linux-kernel - Re: [v6 04/15] mm/huge_memory: implement device-private THP splitting

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Message-ID: <c9a0f531-bba6-48a1-bb50-639fb4f2dfb6@redhat.com>
Date: Thu, 25 Sep 2025 12:06:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Balbir Singh <balbirs@...dia.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, damon@...ts.linux.dev,
 dri-devel@...ts.freedesktop.org, Joshua Hahn <joshua.hahnjy@...il.com>,
 Rakie Kim <rakie.kim@...com>, Byungchul Park <byungchul@...com>,
 Gregory Price <gourry@...rry.net>, Ying Huang
 <ying.huang@...ux.alibaba.com>, Alistair Popple <apopple@...dia.com>,
 Oscar Salvador <osalvador@...e.de>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Lyude Paul <lyude@...hat.com>,
 Danilo Krummrich <dakr@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Ralph Campbell <rcampbell@...dia.com>,
 Mika Penttilä <mpenttil@...hat.com>,
 Matthew Brost <matthew.brost@...el.com>,
 Francois Dugast <francois.dugast@...el.com>
Subject: Re: [v6 04/15] mm/huge_memory: implement device-private THP splitting

>>> Even if this is the only call site, there is no guarantee that
>>> there will be none in the future. I am not sure why we want caller
>>> to handle this special case. Who is going to tell the next user
>>> of RMP_USE_SHARED_ZEROPAGE or caller to try_to_map_unused_to_zeropage()
>>> that device private is incompatible with them?
>>>
>>
>> I don't disagree, but the question was why are device private pages even making
>> it to try_to_map_unused_to_zeropage()>>
> 
> Then, it could be done in remove_migration_pte():
> 
> if (rmap_walk_arg->map_unused_to_zeropage &&
> 	!folio_is_device_private(folio) &&
> 	try_to_map_unused_to_zeropage(&pvmw, folio, idx))
> 	continue;
> 
> Maybe I am too hung up on this and someone else could pat on my back and
> tell me it is OK to just do this at the only caller instead. :)

I think we shouldn't set a flag for a folio that does not make any 
sense. Just like we don't set the flag for non-anon folios?

In addition, we could add a 
VM_WARN_ON_ONCE(folio_is_device_private(folio)) in 
try_to_map_unused_to_zeropage(), to catch any future abuse.

-- 
Cheers

David / dhildenb