[PATCH v3 RESEND 0/4] btrfs: removal of on-stack paddrs[], final part

[CHANGELOG]
v3 RESEND:
- Minor commit message change
  The resend is most for proper sashiko review.

v3:
- Rebased to the latest for-next
  There is a fix in RAID56, which can cause conflicts with the last
  patch

- Remove all remaining on-stack paddrs[] usage
  There are two last ones in RAID56, one can be converted to use bio
  interface, the other is not using on-stack paddrs[] array.

  So we can finally remove all on-stack paddrs[] usage.

v2:
- Fix a missing assignment for metadata repair
  The logical should be assigned before passing it to
  btrfs_repair_bbio_failure().

- Move the commit message of error message change to the correct patch
  It's changed in the first patch not the last one.

Since the experimental bs > ps support, several on-stack fixed paddrs[]
arrays are introduced, for assemble mutli-page sized fs blocks.

However that on-stack memory usage is always there for 4K page sized
systems, no matter if the block size of the filesystem.

The idea is to use bio interface for page iterations, the core idea is
to use a const bvec_iter as the pointer to where the block is.

Then we save a local bevc_iter, and use the local iter to check the next
few pages until we fill a full block.

Furthermore, with the help of bvec_iter, we can remove a lot of
parameters:

- file_offset
- logical
- bio_offset
  All can be generated by using the @iter passed in and the
  bbio->saved_iter to calculate the old @bio_offset.

  @bio_offset is the (iter.bi_sector - saved_iter.bi_sector) <<
  SECTOR_SHIFT.
  As when bvec_iter is advanced, its bi_sector is also increased.

  @logical is simpler, just iter.bi_sector << SECTOR_SHIFT.

  @file_offset is the bbio->file_offset + bio_offset.

This means we no longer need to use on-stack paddrs[] to csum
generation.

With bio interfaces, the iteration of an fs block is as simple as the
following: (I tried to change the page/pg_off/cur_len into a macro just
like btrfs_bio_for_each_block(), but failed)

	u32 cur = 0;

	btrfs_csum_init(&cctx, fs_info->csum_type);
	while (cur < blocksize) {
		struct page *page = bio_iter_page(&bbio->bio, iter);
		const u32 pg_off = bio_iter_offset(&bbio->bio, iter);
		const u32 cur_len = min(bio_iter_len(&bbio->bio, iter), blocksize - cur);
		void *kaddr;

		kaddr = kmap_local_page(page) + pg_off;
		btrfs_csum_update(&cctx, kaddr, cur_len);
		kunmap_local(kaddr);

		bio_advance_iter_single(&bbio->bio, &iter, cur_len);
		cur += cur_len;
	}
	btrfs_csum_final(&cctx, csum);

However there is still one caller left:

- RAID56
  That will be only location left without a bio.
  In that case we can easily craft a local helper to do csum generation
  without using on-stack paddrs[].

Qu Wenruo (4):
  btrfs: replace btrfs_repair_io_failure() to use bio for page iteration
  btrfs: enhance btrfs_data_csum_ok() to use bio for page iteration
  btrfs: use a shared helper to calculate data checksum for a bio
  btrfs: remove on-stack paddrs[] array usage

 fs/btrfs/bio.c         | 140 +++++++++++++++++++----------------------
 fs/btrfs/bio.h         |   5 +-
 fs/btrfs/btrfs_inode.h |  12 ++--
 fs/btrfs/disk-io.c     |  25 +++++---
 fs/btrfs/file-item.c   |  20 ++----
 fs/btrfs/inode.c       | 114 +++++++++++----------------------
 fs/btrfs/raid56.c      |  47 ++++++++------
 7 files changed, 159 insertions(+), 204 deletions(-)

-- 
2.54.0