[PATCH v5 0/3] Optimize folio split in memory failure

[Zi Yan <ziy@nvidia.com>]

This patchset optimizes folio split operations in memory failure code by
always splitting a folio to min_order_for_split() to minimize unusable
pages, even if min_order_for_split() is non zero and memory failure code
would take the failed path eventually for a successfully split folio.
This means instead of making the entire original folio unusable memory
failure code would only make its after-split folio, which has order of
min_order_for_split() and contains HWPoison page, unusable.
For soft offline case, since the original folio is still accessible,
no split is performed if the folio cannot be split to order-0 to prevent
potential performance loss. In addition, add split_huge_page_to_order()
to improve code readability and fix kernel-doc comment format for
folio_split() and other related functions.

It is based on mm-new without V4 of this patchset.

Background
===

This patchset is a follow-up of "[PATCH v3] mm/huge_memory: do not change
split_huge_page*() target order silently."[1] and
[PATCH v4] mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split
to >0 order[2], since both are separated out as hotfixes. It improves how
memory failure code handles large block size(LBS) folios with
min_order_for_split() > 0. By splitting a large folio containing HW
poisoned pages to min_order_for_split(), the after-split folios without
HW poisoned pages could be freed for reuse. To achieve this, folio split
code needs to set has_hwpoisoned on after-split folios containing HW
poisoned pages and it is done in the hotfix in [2].

This patchset includes:
1. A patch adds split_huge_page_to_order(),
2. Patch 2 and Patch 3 of "[PATCH v2 0/3] Do not change split folio target
   order"[3],


Changelog
===
From V4[5]:
1. updated cover letter.
2. updated __split_unmapped_folio() comment and removed stale text.

From V3[4]:
1. Patch, mm/huge_memory: preserve PG_has_hwpoisoned if a folio is split
   to >0 order, is sent separately as a hotfix[2].
2. made newly added new_order const in memory_failure() and
   soft_offline_in_use_page().
3. explained in a comment why in memory_failure() after-split >0 order
   folios are still treated as if the split failed.


From V2[3]:
1. Patch 1 is sent separately as a hotfix[1].
2. set has_hwpoisoned on after-split folios if any contains HW poisoned
   pages.
3. added split_huge_page_to_order().
4. added a missing newline after variable decalaration.
5. added /* release= */ to try_to_split_thp_page().
6. restructured try_to_split_thp_page() in memory_failure().
7. fixed a typo.
8. reworded the comment in soft_offline_in_use_page() for better
   understanding.


Link: https://lore.kernel.org/all/20251017013630.139907-1-ziy@nvidia.com/ [1]
Link: https://lore.kernel.org/all/20251023030521.473097-1-ziy@nvidia.com/ [2]
Link: https://lore.kernel.org/all/20251016033452.125479-1-ziy@nvidia.com/ [3]
Link: https://lore.kernel.org/all/20251022033531.389351-1-ziy@nvidia.com/ [4]
Link: https://lore.kernel.org/all/20251030014020.475659-1-ziy@nvidia.com/ [5]

Zi Yan (3):
  mm/huge_memory: add split_huge_page_to_order()
  mm/memory-failure: improve large block size folio handling.
  mm/huge_memory: fix kernel-doc comments for folio_split() and related.

 include/linux/huge_mm.h | 22 ++++++++++++++------
 mm/huge_memory.c        | 45 ++++++++++++++++++++++-------------------
 mm/memory-failure.c     | 31 ++++++++++++++++++++++++----
 3 files changed, 67 insertions(+), 31 deletions(-)

-- 
2.51.0