FUSE简介
什么是FUSE
傳統(tǒng)的文件系統(tǒng)是操作系統(tǒng)的一部分，放在操作系統(tǒng)內(nèi)核里面實(shí)現(xiàn)。Fuse(Filesystem in Userspace), 一個(gè)用戶空間文件系統(tǒng)框架，提供給我們一組用于實(shí)現(xiàn)一個(gè)文件系統(tǒng)的API，使我們可以在用戶態(tài)實(shí)現(xiàn)自已的文件系統(tǒng)。
FUSE的優(yōu)缺點(diǎn)
1）傳統(tǒng)文件系統(tǒng)都是定義在操作系統(tǒng)內(nèi)核層面上的，要操作系統(tǒng)識(shí)別一種新的文件系統(tǒng)，必需重寫(xiě)內(nèi)核，而內(nèi)核態(tài)代碼難以調(diào)試，生產(chǎn)率較低；但是用戶空間編程和調(diào)試難度較小，有更多的語(yǔ)言可以選擇（目前FUSE已經(jīng)綁定了很多語(yǔ)言，比如c++、java等），還可以復(fù)用已有的庫(kù)），從而能夠大幅提高生產(chǎn)率，極大地簡(jiǎn)少了為操作系統(tǒng)提供新的文件系統(tǒng)的工作量。
2）一些服務(wù)可以通過(guò)統(tǒng)一的文件系統(tǒng)接口來(lái)進(jìn)行訪問(wèn)，比如說(shuō)ftp、sftp、samba
3）可以把非文件的服務(wù)當(dāng)做文件來(lái)實(shí)現(xiàn)，比如把gmail提供的巨大的空間用來(lái)進(jìn)行文件存儲(chǔ)的Gmail Filesystem
4）在用戶態(tài)實(shí)現(xiàn)文件系統(tǒng)必然會(huì)引入額外的內(nèi)核態(tài)/用戶態(tài)切換帶來(lái)的開(kāi)銷(xiāo)，對(duì)性能會(huì)產(chǎn)生一定影響。
FUSE的結(jié)構(gòu)
fuse包括三個(gè)模塊：用戶空間庫(kù)，內(nèi)核模塊以及mount工具
1)用戶空間庫(kù)給程序員提供編程接口，程序員通過(guò)實(shí)現(xiàn)fuse提供的兩組接口fuse_lowlevel_ops，fuse_operations之一即可實(shí)現(xiàn)一個(gè)用戶空間文件系統(tǒng)
2)內(nèi)核模塊實(shí)現(xiàn)了一個(gè)完整文件系統(tǒng)的框架，但具體操作沒(méi)有實(shí)現(xiàn)（由程序員在用戶空間實(shí)現(xiàn)）
3)mount工具fusermount用于掛載基于fuse的文件系統(tǒng)
定義FUSE需要的函數(shù)
fuse為開(kāi)發(fā)者提供了兩組接口，分別是fuse_lowlevel_ops以及fuse_operations，開(kāi)發(fā)者只需要實(shí)現(xiàn)這兩組接口的一種即可實(shí)現(xiàn)一個(gè)用戶空間文件系統(tǒng)。
struct fuse_lowlevel_ops的成員如下所示，其中init方法在其它所有方法之前調(diào)用，用于初始化文件系統(tǒng)，fuse已經(jīng)實(shí)現(xiàn)，destroy則是在文件系統(tǒng)被卸載時(shí)做一些清理工作。用于大多數(shù)請(qǐng)求的參數(shù)都是fuse_ino_t類(lèi)型的ino，而文件系統(tǒng)提供給用戶的視圖是以文件名呈現(xiàn)的，故lookup是實(shí)現(xiàn)文件系統(tǒng)的關(guān)鍵，它在parent中查找名字name對(duì)應(yīng)的文件，并返回相應(yīng)的信息，可使用fuse_reply_entry或fuse_reply_err作為請(qǐng)求的返回。
接口中的方法對(duì)于了解過(guò)VFS的人應(yīng)該都不難理解，只要按需實(shí)現(xiàn)這些接口，你就可以定制出屬于自己的文件系統(tǒng)，這組接口的詳細(xì)說(shuō)明見(jiàn)fuse_lowlevel.h。
void(* init )(void *userdata, struct fuse_conn_info *conn) void(* destroy )(void *userdata) void(* lookup )(fuse_req_t req, fuse_ino_t parent, const char *name) void(* forget )(fuse_req_t req, fuse_ino_t ino, unsigned long nlookup) void(* getattr )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* setattr )(fuse_req_t req, fuse_ino_t ino, struct stat *attr, int to_set, struct fuse_file_info *fi) void(* readlink )(fuse_req_t req, fuse_ino_t ino) void(* mknod )(fuse_req_t req, fuse_ino_t parent, const char *name, mode_t mode, dev_t rdev) void(* mkdir )(fuse_req_t req, fuse_ino_t parent, const char *name, mode_t mode) void(* unlink )(fuse_req_t req, fuse_ino_t parent, const char *name) void(* rmdir )(fuse_req_t req, fuse_ino_t parent, const char *name) void(* symlink )(fuse_req_t req, const char *link, fuse_ino_t parent, const char *name) void(* rename )(fuse_req_t req, fuse_ino_t parent, const char *name, fuse_ino_t newparent, const char *newname) void(* link )(fuse_req_t req, fuse_ino_t ino, fuse_ino_t newparent, const char *newname) void(* open )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* read )(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off, struct fuse_file_info *fi) void(* write )(fuse_req_t req, fuse_ino_t ino, const char *buf, size_t size, off_t off, structfuse_file_info *fi) void(* flush )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* release )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* fsync )(fuse_req_t req, fuse_ino_t ino, int datasync, struct fuse_file_info *fi) void(* opendir )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* readdir )(fuse_req_t req, fuse_ino_t ino, size_t size, off_t off, struct fuse_file_info *fi) void(* releasedir )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) void(* fsyncdir )(fuse_req_t req, fuse_ino_t ino, int datasync, struct fuse_file_info *fi) void(* statfs )(fuse_req_t req, fuse_ino_t ino) void(* setxattr )(fuse_req_t req, fuse_ino_t ino, const char *name, const char *value, size_t size, int flags) void(* getxattr )(fuse_req_t req, fuse_ino_t ino, const char *name, size_t size) void(* listxattr )(fuse_req_t req, fuse_ino_t ino, size_t size) void(* removexattr )(fuse_req_t req, fuse_ino_t ino, const char *name) void(* access )(fuse_req_t req, fuse_ino_t ino, int mask) void(* create )(fuse_req_t req, fuse_ino_t parent, const char *name, mode_t mode, struct fuse_file_info*fi) void(* getlk )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi, struct flock *lock) void(* setlk )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi, struct flock *lock, int sleep) void(* bmap )(fuse_req_t req, fuse_ino_t ino, size_t blocksize, uint64_t idx) void(* ioctl )(fuse_req_t req, fuse_ino_t ino, int cmd, void *arg, struct fuse_file_info *fi, unsigned *flagsp, const void *in_buf, size_t in_bufsz, size_t out_bufszp) void(* poll )(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi, struct fuse_pollhandle *ph)
用戶實(shí)現(xiàn)的接口是如何跟這個(gè)結(jié)構(gòu)關(guān)聯(lián)起來(lái)的？
其實(shí)fuse中已經(jīng)實(shí)現(xiàn)了一組接口，在fuse_lowlevel.c中，定義了一個(gè)靜態(tài)的結(jié)構(gòu)數(shù)組，該數(shù)組的元素為一組（函數(shù)，名字）的結(jié)構(gòu)，但沒(méi)做什么實(shí)際的工作，當(dāng)fuse用戶空間的daemon從/fuse/dev中讀取到請(qǐng)求之后，它通過(guò)請(qǐng)求號(hào)來(lái)判別各個(gè)請(qǐng)求，并調(diào)用這里相應(yīng)的處理函數(shù)，如讀取到read調(diào)用時(shí)，會(huì)調(diào)用do_read進(jìn)行處理。
