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Mon, 10 Aug 2026 22:43:02 -0700 (PDT) Received: from MacBook-Pro-von-Karl (dynamic-2a02-3100-a133-e401-bc8e-d36c-2d08-c5e0.310.pool.telefonica.de. [2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-499740c46efsm39080525e9.4.2026.08.10.22.43.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 10 Aug 2026 22:43:02 -0700 (PDT) Date: Tue, 11 Aug 2026 07:43:00 +0200 From: Karl Mehltretter To: Catalin Marinas Cc: Vlastimil Babka , Harry Yoo , Andrew Morton , Rasmus Villemoes , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN Message-ID: References: <20260808105622.62026-1-kmehltretter@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 7A7A7A000A X-Stat-Signature: txr7wfo7x5jzxem4m9d5xm3je5ifpj6j X-HE-Tag: 1786426984-522259 X-HE-Meta: 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 S00DpeGo 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 10, 2026 at 03:26:37PM +0100, Catalin Marinas wrote: > On Sat, Aug 08, 2026 at 12:56:22PM +0200, Karl Mehltretter wrote: > > diff --git a/include/linux/slab.h b/include/linux/slab.h > > index 32c9f8ed7ae20..0798a714da87f 100644 > > --- a/include/linux/slab.h > > +++ b/include/linux/slab.h > > @@ -259,13 +259,16 @@ enum _slab_flag_bits { > > > > /* > > * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests. > > + * It satisfies the alignment promised by __assume_kmalloc_alignment > > + * and keeps the historic value 16 where that is already aligned. > > * > > * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault. > > * > > * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can. > > * Both make kfree a no-op. > > */ > > -#define ZERO_SIZE_PTR ((void *)16) > > +#define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \ > > + ARCH_KMALLOC_MINALIGN : 16)) > > > > #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \ > > (unsigned long)ZERO_SIZE_PTR) > > @@ -622,6 +625,12 @@ static inline bool kmem_dump_obj(void *object) { return false; } > > #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE) > > #endif > > > > +/* > > + * Keep ZERO_SIZE_PTR below PAGE_SIZE and the low pointer poison values. > > + * 128 is the largest in-tree ARCH_KMALLOC_MINALIGN. > > + */ > > +static_assert(ARCH_KMALLOC_MINALIGN <= 128); > > I'm not sure we should bother with this, or at least make it strictly > less than PAGE_SIZE since 128 doesn't have any meaning for the slab > allocator. > Thanks for the review! The case I had in mind when choosing 128 was POISON_POINTER_DELTA == 0, where LIST_POISON1 is 256 (0x100). With the current range check, an architecture with non-coherent DMA and 256-byte cache lines would make kfree(LIST_POISON1) a silent no-op. Looking at this again, I think ZERO_OR_NULL_PTR() should match only NULL and ZERO_SIZE_PTR, rather than the whole range below the sentinel. This would also stop unrelated low pointer values from being silently accepted. I'll include that change in v2. Thanks, Karl