linux-kernel - Re: [v5 02/15] mm/huge_memory: add device-private THP support to PMD operations

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Message-ID: <4de00290-7c29-42f5-b8aa-58a77b17c1d5@redhat.com>
Date: Mon, 15 Sep 2025 10:10:37 +0200
From: David Hildenbrand <david@...hat.com>
To: Balbir Singh <balbirs@...dia.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: damon@...ts.linux.dev, dri-devel@...ts.freedesktop.org,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 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: [v5 02/15] mm/huge_memory: add device-private THP support to PMD
 operations

>>> +
>>
>> Wrong indentation.
> 
> checkpatch.pl did not complain, what I see is
> 
> +		VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
> +				!is_pmd_device_private_entry(pmd));
> 

David complains :)

> It looks different in your reply (is your email converting tabs to space?
> did you want me to align the conditions?

Indeed, looks wrong, not sure why. Likely I messed it up.

> 
> +		VM_WARN_ON(!is_pmd_migration_entry(pmd) &&
> +			   !is_pmd_device_private_entry(pmd));
> 
> 

Exactly.

> 
>>
>>> +        if (is_migration_entry(entry) &&
>>> +            !is_readable_migration_entry(entry)) {
>>
>> Dito.
>>
> 
> Same as above :)
> 
> 
>> Wonder if we want to be more explicit.
>>
>> if (is_readable_migration_entry(enrty) ||
>>      is_readable_exclusive_migration_entry)) {
>>
> 
> !is_readable_migration_entry => writable entry or read exclusive, did you mean is_writable_migration_entry()
> above?

Yes, sorry, my brain was dizzy after all the review lately.

if (is_writable_migration_entry(enrty) ||
     is_readable_exclusive_migration_entry))

[...]

>>
>> Couldn't we do here
>>
>> if (!pmd_present(pmdval))
>>      goto nomap;
>>
>> To replace the original pmd_none() .. check.
>>
>> A page table must always be present IIRC.
>>
> 
> I am not sure about the pmd_none(), a page table may not be present, I've not audited
> the callers. But I think we can do

IIRC page tables must always have the present bit set. So we can just 
simplify to the single pmd_present() check.

-- 
Cheers

David / dhildenb