linux-kernel - Re: [v2 01/11] mm/zone_device: support large zone device private folios

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Message-ID: <0e9db982-3dd6-49ef-95a4-2862be5aeefd@nvidia.com>
Date: Wed, 6 Aug 2025 22:19:44 +1000
From: Balbir Singh <balbirs@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>,
 David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Jérôme Glisse
 <jglisse@...hat.com>, Shuah Khan <shuah@...nel.org>,
 Barry Song <baohua@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ryan Roberts <ryan.roberts@....com>, Matthew Wilcox <willy@...radead.org>,
 Peter Xu <peterx@...hat.com>, Zi Yan <ziy@...dia.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Jane Chu <jane.chu@...cle.com>,
 Alistair Popple <apopple@...dia.com>, Donet Tom <donettom@...ux.ibm.com>,
 Ralph Campbell <rcampbell@...dia.com>, Mika Penttilä
 <mpenttil@...hat.com>, Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v2 01/11] mm/zone_device: support large zone device private
 folios

On 8/6/25 07:15, Matthew Brost wrote:
> On Tue, Aug 05, 2025 at 02:58:42PM +0200, David Hildenbrand wrote:
>> On 05.08.25 13:01, Balbir Singh wrote:
>>> On 8/5/25 20:57, David Hildenbrand wrote:
>>>> On 05.08.25 06:22, Balbir Singh wrote:
>>>>> On 7/30/25 19:50, David Hildenbrand wrote:
>>>>>
>>>>>> I think I asked that already but maybe missed the reply: Should these folios ever be added to the deferred split queue and is there any value in splitting them under memory pressure in the shrinker?
>>>>>>
>>>>>> My gut feeling is "No", because the buddy cannot make use of these folios, but maybe there is an interesting case where we want that behavior?
>>>>>>
>>>>>
>>>>> I realized I did not answer this
>>>>>
>>>>> deferred_split() is the default action when partial unmaps take place. Anything that does
>>>>> folio_rmap_remove_ptes can cause the folio to be deferred split if it gets partially
>>>>> unmapped.
>>>>
>>>> Right, but it's easy to exclude zone-device folios here. So the real question is: do you want to deal with deferred splits or not?
>>>>
>>>> If not, then just disable it right from the start.
>>>>
>>>
>>> I agree, I was trying to avoid special casing device private folios unless needed to the extent possible
>>
>> By introducing a completely separate split logic :P
>>
>> Jokes aside, we have plenty of zone_device special-casing already, no harm
>> in adding one more folio_is_zone_device() there.
>>
>> Deferred splitting is all weird already that you can call yourself fortunate
>> if you don't have to mess with that for zone-device folios.
>>
>> Again, unless there is a benefit in having it.
> 
> +1 on no deferred split for device folios.
> 
>

I'll add it to v3 to check that we do not do deferred splits on zone device folios

Balbir