linux-kernel - Re: [PATCH 0/7] mm/damon: define and use DAMON initialization check function

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Message-Id: <20250915213312.12892156442f3a795a0a01f5@linux-foundation.org>
Date: Mon, 15 Sep 2025 21:33:12 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: SeongJae Park <sj@...nel.org>
Cc: damon@...ts.linux.dev, kernel-team@...a.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/7] mm/damon: define and use DAMON initialization check
 function

On Mon, 15 Sep 2025 20:35:04 -0700 SeongJae Park <sj@...nel.org> wrote:

> If DAMON is tried to be used by its API callers when it is not yet
> successfully initialized, the callers could be crashed.  Such issues
> actually happened and were fixed [1].  DAMON API callers are therefore
> having their own hacks for seeing if it is safe to use DAMON or not.
> Those built on an untreliable assumption that DAMON should be ready to
> be used on module init time.  DAMON initialization could fail if
> KMEM_CACHE() fails, though.

Wait.  Is there any realistic expectation that KMEM_CACHE() will fail
when DAMON uses it?  We do have the convention of assuming that
__init-time allocations do not fail.  If they do, an oops or panic is
an acceptable response.

Are these problems actually real-world demonstrable things, or has
someone been playing with fault injection or, ...?

> Also those are basically duplications that
> make their maintenance difficult.

Unclear.  This means that the client hacks are no longer necessary after
these changes?