static struct {
void (*func)(fuse_req_t, fuse_ino_t, const void *);
const char *name;
} fuse_ll_ops[] = {
//只列舉了部分
[FUSE_LOOKUP] = { do_lookup, "LOOKUP" },
[FUSE_OPEN] = { do_open, "OPEN" },
[FUSE_READ] = { do_read, "READ" },
[FUSE_WRITE] = { do_write, "WRITE" },
[FUSE_STATFS] = { do_statfs, "STATFS" },
[FUSE_FLUSH] = { do_flush, "FLUSH" },
[FUSE_INIT] = { do_init, "INIT" },
[FUSE_OPENDIR] = { do_opendir, "OPENDIR" },
[FUSE_READDIR] = { do_readdir, "READDIR" },
[FUSE_RELEASEDIR] = { do_releasedir, "RELEASEDIR" },
[FUSE_DESTROY] = { do_destroy, "DESTROY" }
};
接下來(lái)看一下do_read的實(shí)現(xiàn)
static void do_read(fuse_req_t req, fuse_ino_t nodeid, const void *inarg)
{
struct fuse_read_in *arg = (struct fuse_read_in *) inarg;
// 如果用戶實(shí)現(xiàn)了read操作，則調(diào)用用戶空間的read，否則以沒(méi)有實(shí)現(xiàn)該調(diào)用為錯(cuò)誤響應(yīng)，這里的op就是用戶實(shí)現(xiàn)文件系統(tǒng)時(shí)實(shí)現(xiàn)的，并傳遞給fuse。
if (req->f->op.read) {
struct fuse_file_info fi;
memset(&fi, 0, sizeof(fi));
fi.fh = arg->fh;
fi.fh_old = fi.fh;
req->f->op.read(req, nodeid, arg->size, arg->offset, &fi);
} else
fuse_reply_err(req, ENOSYS);
}
從這里的實(shí)現(xiàn)可以看出，這些操作是沒(méi)有加任何鎖的，如果開(kāi)發(fā)者需要文件系統(tǒng)鎖，需要在實(shí)現(xiàn)文件系統(tǒng)時(shí)自行考慮。
fuse_operations又是怎么一回事？
對(duì)于實(shí)現(xiàn)fuse_lowlevel_ops這組接口，沒(méi)有內(nèi)核VFS相關(guān)知識(shí)的開(kāi)發(fā)者是不可能完成的，為了增強(qiáng)fuse的通用性，使更多的用戶能夠使用fuse開(kāi)發(fā)文件系統(tǒng)，fuse提供了一組更簡(jiǎn)單的接口fuse_operations，詳細(xì)說(shuō)明請(qǐng)參考fuse.h。這組接口的參數(shù)跟unix提供的系統(tǒng)調(diào)用的參數(shù)很類(lèi)似，開(kāi)發(fā)者更易理解，fuse想開(kāi)發(fā)者屏蔽了底層的相關(guān)對(duì)象，直接以文件名作為參數(shù)，只有開(kāi)發(fā)者按照自己的方式，把這組接口實(shí)現(xiàn)就可以，顯然這比上面那組接口的實(shí)現(xiàn)要簡(jiǎn)單得多。
struct fuse_operations {
int (*getattr) (const char *, struct stat *);
int (*readlink) (const char *, char *, size_t);
int (*getdir) (const char *, fuse_dirh_t, fuse_dirfil_t);
int (*mknod) (const char *, mode_t, dev_t);
int (*mkdir) (const char *, mode_t);
int (*unlink) (const char *);
int (*rmdir) (const char *);
int (*symlink) (const char *, const char *);
int (*rename) (const char *, const char *);
int (*link) (const char *, const char *);
int (*chmod) (const char *, mode_t);
int (*chown) (const char *, uid_t, gid_t);
int (*truncate) (const char *, off_t);
int (*utime) (const char *, struct utimbuf *);
int (*open) (const char *, struct fuse_file_info *);
int (*read) (const char *, char *, size_t, off_t, struct fuse_file_info *);
