (!error) {
3981 dont_mount(dentry);
3982 detach_mounts(dentry);
3983 }
}
3985 }
3986 out:
3987 inode_unlock(target);
3988
3989 /* We don't d_delete() NFS sillyrenamed files--they still exist. */
3990 if (!error && !(dentry->d_flags & DCACHE_NFSFS_RENAMED)) {
3991 fsnotify_link_count(target);
3992 d_delete(dentry);
3993 }
3994
3995 return error;
3996 }
3997 EXPORT_SYMBOL(vfs_unlink);
我们看到，3979行，调用inode实例中i_op成员的unlink函数指针，这个指针才指向了真正的HAL层实现。
现在看inode结构的定义：
/*
* Keep mostly read-only and often accessed (especially for
* the RCU path lookup and 'stat' data) fields at the beginning
* of the 'struct inode'
*/
struct inode {
umode_t i_mode;
...
const struct inode_operations *i_op;
struct super_block *i_sb;
/* Stat data, not accessed from path walking */
unsigned long i_ino;
...
#ifdef CONFIG_FSNOTIFY
__u32 i_fsnotify_mask; /* all events this inode cares about */
struct fsnotify_mark_connector __rcu *i_fsnotify_marks;
#endif
#if IS_ENABLED(CONFIG_FS_ENCRYPTION)
struct fscrypt_info *i_crypt_info;
#endif
void *i_private; /* fs or device private pointer */
};
可以看到上文的inode实例中的i_op成员是一个inode_operations结构指针。
现在看inode_operations的定义：
struct inode_operations {
struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
...
int (*create) (struct inode *,struct dentry *, umode_t, bool);
int (*link) (struct dentry *,struct inode *,struct dentry *);
int (*unlink) (struct inode *,struct dentry *);
int (*symlink) (struct inode *,struct dentry *,const char *);
...
} ____cacheline_aligned;
vfs下层的各种文件系统，需要按照inode_operations中的规范，完成unlink的实现，向kernel vfs注册。
这里不展开bootloader自举之后的硬件初始化，也忽略kernel接管机器资源之后的一些register机制，直接看当前机器是怎么向vfs最终注册。
看了一下，我机器上挂载的是ext4文件系统，直接看ext4的unlink的最终注册过程：
...
3845 /*
3846 * directories can handle most operations...
3847 */
3848 const struct inode_operations ext4_dir_inode_operations = {
...
3851 .link = ext4_link,
3852 .unlink = ext4_unlink,
3853 .symlink = ext4_symlink,
...
3865 }
ext4_dir_inode_operations实例中，完成了函数指针的赋值。
直接看ext4_unlink的实现：
static int ext4_unlink(struct inode *dir, struct dentry *dentry)
{
int retval;
struct inode *inode;
struct buffer_head *bh;
struct ext4_dir_entry_2 *de;
handle_t *handle = NULL;
if (unlikely(ext4_forced_shutdown(EXT4_SB(dir->i_sb))))
return -EIO;
trace_ext4_unlink_enter(dir, dentry);
/* Initialize quotas before so that eventual writes go
* in separate transaction */
retval = dquot_initialize(dir);
if (retval)
return retval;
retval = dquot_initialize(d_inode(dentry));
if (retval)
return retval;
retval = -ENOENT;
bh = ext4_find_entry(dir, &dentry->d_name, &de, NULL);
if (IS_ERR(bh))
return PTR_ERR(bh);
if (!bh)
goto end_unlink;
inode = d_inode(dentry);
retval = -EFSCORRUPTED;
if (le32_to_cpu(de->inode) != inode->i_ino)
goto end_unlink;
handle = ext4_journal_start(dir, EXT4_HT_DIR,
EXT4_DATA_TRANS_BLOCKS(dir->i_sb));
if (IS_ERR(handle)) {
retval = PTR_ERR(handle);
handle = NULL;
goto end_unlink;
}
if (IS_DIRSYNC(dir))
ext4_handle_sync(handle);
if (inode->i_nlink == 0) {
ext4_warning_inode(inode, "Deleting file '%.*s' with no links",
dentry->d_name.len, dentry->d_name.name);
set_nlink(inode, 1);
}
retval = ext4_delete_entry(handle, dir, de, bh);
if (retval)
goto end_unlink;
dir->i_ctime = dir->i_mtime = current_time(dir);
ext4_update_dx_flag(dir);
ext4_mark_inode_dirty(handle, dir);
drop_nlink(inode);
if (!inode->i_nlink)
ext4_orphan_add(handle, inode);
inode->i_ctime = current_time(inode);
ext4_mark_inode_dirty(handle, inode);
end_unlink:
brelse(bh);
if (handle)
ext4_journal_stop(handle);
trace_ext4_unlink_exit(dentry, retval);
return retval;
}
看d_inode的实现：
static inline struct inode *d_inode(const struct dentry *dentry)
{
return dentry->d_inode;
}
d_inode(dentry)将inode信息从dentry结构中取出来，dentry结构定义如下：
struct dentry {
/* RCU lookup touched fields */
...
struct qstr d_name;
struct inode *d_inode; /* Where the name belongs to - NULL is
...
union {
struct hlist_node d_alias; /* inode alias list */
struct hlist_bl_node d_in_lookup_hash; /* only for in-lookup ones */
struct rcu_head d_rcu;
} d_u;
};
dentry这一层，不是简单的从硬盘中移除。为了高性能，当前ext4对目录做了一些缓存处理。应该是先设置标志位，然后根据sync机制回写存储。
vfs之下的机制就先不详述了，因为我也不太清楚，蛤蛤。
Linkerist
2018年1月19日于北京酒仙桥
