Re: [PATCH 2/2] block: remove the per-bio/request write hint

[Jens Axboe <axboe@kernel.dk>]

On 3/5/22 2:40 PM, Dave Chinner wrote:
> On Sat, Mar 05, 2022 at 06:19:29AM +0100, Christoph Hellwig wrote:
>> On Sat, Mar 05, 2022 at 09:12:55AM +1100, Dave Chinner wrote:
>>> AFAICT, all the filesystem/IO path passthrough plumbing for hints is
>>> now gone, and no hardware will ever receive hints.  Doesn't this
>>> mean that file_write_hint(), file->f_write_hint and iocb->ki_hint
>>> are now completely unused, too?
>>
>> No, for the reason tha you state below.  f2fs still uses it.
> 
> My point is that f2fs uses i_write_hint, not f_write_hint or
> ki_hint. IOWs, nothing in the IO path use the iocb or file write
> hints anymore because they only ever got used to set the hint for
> bios. It's now unused information.
> 
> According to the io_uring ppl, setup of unnecessary fields in the
> iocb has a measurable cost and they've done work to minimise it in
> the past. So if these fields are not actually used by anyone in the
> IO path, why should we still pay the cost calling
> ki_hint_validate(file_write_hint(file)) when setting up an iocb?

Yes, I think we should kill it. If we retain the inode hint, the f2fs
doesn't need a any changes. And it should be safe to make the per-file
fcntl hints return EINVAL, which they would on older kernels anyway.
Untested, but something like the below.


diff --git a/fs/aio.c b/fs/aio.c
index 4ceba13a7db0..eb0948bb74f1 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1478,7 +1478,6 @@ static int aio_prep_rw(struct kiocb *req, const struct iocb *iocb)
 	req->ki_flags = iocb_flags(req->ki_filp);
 	if (iocb->aio_flags & IOCB_FLAG_RESFD)
 		req->ki_flags |= IOCB_EVENTFD;
-	req->ki_hint = ki_hint_validate(file_write_hint(req->ki_filp));
 	if (iocb->aio_flags & IOCB_FLAG_IOPRIO) {
 		/*
 		 * If the IOCB_FLAG_IOPRIO flag of aio_flags is set, then
diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
index 753986ea1583..bc7c7a7d9260 100644
--- a/fs/cachefiles/io.c
+++ b/fs/cachefiles/io.c
@@ -138,7 +138,6 @@ static int cachefiles_read(struct netfs_cache_resources *cres,
 	ki->iocb.ki_filp	= file;
 	ki->iocb.ki_pos		= start_pos + skipped;
 	ki->iocb.ki_flags	= IOCB_DIRECT;
-	ki->iocb.ki_hint	= ki_hint_validate(file_write_hint(file));
 	ki->iocb.ki_ioprio	= get_current_ioprio();
 	ki->skipped		= skipped;
 	ki->object		= object;
@@ -313,7 +312,6 @@ static int cachefiles_write(struct netfs_cache_resources *cres,
 	ki->iocb.ki_filp	= file;
 	ki->iocb.ki_pos		= start_pos;
 	ki->iocb.ki_flags	= IOCB_DIRECT | IOCB_WRITE;
-	ki->iocb.ki_hint	= ki_hint_validate(file_write_hint(file));
 	ki->iocb.ki_ioprio	= get_current_ioprio();
 	ki->object		= object;
 	ki->inval_counter	= cres->inval_counter;
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 3c98ef6af97d..45076c01a2ba 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -4479,10 +4479,8 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
 	struct f2fs_inode_info *fi = F2FS_I(inode);
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
 	const bool do_opu = f2fs_lfs_mode(sbi);
-	const int whint_mode = F2FS_OPTION(sbi).whint_mode;
 	const loff_t pos = iocb->ki_pos;
 	const ssize_t count = iov_iter_count(from);
-	const enum rw_hint hint = iocb->ki_hint;
 	unsigned int dio_flags;
 	struct iomap_dio *dio;
 	ssize_t ret;
@@ -4515,8 +4513,6 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
 		if (do_opu)
 			down_read(&fi->i_gc_rwsem[READ]);
 	}
-	if (whint_mode == WHINT_MODE_OFF)
-		iocb->ki_hint = WRITE_LIFE_NOT_SET;
 
 	/*
 	 * We have to use __iomap_dio_rw() and iomap_dio_complete() instead of
@@ -4539,8 +4535,6 @@ static ssize_t f2fs_dio_write_iter(struct kiocb *iocb, struct iov_iter *from,
 		ret = iomap_dio_complete(dio);
 	}
 