int (*write) (const char *, const char *, size_t, off_t,struct fuse_file_info *);
int (*statfs) (const char *, struct statfs *);
int (*flush) (const char *, struct fuse_file_info *);
int (*release) (const char *, struct fuse_file_info *);
int (*fsync) (const char *, int, struct fuse_file_info *);
int (*setxattr) (const char *, const char *, const char *, size_t, int);
int (*getxattr) (const char *, const char *, char *, size_t);
int (*listxattr) (const char *, char *, size_t);
int (*removexattr) (const char *, const char *);
};
這些操作并非都是必需的，但是一個(gè)文件系統(tǒng)要想正常工作，就需要其中的很多函數(shù)。
提供這組接口，fuse做了什么？
fuse還是實(shí)現(xiàn)了一組fuse_lowlevel_ops的接口，在fuse.c中
static struct fuse_lowlevel_ops fuse_path_ops = {
//只列舉了部分方法
.init = fuse_lib_init,
.destroy = fuse_lib_destroy,
.lookup = fuse_lib_lookup,
.forget = fuse_lib_forget,
.getattr = fuse_lib_getattr,
.setattr = fuse_lib_setattr,
.access = fuse_lib_access,
.read = fuse_lib_read,
.readlink = fuse_lib_readlink
};
fuse實(shí)現(xiàn)的這組接口跟之前的方法不一樣，不是什么都不做，它完成了部分工作，主要是文件節(jié)點(diǎn)與文件名的轉(zhuǎn)換關(guān)系，然后將文件名作為參數(shù)，調(diào)用用戶實(shí)現(xiàn)的fuse_operations的接口。
如fuse_lib_read的實(shí)現(xiàn)
int fuse_fs_read(struct fuse_fs *fs, const char *path, char *buf, size_t size,
off_t off, struct fuse_file_info *fi)
{
fuse_get_context()->private_data = fs->user_data;
//用戶實(shí)現(xiàn)的方法
if (fs->op.read)
return fs->op.read(path, buf, size, off, fi);
else
return -ENOSYS;
}
static void fuse_lib_read(fuse_req_t req, fuse_ino_t ino, size_t size,
off_t off, struct fuse_file_info *fi)
{
struct fuse *f = req_fuse_prepare(req);
char *path;
char *buf;
int res;
buf = (char *) malloc(size);
if (buf == NULL) {
reply_err(req, -ENOMEM);
return;
}
res = -ENOENT;
pthread_rwlock_rdlock(&f->tree_lock); //fuse_operations使用了讀寫(xiě)鎖
//由ino獲取path
path = get_path(f, ino);
if (path != NULL) {
struct fuse_intr_data d;
if (f->conf.debug)
fprintf(stderr, "READ[%llu] %lu bytes from %llun",
(unsigned long long) fi->fh, (unsigned long) size,
(unsigned long long) off);
fuse_prepare_interrupt(f, req, &d);
res = fuse_fs_read(f->fs, path, buf, size, off, fi); //通過(guò)這個(gè)方法調(diào)用用戶實(shí)現(xiàn)的方法
fuse_finish_interrupt(f, req, &d);
free(path);
}
pthread_rwlock_unlock(&f->tree_lock);
if (res >= 0) {
if (f->conf.debug)
fprintf(stderr, " READ[%llu] %u bytesn",
(unsigned long long)fi->fh, res);
if ((size_t) res > size)
fprintf(stderr, "fuse: read too many bytes");
fuse_reply_buf(req, buf, res); //返回結(jié)果
} else
reply_err(req, res);
