Re: [PATCH v2 1/8] mm/gup: reintroduce FOLL_NUMA as FOLL_HONOR_NUMA_FAULT

[David Hildenbrand <david@redhat.com>]

>> Reported-by: liubo <liubo254@huawei.com>
>> Closes: https://lore.kernel.org/r/20230726073409.631838-1-liubo254@huawei.com
>> Reported-by: Peter Xu <peterx@redhat.com>
>> Closes: https://lore.kernel.org/all/ZMKJjDaqZ7FW0jfe@x1n/
>> Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> I agree that FOLL_REMOTE probably needs separate treatment but also agree
> that it's outside the context of this patch, particularly as a -stable
> candidate so
> 
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
> 
> I've a minor nit below that would be nice to get fixed up, but not
> mandatory.

Thanks Mel for taking a look, so I don't mess up once more :)

> 
>> ---
>>   include/linux/mm.h       | 21 +++++++++++++++------
>>   include/linux/mm_types.h |  9 +++++++++
>>   mm/gup.c                 | 29 +++++++++++++++++++++++------
>>   mm/huge_memory.c         |  2 +-
>>   4 files changed, 48 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 2493ffa10f4b..f463d3004ddc 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -2240,6 +2244,12 @@ static bool is_valid_gup_args(struct page **pages, int *locked,
>>   		gup_flags |= FOLL_UNLOCKABLE;
>>   	}
>>   
>> +	/*
>> +	 * For now, always trigger NUMA hinting faults. Some GUP users like
>> +	 * KVM really require it to benefit from autonuma.
>> +	 */
>> +	gup_flags |= FOLL_HONOR_NUMA_FAULT;
>> +
>>   	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
>>   	if (WARN_ON_ONCE((gup_flags & (FOLL_PIN | FOLL_GET)) ==
>>   			 (FOLL_PIN | FOLL_GET)))
> 
> Expand on *why* KVM requires it even though I suspect this changes later
> in the series. Maybe "Some GUP users like KVM require the hint to be as
> the calling context of GUP is functionally similar to a memory reference
> from task context"?

It's raised later in this series but it doesn't hurt to discuss it here 
in a bit more detail.

Sounds good to me.

> 
> Also minor nit -- s/autonuma/NUMA Balancing/ or numab. autonuma refers to
> a specific implementation of automatic balancing that operated similar to
> khugepaged but not merged. If you grep for it, you'll find that autonuma
> is only referenced in powerpc-specific code. It's not important these
> days but very early on, it was very confusing if AutoNUMA was mentioned
> when NUMAB was intended.

Ah, yes, thanks. That's the one of the only place where that terminology 
accidentally slipped in.

I'll wait for more feedback and resend!

-- 
Cheers,

David / dhildenb