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

[Jens Axboe <axboe@kernel.dk>]

On 8/15/24 7:45 PM, Ming Lei wrote:
> On Thu, Aug 15, 2024 at 07:24:16PM -0600, Jens Axboe wrote:
>> On 8/15/24 5:44 PM, Ming Lei wrote:
>>> On Thu, Aug 15, 2024 at 06:11:13PM +0100, Pavel Begunkov wrote:
>>>> On 8/15/24 15:33, Jens Axboe wrote:
>>>>> 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?
>>>>
>>>> Which is a good question, I haven't thought about it, but I tend to
>>>> agree with Jens. Because vfs_fallocate is created synchronous
>>>> IORING_OP_FALLOCATE is slow for anything but pretty large requests.
>>>> Probably can be patched up, which would  involve changing the
>>>> fops->fallocate protot, but I'm not sure async there makes sense
>>>> outside of bdev (?), and cmd approach is simpler, can be made
>>>> somewhat more efficient (1 less layer in the way), and it's not
>>>> really something completely new since we have it in ioctl.
>>>
>>> Yeah, we have ioctl(DISCARD), which acquires filemap_invalidate_lock,
>>> same with blkdev_fallocate().
>>>
>>> But this patch drops this exclusive lock, so it becomes async friendly,
>>> but may cause stale page cache. However, if the lock is required, it can't
>>> be efficient anymore and io-wq may be inevitable, :-)
>>
>> If you want to grab the lock, you can still opportunistically grab it.
>> For (by far) the common case, you'll get it, and you can still do it
>> inline.
> 
> If the lock is grabbed in the whole cmd lifetime, it is basically one sync
> interface cause there is at most one async discard cmd in-flight for each
> device.

Oh for sure, you could not do that anyway as you'd be holding a lock
across the syscall boundary, which isn't allowed.

> Meantime the handling has to move to io-wq for avoiding to block current
> context, the interface becomes same with IORING_OP_FALLOCATE?

I think the current truncate is overkill, we should be able to get by
without. And no, I will not entertain an option that's "oh just punt it
to io-wq".

-- 
Jens Axboe