Re: [PATCH v4 02/10] block: Add copy offload support infrastructure

Nitesh Shetty
2022-04-27T18:09:32Z

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On Wed, Apr 27, 2022 at 12:29:15PM +0200, Hannes Reinecke wrote:
> On 4/26/22 12:12, Nitesh Shetty wrote:
> > Introduce blkdev_issue_copy which supports source and destination bdevs,
> > and an array of (source, destination and copy length) tuples.
> > Introduce REQ_COPY copy offload operation flag. Create a read-write
> > bio pair with a token as payload and submitted to the device in order.
> > Read request populates token with source specific information which
> > is then passed with write request.
> > This design is courtesy Mikulas Patocka's token based copy
> >
> > Larger copy will be divided, based on max_copy_sectors,
> > max_copy_range_sector limits.
> >
> > Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
> > Signed-off-by: Arnav Dawn <arnav.dawn@samsung.com>
> > ---
> > block/blk-lib.c | 232 ++++++++++++++++++++++++++++++++++++++
> > block/blk.h | 2 +
> > include/linux/blk_types.h | 21 ++++
> > include/linux/blkdev.h | 2 +
> > include/uapi/linux/fs.h | 14 +++
> > 5 files changed, 271 insertions(+)
> >
> > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > index 09b7e1200c0f..ba9da2d2f429 100644
> > --- a/block/blk-lib.c
> > +++ b/block/blk-lib.c
> > @@ -117,6 +117,238 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> > }
> > EXPORT_SYMBOL(blkdev_issue_discard);
> > +/*
> > + * Wait on and process all in-flight BIOs. This must only be called once
> > + * all bios have been issued so that the refcount can only decrease.
> > + * This just waits for all bios to make it through bio_copy_end_io. IO
> > + * errors are propagated through cio->io_error.
> > + */
> > +static int cio_await_completion(struct cio *cio)
> > +{
> > + int ret = 0;
> > + unsigned long flags;
> > +
> > + spin_lock_irqsave(&cio->lock, flags);
> > + if (cio->refcount) {
> > + cio->waiter = current;
> > + __set_current_state(TASK_UNINTERRUPTIBLE);
> > + spin_unlock_irqrestore(&cio->lock, flags);
> > + blk_io_schedule();
> > + /* wake up sets us TASK_RUNNING */
> > + spin_lock_irqsave(&cio->lock, flags);
> > + cio->waiter = NULL;
> > + ret = cio->io_err;
> > + }
> > + spin_unlock_irqrestore(&cio->lock, flags);
> > + kvfree(cio);
> > +
> > + return ret;
> > +}
> > +
> > +static void bio_copy_end_io(struct bio *bio)
> > +{
> > + struct copy_ctx *ctx = bio->bi_private;
> > + struct cio *cio = ctx->cio;
> > + sector_t clen;
> > + int ri = ctx->range_idx;
> > + unsigned long flags;
> > + bool wake = false;
> > +
> > + if (bio->bi_status) {
> > + cio->io_err = bio->bi_status;
> > + clen = (bio->bi_iter.bi_sector << SECTOR_SHIFT) - ctx->start_sec;
> > + cio->rlist[ri].comp_len = min_t(sector_t, clen, cio->rlist[ri].comp_len);
> > + }
> > + __free_page(bio->bi_io_vec[0].bv_page);
> > + kfree(ctx);
> > + bio_put(bio);
> > +
> > + spin_lock_irqsave(&cio->lock, flags);
> > + if (((--cio->refcount) <= 0) && cio->waiter)
> > + wake = true;
> > + spin_unlock_irqrestore(&cio->lock, flags);
> > + if (wake)
> > + wake_up_process(cio->waiter);
> > +}
> > +
> > +/*
> > + * blk_copy_offload - Use device's native copy offload feature
> > + * Go through user provide payload, prepare new payload based on device's copy offload limits.
> > + */
> > +int blk_copy_offload(struct block_device *src_bdev, int nr_srcs,
