mm: khugepaged: export set_recommended_min_free_kbytes()

Patch series "mm: thp: reduce unnecessary start_stop_khugepaged()", v7.

Writing to /sys/kernel/mm/transparent_hugepage/enabled causes
start_stop_khugepaged() called independent of any change.
start_stop_khugepaged() SPAMs the printk ring buffer overflow with the
exact same message, even when nothing changes.

For instance, if you have a custom vm.min_free_kbytes, just touching
/sys/kernel/mm/transparent_hugepage/enabled causes a printk message.
Example:

# sysctl -w vm.min_free_kbytes=112382
# for i in $(seq 100); do echo never > /sys/kernel/mm/transparent_hugepage/enabled ; done

and you have 100 WARN messages like the following, which is pretty dull:

khugepaged: min_free_kbytes is not updated to 112381 because user defined value 112382 is preferred

A similar message shows up when setting thp to "always":

# for i in $(seq 100); do
# echo 1024 > /proc/sys/vm/min_free_kbytes
# echo always > /sys/kernel/mm/transparent_hugepage/enabled
# done

And then, we have 100 messages like:

khugepaged: raising min_free_kbytes from 1024 to 67584 to help transparent hugepage allocations

This is more common when you have a configuration management system that
writes the THP configuration without an extra read, assuming that nothing
will happen if there is no change in the configuration, but it prints
these annoying messages.

For instance, at Meta's fleet, ~10K servers were producing 3.5M of these
messages per day.

Fix this by making the sysfs _store helpers easier to digest and
ratelimiting the message.


This patch (of 4):

Make set_recommended_min_free_kbytes() callable from outside khugepaged.c
by removing the static qualifier and adding a declaration in
mm/internal.h.

This allows callers that change THP settings to recalculate watermarks
without going through start_stop_khugepaged().

Link: https://lkml.kernel.org/r/20260317-thp_logs-v7-0-31eb98fa5a8b@debian.org
Link: https://lkml.kernel.org/r/20260317-thp_logs-v7-1-31eb98fa5a8b@debian.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Suggested-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Usama Arif <usamaarif642@gmail.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>