linux-kernel - Re: [PATCH v3 0/2] mm/ksm: add fork-exec support for prctl

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Message-Id: <20230921173251.54b854fb0ec7af2bf3e3ec3b@linux-foundation.org>
Date:   Thu, 21 Sep 2023 17:32:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Stefan Roesch <shr@...kernel.io>
Cc:     kernel-team@...com, david@...hat.com, hannes@...xchg.org,
        riel@...riel.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 0/2] mm/ksm: add fork-exec support for prctl

On Thu, 21 Sep 2023 09:47:07 -0700 Stefan Roesch <shr@...kernel.io> wrote:

> A process can enable KSM with the prctl system call. When the process is
> forked the KSM flag is inherited by the child process.

I guess that's logical, as it's still the same program.

> However if the
> process is executing an exec system call directly after the fork, the
> KSM setting is cleared. This patch series addresses this problem.

Well...  who said it's a problem?  There's nothing in our documentation
about this(?).  Why is the current behavior wrong?  If the new program
wants KSM, it can turn on KSM.

This significant change in user-visible behavior deserves much more
explanation and justification, please.  Including an explanation of why
it's OK to change kernel behavior under existing users' feet like this,