linux-kernel - Re: [PATCH v2] mm/memory-failure: Support disabling soft offline for HugeTLB pages

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Message-ID: <cd71fac2-bb9d-4e84-a074-2b695654e655@redhat.com>
Date: Wed, 17 Sep 2025 21:05:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Kyle Meyer <kyle.meyer@....com>
Cc: Anshuman Khandual <anshuman.khandual@....com>, akpm@...ux-foundation.org,
 corbet@....net, linmiaohe@...wei.com, shuah@...nel.org, tony.luck@...el.com,
 jane.chu@...cle.com, jiaqiyan@...gle.com, Liam.Howlett@...cle.com,
 bp@...en8.de, hannes@...xchg.org, jack@...e.cz, joel.granados@...nel.org,
 laoar.shao@...il.com, lorenzo.stoakes@...cle.com, mclapinski@...gle.com,
 mhocko@...e.com, nao.horiguchi@...il.com, osalvador@...e.de,
 rafael.j.wysocki@...el.com, rppt@...nel.org, russ.anderson@....com,
 shawn.fan@...el.com, surenb@...gle.com, vbabka@...e.cz,
 linux-acpi@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v2] mm/memory-failure: Support disabling soft offline for
 HugeTLB pages

On 17.09.25 20:51, Kyle Meyer wrote:
> On Wed, Sep 17, 2025 at 09:02:55AM +0200, David Hildenbrand wrote:
>>
>>>> +
>>>> +	0 - Enable soft offline
>>>> +	1 - Disable soft offline for HugeTLB pages
>>>> +
>>>> +Supported values::
>>>> +
>>>> +	0 - Soft offline is disabled
>>>> +	1 - Soft offline is enabled
>>>> +	3 - Soft offline is enabled (disabled for HugeTLB pages)
>>>
>>> This looks very adhoc even though existing behavior is preserved.
>>>
>>> - Are HugeTLB pages the only page types to be considered ?
>>> - How the remaining bits here are going to be used later ?
>>>
>>
>> What I proposed (that could be better documented here) is that all other
>> bits except the first one will be a disable mask when bit 0 is set.
>>
>> 2 - ... but yet disabled for hugetlb
>> 4 - ... but yet disabled for $WHATEVER
>> 8 - ... but yet disabled for $WHATEVERELSE
>>
>>> Also without a bit-wise usage roadmap, is not changing a procfs
>>> interface (ABI) bit problematic ?
>>
>> For now we failed setting it to values that are neither 0 or 1, IIUC
>> set_enable_soft_offline() correctly?
> 
> Yes, -EINVAL will be returned.
> 
>> So there should not be any problem, or which scenario do you have in mind?
> 
> Here's an alternative approach.
> 
> Do not modify the existing sysctl parameter:
> 
> /proc/sys/vm/enable_soft_offline
> 
> 0 - Soft offline is disabled
> 1 - Soft offline is enabled
> 
> Instead, introduce a new sysctl parameter:
> 
> /proc/sys/vm/enable_soft_offline_hugetlb
> 
> 0 - Soft offline is disabled for HugeTLB pages
> 1 - Soft offline is enabled for HugeTLB pages
> 
> and note in documentation that this setting only takes effect if
> enable_soft_offline is enabled.
> 
> Anshuman (and David), would you prefer this?

Hmm, at least I don't particularly like that. For each new exception we 
would create a new file, and the file has weird semantics such that it 
has no meaning when enable_soft_offline=0.

-- 
Cheers

David / dhildenb