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

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Message-ID: <46d418f9-74ae-4801-99da-2e31c019bc79@kernel.org>
Date: Tue, 30 Dec 2025 22:29:45 +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 08:42, zhangqilong wrote:
>> 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?
> 
> It's really a good idea. Maybe could setting a xx flag to VMA, but it need hold write
> lock and touch VMA. let me have a think how to implement per-VMA or per-file policy
> or other better hints.
> 
> I have a thought that, how about adding a new flag(in ptrace, uprobes...) and passing
> to faultin_page() to mark the exec PMD COW policy?
> 
>>
>> Note that your proposal will likely interact in bad ways with uprobes, after
>> removing uprobes again.
> 
> Year, good catch here, it's really need be seriously considered.

I'll reply on v2 with the issue I have in mind.

-- 
Cheers

David