-	if (whint_mode == WHINT_MODE_OFF)
-		iocb->ki_hint = hint;
 	if (do_opu)
 		up_read(&fi->i_gc_rwsem[READ]);
 	up_read(&fi->i_gc_rwsem[WRITE]);
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 9c6c6a3e2de5..e26444e977e1 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -292,21 +292,8 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
 
 	switch (cmd) {
 	case F_GET_FILE_RW_HINT:
-		h = file_write_hint(file);
-		if (copy_to_user(argp, &h, sizeof(*argp)))
-			return -EFAULT;
-		return 0;
 	case F_SET_FILE_RW_HINT:
-		if (copy_from_user(&h, argp, sizeof(h)))
-			return -EFAULT;
-		hint = (enum rw_hint) h;
-		if (!rw_hint_valid(hint))
-			return -EINVAL;
-
-		spin_lock(&file->f_lock);
-		file->f_write_hint = hint;
-		spin_unlock(&file->f_lock);
-		return 0;
+		return -EINVAL;
 	case F_GET_RW_HINT:
 		h = inode->i_write_hint;
 		if (copy_to_user(argp, &h, sizeof(*argp)))
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 23e7f93d3956..02400fd00501 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3790,7 +3790,6 @@ static int io_write_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	if (unlikely(!(req->file->f_mode & FMODE_WRITE)))
 		return -EBADF;
-	req->rw.kiocb.ki_hint = ki_hint_validate(file_write_hint(req->file));
 	return io_prep_rw(req, sqe);
 }
 
diff --git a/fs/open.c b/fs/open.c
index 9ff2f621b760..1315253e0247 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -835,7 +835,6 @@ static int do_dentry_open(struct file *f,
 	     likely(f->f_op->write || f->f_op->write_iter))
 		f->f_mode |= FMODE_CAN_WRITE;
 
-	f->f_write_hint = WRITE_LIFE_NOT_SET;
 	f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
 
 	file_ra_state_init(&f->f_ra, f->f_mapping->host->i_mapping);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e2d892b201b0..a1fc3b41cd82 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -327,7 +327,6 @@ struct kiocb {
 	void (*ki_complete)(struct kiocb *iocb, long ret);
 	void			*private;
 	int			ki_flags;
-	u16			ki_hint;
 	u16			ki_ioprio; /* See linux/ioprio.h */
 	struct wait_page_queue	*ki_waitq; /* for async buffered IO */
 	randomized_struct_fields_end
@@ -967,7 +966,6 @@ struct file {
 	 * Must not be taken from IRQ context.
 	 */
 	spinlock_t		f_lock;
-	enum rw_hint		f_write_hint;
 	atomic_long_t		f_count;
 	unsigned int 		f_flags;
 	fmode_t			f_mode;
@@ -2215,31 +2213,13 @@ static inline bool HAS_UNMAPPED_ID(struct user_namespace *mnt_userns,
 	       !gid_valid(i_gid_into_mnt(mnt_userns, inode));
 }
 
-static inline enum rw_hint file_write_hint(struct file *file)
-{
-	if (file->f_write_hint != WRITE_LIFE_NOT_SET)
-		return file->f_write_hint;
-
-	return file_inode(file)->i_write_hint;
-}
-
 static inline int iocb_flags(struct file *file);
 
-static inline u16 ki_hint_validate(enum rw_hint hint)
-{
-	typeof(((struct kiocb *)0)->ki_hint) max_hint = -1;
-
-	if (hint <= max_hint)
-		return hint;
-	return 0;
-}
-
 static inline void init_sync_kiocb(struct kiocb *kiocb, struct file *filp)
 {
 	*kiocb = (struct kiocb) {
 		.ki_filp = filp,
 		.ki_flags = iocb_flags(filp),
-		.ki_hint = ki_hint_validate(file_write_hint(filp)),
 		.ki_ioprio = get_current_ioprio(),
 	};
 }
@@ -2250,7 +2230,6 @@ static inline void kiocb_clone(struct kiocb *kiocb, struct kiocb *kiocb_src,
 	*kiocb = (struct kiocb) {
 		.ki_filp = filp,
 		.ki_flags = kiocb_src->ki_flags,
-		.ki_hint = kiocb_src->ki_hint,
 		.ki_ioprio = kiocb_src->ki_ioprio,
 		.ki_pos = kiocb_src->ki_pos,
 	};
diff --git a/include/trace/events/f2fs.h b/include/trace/events/f2fs.h
index f701bb23f83c..1779e133cea0 100644
--- a/include/trace/events/f2fs.h
+++ b/include/trace/events/f2fs.h
@@ -956,12 +956,11 @@ TRACE_EVENT(f2fs_direct_IO_enter,
 		__entry->rw	= rw;
 	),
 
-	TP_printk("dev = (%d,%d), ino = %lu pos = %lld len = %lu ki_flags = %x ki_hint = %x ki_ioprio = %x rw = %d",
+	TP_printk("dev = (%d,%d), ino = %lu pos = %lld len = %lu ki_flags = %x ki_ioprio = %x rw = %d",
 		show_dev_ino(__entry),
 		__entry->iocb->ki_pos,
 		__entry->len,
 		__entry->iocb->ki_flags,
-		__entry->iocb->ki_hint,
 		__entry->iocb->ki_ioprio,
 		__entry->rw)
 );

-- 
Jens Axboe