Re: [RFC PATCH v5 2/4] block: add simple copy support

[Selva Jove]

I agree with you. Will remove BLKDEV_COPY_NOEMULATION.

On Tue, Apr 13, 2021 at 6:03 AM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
>
> On 2021/04/12 23:35, Selva Jove wrote:
> > On Mon, Apr 12, 2021 at 5:55 AM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
> >>
> >> On 2021/04/07 20:33, Selva Jove wrote:
> >>> Initially I started moving the dm-kcopyd interface to the block layer
> >>> as a generic interface.
> >>> Once I dig deeper in dm-kcopyd code, I figured that dm-kcopyd is
> >>> tightly coupled with dm_io()
> >>>
> >>> To move dm-kcopyd to block layer, it would also require dm_io code to
> >>> be moved to block layer.
> >>> It would cause havoc in dm layer, as it is the backbone of the
> >>> dm-layer and needs complete
> >>> rewriting of dm-layer. Do you see any other way of doing this without
> >>> having to move dm_io code
> >>> or to have redundant code ?
> >>
> >> Right. Missed that. So reusing dm-kcopyd and making it a common interface will
> >> take some more efforts. OK, then. For the first round of commits, let's forget
> >> about this. But I still think that your emulation could be a lot better than a
> >> loop doing blocking writes after blocking reads.
> >>
> >
> > Current implementation issues read asynchronously and once all the reads are
> > completed, then the write is issued as whole to reduce the IO traffic
> > in the queue.
> > I agree that things can be better. Will explore another approach of
> > sending writes
> > immediately once reads are completed and with  plugging to increase the chances
> > of merging.
> >
> >> [...]
> >>>>> +int blkdev_issue_copy(struct block_device *src_bdev, int nr_srcs,
> >>>>> +             struct range_entry *src_rlist, struct block_device *dest_bdev,
> >>>>> +             sector_t dest, gfp_t gfp_mask, int flags)
> >>>>> +{
> >>>>> +     struct request_queue *q = bdev_get_queue(src_bdev);
> >>>>> +     struct request_queue *dest_q = bdev_get_queue(dest_bdev);
> >>>>> +     struct blk_copy_payload *payload;
> >>>>> +     sector_t bs_mask, copy_size;
> >>>>> +     int ret;
> >>>>> +
> >>>>> +     ret = blk_prepare_payload(src_bdev, nr_srcs, src_rlist, gfp_mask,
> >>>>> +                     &payload, &copy_size);
> >>>>> +     if (ret)
> >>>>> +             return ret;
> >>>>> +
> >>>>> +     bs_mask = (bdev_logical_block_size(dest_bdev) >> 9) - 1;
> >>>>> +     if (dest & bs_mask) {
> >>>>> +             return -EINVAL;
> >>>>> +             goto out;
> >>>>> +     }
> >>>>> +
> >>>>> +     if (q == dest_q && q->limits.copy_offload) {
> >>>>> +             ret = blk_copy_offload(src_bdev, payload, dest, gfp_mask);
> >>>>> +             if (ret)
> >>>>> +                     goto out;
> >>>>> +     } else if (flags & BLKDEV_COPY_NOEMULATION) {
> >>>>
> >>>> Why ? whoever calls blkdev_issue_copy() wants a copy to be done. Why would that
> >>>> user say "Fail on me if the device does not support copy" ??? This is a weird
> >>>> interface in my opinion.
> >>>>
> >>>
> >>> BLKDEV_COPY_NOEMULATION flag was introduced to allow blkdev_issue_copy() callers
> >>> to use their native copying method instead of the emulated copy that I
> >>> added. This way we
> >>> ensure that dm uses the hw-assisted copy and if that is not present,
> >>> it falls back to existing
> >>> copy method.
> >>>
> >>> The other users who don't have their native emulation can use this
> >>> emulated-copy implementation.
> >>
> >> I do not understand. Emulation or not should be entirely driven by the device
> >> reporting support for simple copy (or not). It does not matter which component
> >> is issuing the simple copy call: an FS to a real device, and FS to a DM device
> >> or a DM target driver. If the underlying device reported support for simple
> >> copy, use that. Otherwise, emulate with read/write. What am I missing here ?
> >>
> >
> > blkdev_issue_copy() api will generally complete the copy-operation,
> > either by using
> > offloaded-copy or by using emulated-copy. The caller of the api is not
> > required to
> > figure the type of support. However, it can opt out of emulated-copy
> > by specifying
> > the flag BLKDEV_NOEMULATION. This is helpful for the case when the
> > caller already
> > has got a sophisticated emulation (e.g. dm-kcopyd users).
>
> This does not make any sense to me. If the user has already another mean of
> doing copies, then that user will not call blkdev_issue_copy(). So I really do
> not understand what the "opting out of emulated copy" would be useful for. That
> user can check the simple copy support glag in the device request queue and act
> accordingly: use its own block copy code when simple copy is not supported or
> use blkdev_issue_copy() when the device has simple copy. Adding that
> BLKDEV_COPY_NOEMULATION does not serve any purpose at all.
>
>
>
> --
> Damien Le Moal
> Western Digital Research