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

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Message-ID: <da283a44-e668-4e88-985c-3e2805b556bc@redhat.com>
Date: Tue, 5 Aug 2025 14:58:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, linux-mm@...ck.org
Cc: 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>, Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v2 01/11] mm/zone_device: support large zone device private
 folios

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.

-- 
Cheers,

David / dhildenb