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

Ming Lei
2024-08-16T01:45:24Z

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.

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

thanks,
Ming