linux-kernel - Re: [RFC PATCH V2 03/13] mm: Scan the mm and create a migration list

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Message-ID: <95242ba0-e7a6-40c4-ae43-dab2d61555b6@amd.com>
Date: Tue, 8 Jul 2025 16:47:00 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: AneeshKumar.KizhakeVeetil@....com, Hasan.Maruf@....com,
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Subject: Re: [RFC PATCH V2 03/13] mm: Scan the mm and create a migration list



On 6/26/2025 11:57 AM, Raghavendra K T wrote:
> 
> On 6/26/2025 3:37 AM, Harry Yoo wrote:
>> On Tue, Jun 24, 2025 at 05:56:07AM +0000, Raghavendra K T wrote:
>>> Since we already have the list of mm_struct in the system, add a 
>>> module to
>>> scan each mm that walks VMAs of each mm_struct and scan all the pages
>>> associated with that.
>>>
>>>   In the scan path: Check for the recently acccessed pages (folios) 
>>> belonging
>>> to slowtier nodes. Add all those folios to a list.
>>>
>>> Signed-off-by: Raghavendra K T <raghavendra.kt@....com>
>>> ---
>>
>> Hi, just taking a quick look...
> 
> Hello Harry,
> Thanks for taking a look at the patches.
>>
>>>   mm/kscand.c | 319 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>   1 file changed, 318 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/kscand.c b/mm/kscand.c
>>> index d5b0d3041b0f..0edec1b7730d 100644
>>> --- a/mm/kscand.c
>>> +++ b/mm/kscand.c
>>> @@ -42,6 +55,8 @@ static struct kmem_cache *kscand_slot_cache 
>>> __read_mostly;
>>> @@ -84,11 +122,275 @@ static void kscand_wait_work(void)
>>>               scan_sleep_jiffies);
>>>   }
>>> +static inline bool is_valid_folio(struct folio *folio)
>>> +{
>>> +    if (!folio || folio_test_unevictable(folio) || ! 
>>> folio_mapped(folio) ||
>>> +        folio_is_zone_device(folio) || 
>>> folio_maybe_mapped_shared(folio))
>>> +        return false;
>>> +
>>> +    return true;
>>> +}
>>
>> What makes it undesirable to migrate shared folios?
> 
> This was mostly to avoid shared libraries, but yes this also
> should have accompanied with EXEC flag to refine further.
> This also avoids moving around shared data. I will experiment more
> and add additional filters OR remove the check.
>

Atleast experiment with microbenchmark gives a positive benefit if I
remove check for folio_maybe_mapped_shared() folios. Will respin
accordingly.
(also after checking some real workloads).

- Raghu