[PATCH 0/4] btrfs: refactor scrub entrances for each profile

[Qu Wenruo <wqu@suse.com>]

The branch is based on several recent submitted small cleanups, thus
it's better to fetch the branch from github:

https://github.com/adam900710/linux/tree/refactor_scrub

[CRAP-BUT-IT-WORKS(TM)]

Scrub is one of the area we seldom touch because:

- It's a mess
  Just check scrub_stripe() function.
  It's a function scrubbing a stripe for *all* profiles.

  It's near 400 lines for a single complex function, with double while()
  loop and several different jumps inside the loop.

  Not to mention the lack of comments for various structures.

  This should and will never happen under our current code standard.

- It just works
  I have hit more than 10 bugs during development, and I just want to
  give up the refactor, as even the code is crap, it works, passing the
  existing scrub/replace group.
  While no matter how small code change I'm doing, it always fails to pass
  the same tests.

[REFACTOR-IDEA]

The core idea here, is to get rid of one-fit-all solution for
scrub_stripe().

Instead, we explicitly separate the scrub into 3 groups (the idea is
from my btrfs-fuse project):

- Simple-mirror based profiles
  This includes SINGLE/DUP/RAID1/RAID1C* profiles.
  They have no stripe, and their repair is purely mirror based.

- Simple-stripe based profiles
  This includes RAID0/RAID10 profiles.
  They are just simple stripe (without P/Q nor rotation), with extra
  mirrors to support their repair.

- RAID56
  The most complex profiles, they have extra P/Q, and have rotation.

[REFACTOR-IMPLEMENTATION]

So we have 3 entrances for all those supported profiles:

- scrub_simple_mirror()
  For SINGLE/DUP/RAID1/RAID1C* profiles.
  Just go through each extent and scrub the extent.

- scrub_simple_stripe()
  For RAID0/RAID10 profiles.
  Instead we go each data stripe first, then inside each data stripe, we
  can call scrub_simple_mirror(), since after stripe split, RAID0 is
  just SINGLE and RAID10 is just RAID1.

- scrub_stripe() untouched for RAID56
  RAID56 still has all the complex things to do, but they can still be
  split into two types (already done by the original code)

  * data stripes
    They are no different in the verification part, RAID56 is just
    SINGLE if we ignore the repair path.
    It's only in repair path that our path divides.

    So we can reuse scrub_simple_mirror() again.

  * P/Q stripes
    They already have a dedicated function handling the case.

With all these refactors, although we have several more functions, we
get rid of:

- A double while () loop
- Several jumps inside the double loop
- Complex calculation to try to fit all profiles

And we get:

- Better comments
- More dedicated functions
- A better basis for further refactors

[INCOMING CLEANUPS]

- Use find_first_extent_item() to cleanup the RAID56 code
  This part itself can be as large this patchset already, thus will be
  in its own patchset.

- Refactor scrub_pages/scrub_parity/... structures

Qu Wenruo (4):
  btrfs: introduce a helper to locate an extent item
  btrfs: introduce dedicated helper to scrub simple-mirror based range
  btrfs: introduce dedicated helper to scrub simple-stripe based range
  btrfs: use scrub_simple_mirror() to handle RAID56 data stripe scrub

 fs/btrfs/scrub.c | 702 ++++++++++++++++++++++++++++-------------------
 1 file changed, 422 insertions(+), 280 deletions(-)

-- 
2.34.1