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Thu, 13 May 2021 06:08:53 -0700 (PDT) Received: from hyeyoo ([121.135.181.35]) by smtp.gmail.com with ESMTPSA id w14sm2164690pff.94.2021.05.13.06.08.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 13 May 2021 06:08:53 -0700 (PDT) Date: Thu, 13 May 2021 22:08:48 +0900 From: Hyeonggon Yoo <42.hyeyoo@gmail.com> To: Marco Elver Cc: Vlastimil Babka , Andrew Morton , Joonsoo Kim , David Rientjes , Pekka Enberg , Christoph Lameter , Linux Memory Management List , LKML Subject: Re: [PATCH v3] mm, slub: change run-time assertion in kmalloc_index() to compile-time Message-ID: <20210513130848.GA778609@hyeyoo> References: <20210511173448.GA54466@hyeyoo> <20210512195227.245000695c9014242e9a00e5@linux-foundation.org> <20210513031220.GA133011@hyeyoo> <20210512204024.401ff3de38649d7d0f5a45e8@linux-foundation.org> <20210513062809.GA319973@hyeyoo> <20210513120339.GA772931@hyeyoo> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=IEah8Hvz; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf20.hostedemail.com: domain of 42hyeyoo@gmail.com designates 209.85.210.173 as permitted sender) smtp.mailfrom=42hyeyoo@gmail.com X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 219AF3DF X-Stat-Signature: mrxbtiakpfb3q46cs5d6pgbqzmtrhe69 Received-SPF: none (gmail.com>: No applicable sender policy available) receiver=imf20; identity=mailfrom; envelope-from="<42.hyeyoo@gmail.com>"; helo=mail-pf1-f173.google.com; client-ip=209.85.210.173 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620911335-829075 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, May 13, 2021 at 02:29:13PM +0200, Marco Elver wrote: > This doesn't solve the problem. We want the compiler to complain > whenever kmalloc_index() is used with non-constant in normal code. But > it should be possible to use it in allocator tests regardless of size. > Either that or export kmalloc_slab(), but I think that's worse. I'll > send my patch with an updated comment. to explain in more detail, in include/linux/slab.h: static __always_inline void *kmalloc(size_t size, gfp_t flags) { if (__builtin_constant_p(size)) { #ifndef CONFIG_SLOB unsigned int index; #endif if (size > KMALLOC_MAX_CACHE_SIZE) return kmalloc_large(size, flags); #ifndef CONFIG_SLOB index = kmalloc_index(size); it checks if size is bigger than KMALLOC_MAX_CACHE_SIZE. so kmalloc_index works safely because the size was already checked. and definition of KMALLOC_MAX_CACHE_SIZE is in include/linux/slab.h: #ifdef CONFIG_SLAB #define KMALLOC_SHIFT_HIGH ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \ (MAX_ORDER + PAGE_SHIFT - 1) : 25) #define KMALLOC_SHIFT_MAX KMALLOC_SHIFT_HIGH #ifndef KMALLOC_SHIFT_LOW #define KMALLOC_SHIFT_LOW 5 #endif #endif #ifdef CONFIG_SLUB #define KMALLOC_SHIFT_HIGH (PAGE_SHIFT + 1) #define KMALLOC_SHIFT_MAX (MAX_ORDER + PAGE_SHIFT - 1) #ifndef KMALLOC_SHIFT_LOW #define KMALLOC_SHIFT_LOW 3 #endif #endif #ifdef CONFIG_SLOB #define KMALLOC_SHIFT_HIGH PAGE_SHIFT #define KMALLOC_SHIFT_MAX (MAX_ORDER + PAGE_SHIFT - 1) #ifndef KMALLOC_SHIFT_LOW #define KMALLOC_SHIFT_LOW 3 #endif #endif so if kmalloc_index is called from another place other than kmalloc, it's not safe to assume that the supported size is 32MB. Thanks, Hyeonggon