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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <cd9bae54-a12b-43f0-8014-0c1d95916c0b@redhat.com>
Date: Wed, 17 Sep 2025 12:22:15 +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

>>>>
>>>> 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.
>>
> 
> Not sure about that one, are you happy if we follow it up later with a separate fix

Taking a look at other page table walkers, they couldn't possibly work if
tables would not be pmd_present().

E.g., walk_pmd_range() in mm/pagewalk.c has

if (walk->vma)
	split_huge_pmd(walk->vma, pmd, addr);
else if (pmd_leaf(*pmd) || !pmd_present(*pmd))
	continue; /* Nothing to do. */

err = walk_pte_range(pmd, addr, next, walk);
if (err)
	break;

If a PTE table would not be pmd_present() we would never ever call walk_pte_range().

So I am pretty sure that this can just be:

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 567e2d084071e..1916d22aaf1f1 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -290,7 +290,7 @@ pte_t *___pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)

         if (pmdvalp)
                 *pmdvalp = pmdval;
-       if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
+       if (unlikely(!pmd_present(pmdval)))
                 goto nomap;
         if (unlikely(pmd_trans_huge(pmdval)))
                 goto nomap;

Unless I am missing something important :)

-- 
Cheers

David / dhildenb