free(buf);
FUSE 流程
通過(guò)這幅圖可以看到三個(gè)模塊在fuse工作時(shí)所起的作用
fuse_main() (lib/helper.c)——fuse用戶空間主函數(shù)，用戶程序調(diào)用它時(shí)，fuse_main()函數(shù)解析相關(guān)參數(shù)（如mountpoint，multithreaded），并調(diào)用fuse_mount()函數(shù)，接著調(diào)用fuse_new()函數(shù)，為fuse文件系統(tǒng)數(shù)據(jù)分配存儲(chǔ)空間。最后調(diào)用fuse_loop()函數(shù)實(shí)現(xiàn)會(huì)話的接受與處理。
fuse_mount() (lib/mount.c)——?jiǎng)?chuàng)建UNIX本地套接口，創(chuàng)建并運(yùn)行子進(jìn)程fusermount。
fusermount (util/fusermount.c)——確保fuse模塊已經(jīng)加載，通過(guò)UNIX套接口返回fuse模塊的文件fd給fuse_mount()函數(shù)。
fuse_new() (lib/fuse.c)——為fuse創(chuàng)建數(shù)據(jù)結(jié)構(gòu)空間，用來(lái)存儲(chǔ)文件系統(tǒng)數(shù)據(jù)。
fuse_loop() (lib/fuse.c)( fuse_loop_mt() (lib/fuse_mt.c))——從/dev/fuse (/dev 設(shè)備文件存儲(chǔ)目錄)讀取文件系統(tǒng)調(diào)用，調(diào)用fuse_operations或fuse_lowlevel_ops結(jié)構(gòu)中的處理函數(shù)，返回調(diào)用結(jié)果給/dev/fuse
FUSE Kernel模塊由兩部分組成：
第一部分——proc文件系統(tǒng)組件：Kernel/dev.c——回應(yīng)io請(qǐng)求到/dev/fuse。fuse_dev_read()函數(shù)負(fù)責(zé)讀出文件，并將來(lái)自“l(fā)ist of request”結(jié)構(gòu)體的命令返回到調(diào)用函數(shù)。fuse_dev_write ()負(fù)責(zé)文件寫(xiě)入，并將寫(xiě)入的數(shù)據(jù)置放到“req→out”數(shù)據(jù)結(jié)構(gòu)中。
第二部分——文件系統(tǒng)調(diào)用部分：kernel/file.c，kernel/inode.c，kernel/dir.c——調(diào)用request_send()，將請(qǐng)求加入到“l(fā)ist of request”結(jié)構(gòu)體中，等待回復(fù)(reply)。
Fuse調(diào)用流程
在shell里輸入命令，請(qǐng)求通過(guò)vfs到達(dá)fuse，然后通過(guò)用戶實(shí)現(xiàn)的fuse給出的API返回調(diào)用。
Fuse處理請(qǐng)求的核心工作就是進(jìn)行隊(duì)列管理
兩個(gè)重要的數(shù)據(jù)結(jié)構(gòu) fc, req
/* A Fuse connection.
* This structure is created, when the filesystem is mounted, and is destroyed, when the
* client device is closed and the filesystem is unmounted.
*/
Struct fuse_conn
{
/** Readers of the connection are waiting on this */
wait_queue_head_t waitq; // 等待執(zhí)行請(qǐng)求的進(jìn)程的隊(duì)列
/** The list of pending requests */
struct list_head pending; // 被掛起的請(qǐng)求 的隊(duì)列
/** The list of requests being processed */
struct list_head processing; // 正在被處理的請(qǐng)求的 隊(duì)列
/** Pending interrupts */
struct list_head interrupts; // 執(zhí)行中被中斷的請(qǐng)求的 隊(duì)列
...
}
/*
*A request to the client
*/
struct fuse_req
{
/** Used to wake up the task waiting for completion of request*/
wait_queue_head_t waitq; // 請(qǐng)求的等待隊(duì)列
…
}
fuse通過(guò)fuse_session_loop來(lái)啟動(dòng)守護(hù)程序，守護(hù)程序最終會(huì)調(diào)用fuse_dev_readv, fuse_dev_readv調(diào)用request_wait，使得進(jìn)程在fc的waitq隊(duì)列上睡眠。
Static size_t fuse_dev_readv(struct file *file, const struct iovec *iov, unsigned long nr_segs, loff_t *off)
{
….
request_wait(fc);
….
