linux-kernel - Re: [PATCH 9/9] mm: kmemleak: fix undetected leaks for page aligned objects

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Message-ID: <b081dce4-46a7-c3df-30dc-e064f2515a@gentwo.de>
Date:   Thu, 26 Jan 2023 12:21:14 +0100 (CET)
From:   Christoph Lameter <cl@...two.de>
To:     George Prekas <george@...abrica.net>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>
Subject: Re: [PATCH 9/9] mm: kmemleak: fix undetected leaks for page aligned
 objects

On Mon, 23 Jan 2023, George Prekas wrote:

> If kmalloc returns a page aligned object, then the object has 2
> references: the pointer returned by kmalloc and page->s_mem of the first
> page of the object. Account for this extra reference, so that kmemleak
> can correctly detect leaks for page aligned objects.

s_mem is a reference to the array of slab objects in a SLAB page. It is
not referring to a particular object. s_mem allows access to the Nth
object in a slab page.

See the function index_to_obj() in slab.c



static inline void  *index_to_obj(struct kmem_cache *cache,
                                 const struct slab *slab, unsigned int idx)
{

        return slab->s_mem + cache->size * idx;
}