[PATCH v2 0/8] mm/khugepaged: collapse_file() cleanups

[Pedro Falcato <pfalcato@suse.de>]

Here are a bunch of cleanups for collapse_file() that attempt to aid
code readability and understanding. This is part 1 out of N, and only
attempts to address the main collapse loop. The exit path is still a
mess but I figure it's much easier to review if I send this batch as-is,
versus sending the whole thing.

Tested using the khugepaged shmem and file selftests.

Based on top of latest mm-core/for-next.

Note that some review comments from v1 went unaddressed
(Nico's larger restructuring, some small Lorenzo suggestions) for a couple
of small reasons. The main one being that I want to get this v2 out before
going insane :)) I agreed to work with Nico, and I'll either send out some
of his patches as a followup, or he will.

In any case, I'm tired of this function and I hope this kicks off some
discussion :)

Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org

Changes in v2:
- Rebased on current mm-core/for-next, which contains a number of fixes to
  the amazingly simple and awesome collapse_file() function.
- Fix the "bad unlock balance" reported by syzbot ci (syzbot ci, Lorenzo)
- 1/8:
  - Fold in v1 5/8 (kerneldoc); new helpers get their kerneldoc in
    the patch that introduces them. (Lorenzo)
  - Group the input fields of collapse_file_state and make end and
    is_shmem const; is_shmem is now a bool. (Lorenzo)
  - Wrap the prepare_collapse_file_folio() declaration to fit in 80
    columns. (Lorenzo)
  - Keep the original LRU drain behaviour (drain only after a
    successful shmem_get_folio()) (Lorenzo)
- 2/8: New patch. Turn value entries into NULL before calling
  prepare_collapse_file_folio().
- 3/8 (v1 2/8):
  - Rename collapse_read_folio() to maybe_read_folio(). (Lorenzo)
  - Replace the compound condition with an early-return guard.
    (Lorenzo)
  - Comment on why the xa_lock is dropped. (Lorenzo)
  - Expand the commit message. (Lorenzo)
- 5/8 (v1 4/8): Drop a duplicate blank line.
- 6/8: Add a commit message, noting that errors before try_to_unmap()
  no longer call try_to_unmap_flush() themselves.
- 7/8: Explain in the commit message why the refcount and dirty checks
  don't need the i_pages lock. The flush added by 854e5ff8b3ce now
  happens at out_putback.
- 8/8: Retitle (drop "fix"), since 854e5ff8b3ce already reworded the
  comment; drop a stray blank line.

  Link to v1: https://lore.kernel.org/all/20260720142913.846902-1-pfalcato@suse.de/

Pedro Falcato (8):
  mm/khugepaged: separate out windy folio logic from collapse_file
  mm/khugepaged: sanitize the folio passed to file folio collapse logic
  mm/khugepaged: factor out page cache folio reading
  mm/khugepaged: factor out and simplify dirty/writeback handling
  mm/khugepaged: simplify prepare folio locking and exit paths
  mm/khugepaged: hoist isolation into collapse_isolate_folio()
  mm/khugepaged: hoist more code into collapse_isolate_folio()
  mm/khugepaged: flesh out try_to_unmap_flush() comment

 mm/khugepaged.c | 461 ++++++++++++++++++++++++++++++------------------
 1 file changed, 286 insertions(+), 175 deletions(-)


base-commit: 8d21e3d3faf19f461f6d0380fe8a1f5bdba97f25
-- 
2.56.0