}
* Wait until a request is available on the pending list
*當(dāng)前進(jìn)程一直等待，直到掛起隊(duì)列中有一個(gè)請(qǐng)求
*/
static void request_wait(struct fuse_conn *fc)
{
DECLARE_WAITQUEUE(wait, current); //定義一個(gè)隊(duì)列節(jié)點(diǎn)變量wait，其與當(dāng)前進(jìn)程相關(guān)聯(lián)
add_wait_queue_exclusive(&fc->waitq, &wait); //將wait加入到fc->waitq等待隊(duì)列中
//不斷的檢查fc的pending隊(duì)列及interrupts隊(duì)列，看是否有請(qǐng)求，沒(méi)有請(qǐng)求一直while循環(huán)
while (fc->connected && !request_pending(fc))
{
set_current_state(TASK_INTERRUPTIBLE);
if (signal_pending(current)) break;
spin_unlock(&fc->lock);
schedule(); //選擇一個(gè)進(jìn)程運(yùn)行
spin_lock(&fc->lock);
}
// 有請(qǐng)求，將進(jìn)程設(shè)為T(mén)ASK_RUNNING狀態(tài)(被喚醒，被賦予CPU使用權(quán))
set_current_state(TASK_RUNNING);
remove_wait_queue(&fc->waitq, &wait); // 將wait（當(dāng)前進(jìn)程）從等待隊(duì)列中移除
}
③// fc的pending隊(duì)列及interrupts隊(duì)列，看是否有請(qǐng)求
static int request_pending(struct fuse_conn *fc)
{
return !list_empty(&fc->pending) || !list_empty(&fc->interrupts);
}
request_send是用戶請(qǐng)求經(jīng)過(guò)vfs（如上面的圖），再到fuse operation中被調(diào)用的，它向/dev/fuse發(fā)送請(qǐng)求
void request_send(struct fuse_conn *fc, struct fuse_req *req)
{
……
queue_request(fc, req);
request_wait_answer(fc, req);
……
}
⑤static void queue_request(struct fuse_conn *fc, struct fuse_req *req)
{
list_add_tail(&req->list, &fc->pending); //將請(qǐng)求加入到pending隊(duì)列
req->state = FUSE_REQ_PENDING;
if (!req->waiting)
{
req->waiting = 1;
atomic_inc(&fc->num_waiting);
}
wake_up(&fc->waitq); //喚醒等待等列
kill_fasync(&fc->fasync, SIGIO, POLL_IN);
}
/* Called with fc->lock held. Releases, and then reacquires it. */
//該調(diào)用會(huì)在req的waitq上睡眠，fuse守護(hù)程序處理完請(qǐng)求后，會(huì)將其喚醒
static void request_wait_answer(struct fuse_conn *fc, struct fuse_req *req)
{
if (!fc->no_interrupt)
{
/* Any signal may interrupt this */
wait_answer_interruptible(fc, req);
if (req->aborted)
goto aborted;
if (req->state == FUSE_REQ_FINISHED)
return;
req->interrupted = 1;
if (req->state == FUSE_REQ_SENT)
queue_interrupt(fc, req);
}
if (req->force) {
spin_unlock(&fc->lock);
wait_event(req->waitq, req->state == FUSE_REQ_FINISHED);
spin_lock(&fc->lock);
} else {
sigset_t oldset;
/* Only fatal signals may interrupt this */
block_sigs(&oldset);
wait_answer_interruptible(fc, req);
restore_sigs(&oldset);
}
if (req->aborted)
goto aborted;
if (req->state == FUSE_REQ_FINISHED) return;
req->out.h.error = -EINTR;
req->aborted = 1;
aborted:
if (req->locked) {
/* This is uninterruptible sleep, because data is
being copied to/from the buffers of req. During
locked state, there mustn't be any filesystem
operation (e.g. page fault), since that could lead
to deadlock */
spin_unlock(&fc->lock);
wait_event(req->waitq, !req->locked);
spin_lock(&fc->lock);
}
if (req->state == FUSE_REQ_PENDING) {
list_del(&req->list);
__fuse_put_request(req);
} else if (req->state == FUSE_REQ_SENT) {
spin_unlock(&fc->lock);
wait_event(req->waitq, req->state == FUSE_REQ_FINISHED);
spin_lock(&fc->lock);
}
}
}
(左列七行)fuse守護(hù)程序處理完請(qǐng)求，最終通過(guò)fuse_dev_writev寫(xiě)回/dev/fuse，它將喚醒相應(yīng)req中waitq的等待隊(duì)列元素，從而讓文件系統(tǒng)請(qǐng)求完成request_wait_answer，獲取到結(jié)果。
⑦/**Write a single reply to a request. First the header is copied from the write buffer. The request is then *searched on the processing list by the unique ID found in the header. If found, then remove it from the list *and copy the rest of the buffer to the request. The request is finished by calling request_end()
*/
static ssize_t fuse_dev_writev(struct file *file, const struct iovec *iov, unsigned long nr_segs, loff_t *off)
{
……..
req = request_find(fc, oh.unique);
request_end(fc, req);
….
