selftests/mm: add khugepaged completion barrier helper

A khugepaged test has to tell "not collapsed" from "not scanned yet", and
nothing in the selftests can.  wait_for_scan() in khugepaged.c comes
closest: it polls full_scans until the counter has advanced by two, since
the pass in progress may already have passed the test's mm.  But it only
returns in time if scan_sleep_millisecs happens to be short, and it is
private to that one test.

Add khugepaged_full_pass() to hugepage_settings, built on the same
advance-by-two wait but driven through sysfs: a store to
scan_sleep_millisecs wakes the daemon, so the barrier completes whatever
the scan cadence.  A store made while the daemon is scanning rather than
sleeping is lost, so the helper keeps storing until the pass lands.

One wake completes one pass only if pages_to_scan covers every mm on the
list, so callers need it large.

Settings pushes must not start passes of their own.  A store to either
sleep knob wakes the daemon, so thp_write_settings() now writes a
khugepaged knob only when its value changes.

Link: https://lore.kernel.org/20260919002451.496763-11-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Assisted-by: LLM
Cc: Alexander Gordeev <agordeev@linux.ibm.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