> > + struct range_entry *rlist, struct block_device *dst_bdev, gfp_t gfp_mask)
> > +{
> > + struct request_queue *sq = bdev_get_queue(src_bdev);
> > + struct request_queue *dq = bdev_get_queue(dst_bdev);
> > + struct bio *read_bio, *write_bio;
> > + struct copy_ctx *ctx;
> > + struct cio *cio;
> > + struct page *token;
> > + sector_t src_blk, copy_len, dst_blk;
> > + sector_t remaining, max_copy_len = LONG_MAX;
> > + unsigned long flags;
> > + int ri = 0, ret = 0;
> > +
> > + cio = kzalloc(sizeof(struct cio), GFP_KERNEL);
> > + if (!cio)
> > + return -ENOMEM;
> > + cio->rlist = rlist;
> > + spin_lock_init(&cio->lock);
> > +
> > + max_copy_len = min_t(sector_t, sq->limits.max_copy_sectors, dq->limits.max_copy_sectors);
> > + max_copy_len = min3(max_copy_len, (sector_t)sq->limits.max_copy_range_sectors,
> > + (sector_t)dq->limits.max_copy_range_sectors) << SECTOR_SHIFT;
> > +
> > + for (ri = 0; ri < nr_srcs; ri++) {
> > + cio->rlist[ri].comp_len = rlist[ri].len;
> > + src_blk = rlist[ri].src;
> > + dst_blk = rlist[ri].dst;
> > + for (remaining = rlist[ri].len; remaining > 0; remaining -= copy_len) {
> > + copy_len = min(remaining, max_copy_len);
> > +
> > + token = alloc_page(gfp_mask);
> > + if (unlikely(!token)) {
> > + ret = -ENOMEM;
> > + goto err_token;
> > + }
> > +
> > + ctx = kzalloc(sizeof(struct copy_ctx), gfp_mask);
> > + if (!ctx) {
> > + ret = -ENOMEM;
> > + goto err_ctx;
> > + }
> > + ctx->cio = cio;
> > + ctx->range_idx = ri;
> > + ctx->start_sec = dst_blk;
> > +
> > + read_bio = bio_alloc(src_bdev, 1, REQ_OP_READ | REQ_COPY | REQ_NOMERGE,
> > + gfp_mask);
> > + if (!read_bio) {
> > + ret = -ENOMEM;
> > + goto err_read_bio;
> > + }
> > + read_bio->bi_iter.bi_sector = src_blk >> SECTOR_SHIFT;
> > + __bio_add_page(read_bio, token, PAGE_SIZE, 0);
> > + /*__bio_add_page increases bi_size by len, so overwrite it with copy len*/
> > + read_bio->bi_iter.bi_size = copy_len;
> > + ret = submit_bio_wait(read_bio);
> > + bio_put(read_bio);
> > + if (ret)
> > + goto err_read_bio;
> > +
> > + write_bio = bio_alloc(dst_bdev, 1, REQ_OP_WRITE | REQ_COPY | REQ_NOMERGE,
> > + gfp_mask);
> > + if (!write_bio) {
> > + ret = -ENOMEM;
> > + goto err_read_bio;
> > + }
> > + write_bio->bi_iter.bi_sector = dst_blk >> SECTOR_SHIFT;
> > + __bio_add_page(write_bio, token, PAGE_SIZE, 0);
> > + /*__bio_add_page increases bi_size by len, so overwrite it with copy len*/
> > + write_bio->bi_iter.bi_size = copy_len;
> > + write_bio->bi_end_io = bio_copy_end_io;
> > + write_bio->bi_private = ctx;
> > +
> > + spin_lock_irqsave(&cio->lock, flags);
> > + ++cio->refcount;
> > + spin_unlock_irqrestore(&cio->lock, flags);
> > +
> > + submit_bio(write_bio);
> > + src_blk += copy_len;
> > + dst_blk += copy_len;
> > + }
> > + }
> > +
>
> Hmm. I'm not sure if I like the copy loop.
> What I definitely would do is to allocate the write bio before reading data;
> after all, if we can't allocate the write bio reading is pretty much
> pointless.
>
> But the real issue I have with this is that it's doing synchronous reads,
> thereby limiting the performance.
>
> Can't you submit the write bio from the end_io function of the read bio?
> That would disentangle things, and we should be getting a better
> performance.
>

Agree, it will make code efficient.

--
Thank you
Nitesh Shetty

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