}
⑧/* * This function is called when a request is finished. Either a reply has arrived or it was aborted (and not yet *sent) or some error occurred during communication with userspace, or the device file was closed. The *requester thread is woken up (if still waiting), the 'end' callback is called if given, else the reference to the *request is released Called with fc->lock, unlocks it
*/
static void request_end(struct fuse_conn *fc, struct fuse_req *req)
{
….
wake_up(&req->waitq); //喚醒req上的等待隊(duì)列
……
}
fuse通過(guò)fuse_session_loop（或?qū)?yīng)多線程的方法）來(lái)啟動(dòng)fuse守護(hù)程序，守護(hù)程序不斷的從/dev/fuse上讀取請(qǐng)求，并處理。
int fuse_session_loop(struct fuse_session *se) //在fuse_main中會(huì)被調(diào)用，或其多線程版本
{
int res = 0;
struct fuse_chan *ch = fuse_session_next_chan(se, NULL);
size_t bufsize = fuse_chan_bufsize(ch);
char *buf = (char *) malloc(bufsize); //為channel分配好緩沖區(qū)
if (!buf) {
fprintf(stderr, "fuse: failed to allocate read buffer
");
return -1;
}
//fuse daemon, loops
while (!fuse_session_exited(se)) {
struct fuse_chan *tmpch = ch;
// 從/dev/fuse讀請(qǐng)求，會(huì)等待一直到有請(qǐng)求為止
res = fuse_chan_recv(&tmpch, buf, bufsize);
if (res == -EINTR) continue;
if (res <= 0) break;
fuse_session_process(se, buf, res, tmpch); //處理讀到的請(qǐng)求
}
free(buf);
fuse_session_reset(se);
return res < 0 ? -1 : 0;
}
②int fuse_chan_recv(struct fuse_chan **chp, char *buf, size_t size)
{
struct fuse_chan *ch = *chp;
if (ch->compat)
return ((struct fuse_chan_ops_compat24 *) &ch->op)->receive(ch, buf, size);
else
return ch->op.receive(chp, buf, size); //由下面的一段代碼可以發(fā)現(xiàn)，receive最終是通過(guò)
// fuse_kern_chan_receive實(shí)現(xiàn)的，代碼片段3分析該請(qǐng)求
}
③#define MIN_BUFSIZE 0x21000
struct fuse_chan *fuse_kern_chan_new(int fd)
{
//channel的讀寫(xiě)方法
struct fuse_chan_ops op = {
.receive = fuse_kern_chan_receive,
.send = fuse_kern_chan_send,
.destroy = fuse_kern_chan_destroy,
};
//設(shè)置bufsize大小
size_t bufsize = getpagesize() + 0x1000;
bufsize = bufsize < MIN_BUFSIZE ? MIN_BUFSIZE : bufsize;
return fuse_chan_new(&op, fd, bufsize, NULL);
}
static int fuse_kern_chan_receive(struct fuse_chan **chp, char *buf, size_t size)
{
struct fuse_chan *ch = *chp;
int err;
ssize_t res;
struct fuse_session *se = fuse_chan_session(ch);
assert(se != NULL);
// 一直輪詢，直到讀到請(qǐng)求為止
restart:
//fuse_chan_fd獲取到/dev/fuse的文件描述符，調(diào)用read系統(tǒng)調(diào)用從設(shè)備讀取請(qǐng)求
res = read(fuse_chan_fd(ch), buf, size);
//根據(jù)fuse設(shè)備驅(qū)動(dòng)程序file結(jié)構(gòu)的實(shí)現(xiàn)（dev.c），read將調(diào)用fuse_dev_read,該方法最終通過(guò)fuse_dev_readv
//實(shí)現(xiàn)，根據(jù)代碼中的注釋?zhuān)琭use_dev_read做了如下工作：
// Read a single request into the userspace filesystem's buffer. This function waits until a request is available,
// then removes it from the pending list and copies request data to userspace buffer.
