MADV_COLLAPSE on file-backed mappings fails with -EINVAL when TEXT pages
are dirty. This may affect real scenarios: package/container updates,
executing binaries immediately after writing them, etc.

The issue is that collapse_file() triggers async writeback and returns
SCAN_FAIL (maps to -EINVAL), expecting khugepaged to revisit later. But
MADV_COLLAPSE is synchronous and userspace expects immediate success or a
clear retry signal.

Reproduction:
 - Copy 2MB-aligned executable to freshly mounted XFS/ext4
 - Call MADV_COLLAPSE on .text section
 - First call fails with -EINVAL (text pages dirty from copy)
 - Second call succeeds (async writeback completed)

Issue Report:
https://lore.kernel.org/all/4e26fe5e-7374-467c-a333-9dd48f85d7cc@amd.com

v2:
- Move writeback to madvise_collapse() (better abstraction, proper
  mmap_lock handling and does VMA revalidation after I/O) (Lorenzo)
- Rename to SCAN_PAGE_DIRTY to SCAN_PAGE_NOT_CLEAN and extend its use
  for all dirty/writeback folio cases that previously returned incorrect
  results (Dev) 

v1: https://lore.kernel.org/all/20251110113254.77822-1-shivankg@amd.com

Shivank Garg (2):
  mm/khugepaged: do synchronous writeback for MADV_COLLAPSE
  mm/khugepaged: map dirty/writeback pages failures to EAGAIN

 include/trace/events/huge_memory.h |  3 ++-
 mm/khugepaged.c                    | 34 +++++++++++++++++++++++++++---
 2 files changed, 33 insertions(+), 4 deletions(-)


base-commit: 4a3f8fc3adb7046e44bd1feb2f5c5fe95296894f
-- 
2.43.0