On 12/16/25 03:24, zhangqilong wrote:

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  > On 12/15/25 15:00, Matthew Wilcox wrote:

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On Mon, Dec 15, 2025 at 08:34:05PM +0800, Zhang Qilong wrote:

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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.

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So you need to prove that's now the wrong decision to make.

Microbenchmarks would be a bare minimum, but what are really

needed

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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. 

--
Cheers

David