// 而fuse_dev_read又調(diào)用request_wait,使得進(jìn)程在fc->waitq上睡眠
if no data： goto restart
………
}
以上的分析對(duì)應(yīng)了fuse filesystem daemon做的第一部分工作。當(dāng)用戶從控制臺(tái)輸入"rm /mnt/fuse/file"時(shí)，通過(guò)VFS（sys_unlink），再到fuse（dir.c中實(shí)現(xiàn)的inode_operations，file.c中實(shí)現(xiàn)的file_operations中的方法都會(huì)最終調(diào)用request_send，后面會(huì)講到），這個(gè)請(qǐng)求最終被發(fā)到了/dev/fuse中，該請(qǐng)求的到達(dá)會(huì)喚醒正在等待的fuse守護(hù)程序，fuse守護(hù)程序讀取該請(qǐng)求并進(jìn)行處理，接下來(lái)介紹處理請(qǐng)求所作的工作。
⑤struct fuse_session *fuse_lowlevel_new_common(struct fuse_args *args,
const struct fuse_lowlevel_ops *op,
size_t op_size, void *userdata)
{
//fuse_lowlevel_ops在之前的文章
http://blog.chinaunix.net/u2/87570/showart_2166461.html中已經(jīng)介紹//過(guò)了，開(kāi)發(fā)者實(shí)現(xiàn)了fuse_lowlevel_ops并傳遞給fuse_lowlevel_common
struct fuse_ll *f;
struct fuse_session *se;
struct fuse_session_ops sop = {
//最終調(diào)用的處理方法
.process = fuse_ll_process, //分析見(jiàn)代碼片段5
.destroy = fuse_ll_destroy,
};
…….
}
⑥static void fuse_ll_process(void *data, const char *buf, size_t len, struct fuse_chan *ch)
{
struct fuse_ll *f = (struct fuse_ll *) data;
struct fuse_in_header *in = (struct fuse_in_header *) buf;
const void *inarg = buf + sizeof(struct fuse_in_header);
struct fuse_req *req;
//創(chuàng)建并初始化一個(gè)請(qǐng)求
req = (struct fuse_req *) calloc(1, sizeof(struct fuse_req));
if (req == NULL) {
fprintf(stderr, "fuse: failed to allocate request
");
return;
}
req->f = f;
req->unique = in->unique;
……
//根據(jù)opcode調(diào)用fuse_ll_ops中相應(yīng)的方法，fuse_ll_ops的介紹
// http://blog.chinaunix.net/u2/87570/showart_2166461.html
fuse_ll_ops[in->opcode].func(req, in->nodeid, inarg);
}
}
以上代碼對(duì)應(yīng)中流程中perform unlink的工作，實(shí)際上就是執(zhí)行開(kāi)發(fā)者實(shí)現(xiàn)的一組方法來(lái)完成相關(guān)的工作，接下來(lái)就是把執(zhí)行完請(qǐng)求后需要的數(shù)據(jù)返回，最終是通過(guò)send_reply實(shí)現(xiàn)的
⑦static int send_reply(fuse_req_t req, int error, const void *arg, size_t argsize)
{
struct iovec iov[2];
int count = 1;
if (argsize) {
iov[1].iov_base = (void *) arg;
iov[1].iov_len = argsize;
count++;
}
return send_reply_iov(req, error, iov, count);
}
⑧static int send_reply_iov(fuse_req_t req, int error, struct iovec *iov, int count)
{
……
res = fuse_chan_send(req->ch, iov, count);
free_req(req);
return res;
}
⑨static int fuse_kern_chan_send(struct fuse_chan *ch, const struct iovec iov[], size_t count)
{
if (iov) {
//將數(shù)據(jù)寫(xiě)到/dev/fuse上,最終會(huì)調(diào)用fuse_dev_write
ssize_t res = writev(fuse_chan_fd(ch), iov, count);
……
return 0;
}
另外fuse接收到VFS的請(qǐng)求時(shí)，通過(guò)request_send將請(qǐng)求發(fā)送到/fuse/dev，并調(diào)用request_wait_answer等待返回結(jié)果。至于fuse使用的隊(duì)列的管理，在流程圖中也做了簡(jiǎn)單的說(shuō)明，下一篇文章將詳細(xì)分析隊(duì)列的管理。
void request_send(struct fuse_conn *fc, struct fuse_req *req)
{
req->isreply = 1;
spin_lock(&fc->lock);
if (!fc->connected)
req->out.h.error = -ENOTCONN;
else if (fc->conn_error)
req->out.h.error = -ECONNREFUSED;
else {
//將請(qǐng)求加入請(qǐng)求隊(duì)列
queue_request(fc, req);
/* acquire extra reference, since request is still needed after request_end() */
__fuse_get_request(req);
//等待結(jié)果
request_wait_answer(fc, req);
}
spin_unlock(&fc->lock);
}
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