linux-kernel - Re: [PATCH next 0/2] THP COW support for private executable file mmap

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Message-ID: <d86584ef-d32d-476f-a939-10052ca0372e@kernel.org>
Date: Tue, 16 Dec 2025 03:43:03 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: zhangqilong <zhangqilong3@...wei.com>,
 Matthew Wilcox <willy@...radead.org>
Cc: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
 "lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>,
 "corbet@....net" <corbet@....net>, "ziy@...dia.com" <ziy@...dia.com>,
 "baolin.wang@...ux.alibaba.com" <baolin.wang@...ux.alibaba.com>,
 "Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>,
 "npache@...hat.com" <npache@...hat.com>,
 "ryan.roberts@....com" <ryan.roberts@....com>,
 "dev.jain@....com" <dev.jain@....com>, "baohua@...nel.org"
 <baohua@...nel.org>, "lance.yang@...ux.dev" <lance.yang@...ux.dev>,
 "vbabka@...e.cz" <vbabka@...e.cz>, "rppt@...nel.org" <rppt@...nel.org>,
 "surenb@...gle.com" <surenb@...gle.com>, "mhocko@...e.com"
 <mhocko@...e.com>, "Wangkefeng (OS Kernel Lab)"
 <wangkefeng.wang@...wei.com>, Sunnanyong <sunnanyong@...wei.com>,
 "linux-mm@...ck.org" <linux-mm@...ck.org>,
 "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH next 0/2] THP COW support for private executable file mmap

On 12/16/25 03:24, zhangqilong wrote:
>   > On 12/15/25 15:00, Matthew Wilcox wrote:
>>> On Mon, Dec 15, 2025 at 08:34:05PM +0800, Zhang Qilong wrote:
>>>> This patch series implementate THP COW for private executable file
>>>> mmap. It's major designed to increase the iTLB cache hit rate for hot
>>>> patching application, and we add a new sysfs knob to disable or
>>>> enable it.
>>>
>>> You're going to have to provide data to get this patch in.  We've
>>> deliberately not done this in the past due to memory consumption
>> overhead.
>>> So you need to prove that's now the wrong decision to make.
>>>
>>> Microbenchmarks would be a bare minimum, but what are really needed
>>> are numbers from actual workloads.
>>
>> In addition, the sysfs toggle is rather horrible. It's rather clear that this is not a
>> system-wide setting to be made, as you likely only want that behavior (if at
>> all ...) for a handful of special processes I assume?
> 
> Year, it's not a system-wide setting. We consider enabling this option only when
> applying hot patches to special processes. If the sysfs toggle is unavailable, we will
> evaluate the overall memory impact on the system after removing it. Thanks very
> much for your suggestion.

I don't think we want this as any kind of default behavior. But the 
system toggle is really also not what we want. Could we use some per-VMA 
or per-file hints to affect the policy?

Note that your proposal will likely interact in bad ways with uprobes, 
after removing uprobes again.

-- 
Cheers

David