linux-kernel - Re: [PATCH] mm/huge_memory: fix override of entry in remove_migration

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Message-ID: <5092cfb0-ad25-40ff-9a07-3de86d37665f@nvidia.com>
Date: Fri, 14 Nov 2025 07:55:29 +1100
From: Balbir Singh <balbirs@...dia.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 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>,
 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: [PATCH] mm/huge_memory: fix override of entry in
 remove_migration_pmd

On 11/13/25 23:07, David Hildenbrand (Red Hat) wrote:
> On 13.11.25 12:56, Lorenzo Stoakes wrote:
>> On Thu, Nov 13, 2025 at 04:13:52PM +1100, Balbir Singh wrote:
>>> The softleaf changes exposed a BUG in remove_rmap_pmd() where the
>>> migration entry was being overridden when the folio is device private.
>>>
>>> Use scope local entry for creating the device private pmde. Make the
>>> pmde writable if the migration entry is writable by moving the check
>>> softleaf_is_migration_write() prior to creating the device private entry.
>>>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: David Hildenbrand <david@...hat.com>
>>> Cc: Zi Yan <ziy@...dia.com>
>>> Cc: Joshua Hahn <joshua.hahnjy@...il.com>
>>> Cc: Rakie Kim <rakie.kim@...com>
>>> Cc: Byungchul Park <byungchul@...com>
>>> Cc: Gregory Price <gourry@...rry.net>
>>> Cc: Ying Huang <ying.huang@...ux.alibaba.com>
>>> Cc: Alistair Popple <apopple@...dia.com>
>>> Cc: Oscar Salvador <osalvador@...e.de>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>>> Cc: Nico Pache <npache@...hat.com>
>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>> Cc: Dev Jain <dev.jain@....com>
>>> Cc: Barry Song <baohua@...nel.org>
>>> Cc: Lyude Paul <lyude@...hat.com>
>>> Cc: Danilo Krummrich <dakr@...nel.org>
>>> Cc: David Airlie <airlied@...il.com>
>>> Cc: Simona Vetter <simona@...ll.ch>
>>> Cc: Ralph Campbell <rcampbell@...dia.com>
>>> Cc: Mika Penttilä <mpenttil@...hat.com>
>>> Cc: Matthew Brost <matthew.brost@...el.com>
>>> Cc: Francois Dugast <francois.dugast@...el.com>
>>>
>>> Signed-off-by: Balbir Singh <balbirs@...dia.com>
>>
>> THe logic LGTM but we don't want to have a bisect hazard here by having the bug
>> introduced earlier then resolved here.
> 
> Exactly.
> 

The hazard is a VM_WARN_ON() that checks for is_migration_entry() in the
call to softleaf_is_migration_young(). I am happy to go down that path,
let me send out the fixups, it felt weird to break the rebase, but I
am sure Andrew has a better way of dealing with this stuff.

Balbir