linux-kernel - Re: [PATCH] rust: drm: mm: Add DRM MM Range Allocator abstraction

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Message-Id: <7D78A0E7-9CA7-4E04-AD55-11254DF66ED8@collabora.com>
Date: Wed, 25 Jun 2025 16:16:36 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>
Cc: linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org,
 Asahi Lina <lina+kernel@...hilina.net>
Subject: Re: [PATCH] rust: drm: mm: Add DRM MM Range Allocator abstraction

Hmm, this has an issue

[..]

> 
> +impl<A: AllocInner<T>, T> Drop for MmInner<A, T> {
> +    fn drop(&mut self) {
> +        // SAFETY: If the MmInner is dropped then all nodes are gone (since they hold references),
> +        // so it is safe to tear down the allocator.
> +        unsafe {
> +            bindings::drm_mm_takedown(self.0.get());
> +        }
> +    }
> +}
> +
> 

Where the docs for drm_mm_takedown states:

/**
* drm_mm_takedown - clean up a drm_mm allocator
* @mm: drm_mm allocator to clean up
*
* Note that it is a bug to call this function on an allocator which is not
* clean.
*/
void drm_mm_takedown(struct drm_mm *mm)
{
if (WARN(!drm_mm_clean(mm),
"Memory manager not clean during takedown.\n"))
show_leaks(mm);
}
EXPORT_SYMBOL(drm_mm_takedown);


So perhaps we should remove all nodes before takedown?

This covers the error path, i.e.: something failed somewhere else and the allocator is dirty.

— Daniel