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From: Selva Jove <selvajove@gmail.com>
To: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: SelvaKumar S <selvakuma.s1@samsung.com>,
	 "linux-nvme@lists.infradead.org"
	<linux-nvme@lists.infradead.org>,
	"kbusch@kernel.org" <kbusch@kernel.org>,
	 "axboe@kernel.dk" <axboe@kernel.dk>, "hch@lst.de" <hch@lst.de>,
	"sagi@grimberg.me" <sagi@grimberg.me>,
	 "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	 "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	 "snitzer@redhat.com" <snitzer@redhat.com>,
	"joshiiitr@gmail.com" <joshiiitr@gmail.com>,
	 "nj.shetty@samsung.com" <nj.shetty@samsung.com>,
	"joshi.k@samsung.com" <joshi.k@samsung.com>,
	 "javier.gonz@samsung.com" <javier.gonz@samsung.com>,
	"kch@kernel.org" <kch@kernel.org>,
	 "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [RFC PATCH v5 2/4] block: add simple copy support
Date: Wed, 14 Apr 2021 12:28:39 +0530	[thread overview]
Message-ID: <CAHqX9vYPVwYLT5Bdk_GqKZWxpJNib9fYidEmT0j+bRikwyLgKw@mail.gmail.com> (raw)
In-Reply-To: <BL0PR04MB65146169A9C7527280C15D4AE74F9@BL0PR04MB6514.namprd04.prod.outlook.com>

I agree with you. Will remove BLKDEV_COPY_NOEMULATION.

On Tue, Apr 13, 2021 at 6:03 AM Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> On 2021/04/12 23:35, Selva Jove wrote:
> > On Mon, Apr 12, 2021 at 5:55 AM Damien Le Moal <Damien.LeMoal@wdc.com> 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

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  reply	other threads:[~2021-04-14  6:59 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20210219124555epcas5p1334e7c4d64ada5dc4a2ca0feb48c1d44@epcas5p1.samsung.com>
2021-02-19 12:45 ` [RFC PATCH v5 0/4] " SelvaKumar S
     [not found]   ` <CGME20210219124559epcas5p41da46f1c248e334953d407a154697903@epcas5p4.samsung.com>
2021-02-19 12:45     ` [RFC PATCH v5 1/4] block: make bio_map_kern() non static SelvaKumar S
     [not found]   ` <CGME20210219124603epcas5p33add0f2c1781b2a4d71bf30c9e1ac647@epcas5p3.samsung.com>
2021-02-19 12:45     ` [RFC PATCH v5 2/4] block: add simple copy support SelvaKumar S
2021-02-20  4:59       ` Damien Le Moal
2021-04-07 11:32         ` Selva Jove
2021-04-12  0:24           ` Damien Le Moal
2021-04-12 14:34             ` Selva Jove
2021-04-13  0:32               ` Damien Le Moal
2021-04-14  6:58                 ` Selva Jove [this message]
     [not found]   ` <CGME20210219124608epcas5p2a673f9e00c3e7b5352f115497b0e2d98@epcas5p2.samsung.com>
2021-02-19 12:45     ` [RFC PATCH v5 3/4] nvme: " SelvaKumar S
2021-02-20  3:36       ` Matthew Wilcox
2021-02-22 15:57         ` Selva Jove
     [not found]   ` <CGME20210219124611epcas5p1c775b63b537e75da161556e375fcf05e@epcas5p1.samsung.com>
2021-02-19 12:45     ` [RFC PATCH v5 4/4] dm kcopyd: add simple copy offload support SelvaKumar S
2021-02-20 18:01   ` [RFC PATCH v5 0/4] add simple copy support David Laight
2021-02-20 19:08     ` Matthew Wilcox
2021-02-20 19:19     ` Keith Busch
2021-02-21 23:52   ` Dave Chinner
2021-02-23  9:14     ` Selva Jove
2021-02-22  1:31   ` Ming Lei
2021-02-22  6:52   ` Su Yue
2021-02-23  9:00     ` Selva Jove
2021-04-10  0:21   ` Max Gurtovoy
2021-04-10  0:29     ` Chaitanya Kulkarni
2021-04-10  6:32       ` Javier González
2021-04-11  9:10         ` Max Gurtovoy
2021-04-11 19:26           ` Javier González
2021-04-13 15:38             ` Max Gurtovoy
2021-04-13 18:25               ` Javier González
2021-04-13 18:36                 ` Chaitanya Kulkarni

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