selftests/mm: add khugepaged race harness

Collapse serialises against faults, GUP, fork, mremap and zapping through
a protocol of locks, TLB flushes and refcount checks.  No khugepaged
selftest exercises any of it under contention.

Add khugepaged_race.  Six racing threads work the same address space:

  - two faulters
  - an MADV_DONTNEED thread
  - a transient FOLL_PIN thread (gup_test)
  - a forker
  - an mremap thread

One of three drivers collapses under them:

  stepped   khugepaged, one full pass at a time via
            khugepaged_full_pass(), so each step covers a known extent;
  free      khugepaged left to run (scan_sleep_millisecs=0), for soak;
  madvise   an MADV_COLLAPSE and MADV_DONTNEED loop.

Every mode runs in turn unless -m names one, five seconds each.  Every
supported anon THP order is set to inherit and max_ptes_none is 0, so a
window collapses only once fully populated and the racing MADV_DONTNEED
steers selection across orders.

The rule is that a racing page reads as its pattern or as zero, never
anything else.  The faulters and fork children check it throughout, and a
final sweep checks it again.  The other half of the check is the kernel's
own assertions, so read dmesg too.

The pin thread goes through gup_test, so the harness skips without
CONFIG_GUP_TEST or root.  The default playground is three shared PMD-sized
areas plus the mremap thread's, over two gigabytes at a 512M PMD; -a
shrinks it.

Link: https://lore.kernel.org/20260919002451.496763-17-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Assisted-by: LLM
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "David Hildenbrand (arm)" <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: kernel-team@meta.com
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Nico Pache (Red Hat) <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>

0 files changed, 0 insertions, 0 deletions