Re: [RFC 5/5] block: implement io_uring discard cmd

[Jens Axboe <axboe@kernel.dk>]

On 8/14/24 7:42 PM, Ming Lei wrote:
> On Wed, Aug 14, 2024 at 6:46?PM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>
>> Add ->uring_cmd callback for block device files and use it to implement
>> asynchronous discard. Normally, it first tries to execute the command
>> from non-blocking context, which we limit to a single bio because
>> otherwise one of sub-bios may need to wait for other bios, and we don't
>> want to deal with partial IO. If non-blocking attempt fails, we'll retry
>> it in a blocking context.
>>
>> Suggested-by: Conrad Meyer <conradmeyer@meta.com>
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>> ---
>>  block/blk.h             |  1 +
>>  block/fops.c            |  2 +
>>  block/ioctl.c           | 94 +++++++++++++++++++++++++++++++++++++++++
>>  include/uapi/linux/fs.h |  2 +
>>  4 files changed, 99 insertions(+)
>>
>> diff --git a/block/blk.h b/block/blk.h
>> index e180863f918b..5178c5ba6852 100644
>> --- a/block/blk.h
>> +++ b/block/blk.h
>> @@ -571,6 +571,7 @@ blk_mode_t file_to_blk_mode(struct file *file);
>>  int truncate_bdev_range(struct block_device *bdev, blk_mode_t mode,
>>                 loff_t lstart, loff_t lend);
>>  long blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg);
>> +int blkdev_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags);
>>  long compat_blkdev_ioctl(struct file *file, unsigned cmd, unsigned long arg);
>>
>>  extern const struct address_space_operations def_blk_aops;
>> diff --git a/block/fops.c b/block/fops.c
>> index 9825c1713a49..8154b10b5abf 100644
>> --- a/block/fops.c
>> +++ b/block/fops.c
>> @@ -17,6 +17,7 @@
>>  #include <linux/fs.h>
>>  #include <linux/iomap.h>
>>  #include <linux/module.h>
>> +#include <linux/io_uring/cmd.h>
>>  #include "blk.h"
>>
>>  static inline struct inode *bdev_file_inode(struct file *file)
>> @@ -873,6 +874,7 @@ const struct file_operations def_blk_fops = {
>>         .splice_read    = filemap_splice_read,
>>         .splice_write   = iter_file_splice_write,
>>         .fallocate      = blkdev_fallocate,
>> +       .uring_cmd      = blkdev_uring_cmd,
> 
> Just be curious, we have IORING_OP_FALLOCATE already for sending
> discard to block device, why is .uring_cmd added for this purpose?

I think wiring up a bdev uring_cmd makes sense, because:

1) The existing FALLOCATE op is using vfs_fallocate, which is inherently
   sync and hence always punted to io-wq.

2) There will most certainly be other async ops that would be
   interesting to add, at which point we'd need it anyway.

3) It arguably makes more sense to have a direct discard op than use
   fallocate for this, if working on a raw bdev.

And probably others...

-- 
Jens Axboe