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

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Message-ID: <aMsqdesM1ImJp1yt@hpe.com>
Date: Wed, 17 Sep 2025 16:39:01 -0500
From: Kyle Meyer <kyle.meyer@....com>
To: "Luck, Tony" <tony.luck@...el.com>
Cc: Jiaqi Yan <jiaqiyan@...gle.com>, David Hildenbrand <david@...hat.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        akpm@...ux-foundation.org, corbet@....net, linmiaohe@...wei.com,
        shuah@...nel.org, jane.chu@...cle.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 Wed, Sep 17, 2025 at 12:54:09PM -0700, Luck, Tony wrote:
> On Wed, Sep 17, 2025 at 12:32:47PM -0700, Jiaqi Yan wrote:
> > +1. Given /proc/sys/vm/enable_soft_offline is extensible, I would
> > prefer a compact userspace API.
> > 
> > > would create a new file, and the file has weird semantics such that it
> > > has no meaning when enable_soft_offline=0.
> 
> So the expand the bitmask idea from earlier in this thread?
> 
> Bit0	0 = soft offline disabled. 1 = Enabled (but see other bits)
> Bit1	0 = allow offline of 4K pages, 1 = suppress 4K offline
> Bit2	0 = allow offline of hugetlb, 1 = suppress hugetlb offline
> Bit3	0 = allow breakup of transparent huge pages to just offline 4K, 1 = suppress transparent breakup
> Bit4+	Reserved for suppressing other page types we invent in the future
> 
> Values 0 and 1 keep their original meaning.
> 
> Value 5 means: offline 4K, keep hugetlb, breakup transparent huge pages.

Do you happen to have any use cases or reasoning for why someone might want
to disable soft offline for 4K pages or transparent huge pages? I'd like to
understand the motivation for adding the extra bits.

Thanks,
Kyle Meyer