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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 19, 2024 at 02:01:21PM -0600, Jens Axboe wrote:
> 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.

Indeed.

> 
> > 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".

BTW, the truncate is added by 351499a172c0 ("block: Invalidate cache on discard v2"),
and block/009 serves as regression test for covering page cache
coherency and discard.

Here the issue is actually related with the exclusive lock of
filemap_invalidate_lock(). IMO, it is reasonable to prevent page read during
discard for not polluting page cache. block/009 may fail too without the lock.

It is just that concurrent discards can't be allowed any more by
down_write() of rw_semaphore, and block device is really capable of doing
that. It can be thought as one regression of 7607c44c157d ("block: Hold invalidate_lock in
BLKDISCARD ioctl").

Cc Jan Kara and Shin'ichiro Kawasaki.


Thanks,
Ming