Re: [PATCH v6 2/9] mm/page_owner: add MR_NEVER to enum migrate_reason and use it for last_migrate_reason

[Vlastimil Babka (SUSE)]

On 7/14/26 15:22, David Hildenbrand (Arm) wrote:
> On 7/14/26 15:09, Vlastimil Babka (SUSE) wrote:
>> On 7/14/26 15:05, David Hildenbrand (Arm) wrote:
>>> On 7/14/26 03:51, Ye Liu wrote:
>>>> The last_migrate_reason field uses -1 as a sentinel value to mean "no
>>>> migration has happened".  Replace the four bare -1 occurrences by
>>>> adding a proper MR_NEVER member to enum migrate_reason, defining a
>>>> corresponding "never_migrated" string in the MIGRATE_REASON trace
>>>> macro, and updating the GDB page_owner script to use MR_NEVER instead
>>>> of the hardcoded -1 so that lx-dump-page-owner does not incorrectly
>>>> report unmigrated pages as migrated.
>>>>
>>>> No functional change.
>>>>
>>>> Signed-off-by: Ye Liu <ye.liu@xxxxxxxxx>
>>>> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
>>>> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
>>>> ---
>>>>  include/linux/migrate_mode.h    | 1 +
>>>>  include/trace/events/migrate.h  | 3 ++-
>>>>  mm/page_owner.c                 | 8 ++++----
>>>>  scripts/gdb/linux/page_owner.py | 4 +++-
>>>>  4 files changed, 10 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
>>>> index 265c4328b36a..05102d4d2490 100644
>>>> --- a/include/linux/migrate_mode.h
>>>> +++ b/include/linux/migrate_mode.h
>>>> @@ -25,6 +25,7 @@ enum migrate_reason {
>>>>  	MR_LONGTERM_PIN,
>>>>  	MR_DEMOTION,
>>>>  	MR_DAMON,
>>>> +	MR_NEVER,		/* page has never been migrated */
>>>
>>> Shouldn't this be more like
>>>
>>> "MR_NONE" ?
>>>
>>> As "never" is not really a "reason" ?
>> 
>> It's not used ever as a reason that would be actually passed to migration.
> 
> Right, it's a placeholder for "there is no migrate reason because it is unset"
> 
>> So I think the name is more descriptive this way.
> 
> Not sure I agree. The usual translation of -1 -> unset is NONE or UNSET.

(note it's no longer -1 after the patch.)

> At least I was confused by "NEVER".

I won't bikeshed this, so whatever.

Just to avoid another extra respin, please also say how the following line
should change, as if we only rename MR_NEVER to MR_NONE, it will be:

+	EMe(MR_NONE,		"never_migrated")

Is that ok? Note it should never be actually printed anywhere per the
current code.