Re: [PATCH v7 0/6] block: Introduce REQ_ALLOCATE flag for REQ_OP_WRITE_ZEROES

[Chaitanya Kulkarni]

On 03/26/2020 07:46 AM, Christoph Hellwig wrote:
> On Thu, Mar 26, 2020 at 10:34:42AM -0400, Martin K. Petersen wrote:
>>> I just worry about the proliferation of identical merging and
>>> splitting code throughout the block stack as we add additional
>>> single-range, no payload operations (Verify, etc.). I prefer to
>>> enforce the semantics in the LLD and not in the plumbing. But I
>>> won't object to a separate REQ_OP_ALLOCATE if you find the
>>> resulting code duplication acceptable.
> I find it acceptable for now.  And I think we should find some way
> (e.g. by being table driven) to share code between differnet
> opcodes.
>

With reference to Martin's comment (verify etc) there is a significant
advantage when using payloadless bio to offload the functionality
to the directly attached device and over the fabrics when dealing
with larger disks.

How about we create a helper which is independent of the operations
can accept req_op and issues the payloadless bios. Something like
following totally untested :-

diff --git a/block/blk-lib.c b/block/blk-lib.c
index cf9e75a730b4..d3fccd3211cc 100644
--- a/block/blk-lib.c
+++ b/block/blk-lib.c
@@ -209,6 +209,33 @@ int blkdev_issue_write_same(struct block_device
*bdev, sector_t sector,
  }
  EXPORT_SYMBOL(blkdev_issue_write_same);

+static void __blkdev_issue_payloadless(struct block_device *bdev,
unsigned op,
+               sector_t sector, sector_t nr_sects, gfp_t gfp_mask,
+               struct bio **biop, unsigned bio_opf, unsigned int
max_sectors)
+{
+       struct bio *bio = *biop;
+
+       while (nr_sects) {
+               bio = blk_next_bio(bio, 0, gfp_mask);
+               bio->bi_iter.bi_sector = sector;
+               bio_set_dev(bio, bdev);
+               bio->bi_opf = op;
+               bio->bi_opf |= bio_opf;
+
+               if (nr_sects > max_sectors) {
+                       bio->bi_iter.bi_size = max_sectors << 9;
+                       nr_sects -= max_sectors;
+                       sector += max_sectors;
+               } else {
+                       bio->bi_iter.bi_size = nr_sects << 9;
+                       nr_sects = 0;
+               }
+               cond_resched();
+       }
+
+       *biop = bio;
+}
+
  static int __blkdev_issue_write_zeroes(struct block_device *bdev,
                 sector_t sector, sector_t nr_sects, gfp_t gfp_mask,
                 struct bio **biop, unsigned flags)
@@ -216,6 +243,7 @@ static int __blkdev_issue_write_zeroes(struct
block_device *bdev,
         struct bio *bio = *biop;
         unsigned int max_write_zeroes_sectors;
         struct request_queue *q = bdev_get_queue(bdev);
+       unsigned int unmap = (flags & BLKDEV_ZERO_NOUNMAP) ? REQ_NOUNMAP
: 0;

         if (!q)
                 return -ENXIO;
@@ -229,24 +257,8 @@ static int __blkdev_issue_write_zeroes(struct
block_device *bdev,
         if (max_write_zeroes_sectors == 0)
                 return -EOPNOTSUPP;

-       while (nr_sects) {
-               bio = blk_next_bio(bio, 0, gfp_mask);
-               bio->bi_iter.bi_sector = sector;
-               bio_set_dev(bio, bdev);
-               bio->bi_opf = REQ_OP_WRITE_ZEROES;
-               if (flags & BLKDEV_ZERO_NOUNMAP)
-                       bio->bi_opf |= REQ_NOUNMAP;
-
-               if (nr_sects > max_write_zeroes_sectors) {
-                       bio->bi_iter.bi_size = max_write_zeroes_sectors
<< 9;
-                       nr_sects -= max_write_zeroes_sectors;
-                       sector += max_write_zeroes_sectors;
-               } else {
-                       bio->bi_iter.bi_size = nr_sects << 9;
-                       nr_sects = 0;
-               }
-               cond_resched();
-       }
+       __blkdev_issue_payloadless(bdev, REQ_OP_WRITE_ZEROES, sector,
nr_sects,
+                       gfp_mask, biop, unmap, max_write_zeroes_sectors);

         *biop = bio;
         return 0;

I'll be happy to send out a well tested patch based on the above
suggestion or any feedback I get and re-spin this series or OP can
re-spin this series whatever works.