linux-kernel - Re: [PATCH] mm/huge_memory: softleaf device private fixes in remove_migration

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Message-ID: <096d8a22-3ff7-4ced-b58f-458f9826c7da@nvidia.com>
Date: Thu, 13 Nov 2025 08:07:02 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 akpm@...ux-foundation.org, David Hildenbrand <david@...nel.org>,
 Zi Yan <ziy@...dia.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>, Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH] mm/huge_memory: softleaf device private fixes in
 remove_migration_pmd()

On 11/13/25 00:43, Lorenzo Stoakes wrote:
> On Wed, Nov 12, 2025 at 03:46:34PM +1100, Balbir Singh wrote:
>> commit a6ca2ba46390 ("mm: replace pmd_to_swp_entry() with softleaf_from_pmd()")
>> does not work with device private THP entries. softleaf_is_migration_young()
>> asserts that the entry be a migration entry, but in the current code, the
>> entry might already be replaced by a device private entry by the time the
>> check is made. The issue exists with commit
>> 7385dbdbf841 ("mm/rmap: extend rmap and migration support device-private entries")
> 
> OK this is _hugely_ confusing.
> 
> Is the bug in my patch or in yours?
> 

The bug exists in my series (as pointed out in the the issue exists with), 
but it is exposed by your changes with the VM_WARN_ON in your changes.

> Why are you replying to your own series with this patch?
> 
> You shouldn't reference non-upstream commit messages in general.
> 
> If the bug is in 7385dbdbf841, fix it in your series, then perhaps send a
> suggested fix-patch to the appropriate patch in my series to make life easier
> for Andrew.
> 

OK, let me split it up then

> As mine I think in this case was purely a mechanical replacement of function
> calls I'm guessing it's a bug in yours? So I think this is probably the best
> way.
> 

[...]
Balbir