From nobody Sat Feb 7 20:38:50 2026 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 435CD2DAFBB for ; Mon, 29 Dec 2025 12:25:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.186 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767011108; cv=none; b=tDuv3lANay3+u7VpLRFT9Wf0DhyCxrVlqEIqlUrvv33crglWuGPW/F54Wuhj+cH0KXjFCiIs69RnLjXkaz3ihvPXZ43ALwwAXqDtLJWIK4LBQKmzRKU5oSEmC0QD8ZvC9rC3rOWYwTwvx7mfRJXsKZdMv1zY13SuodeD5b7Z3b0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767011108; c=relaxed/simple; bh=7uhS1Zhm/IaLUinic89dTjRuRE6koCUx+HPZfOE+dBg=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=j3umylD5mV+BQxqtA68ebhiZ1HI8YREvEqHhfhn5n0A4oZOKeKUBWisgfh6AMvUAp5o8J6aW/MN9FFocGpKGUxI8Sj0dm6WVfhap921TEG+3mIbkk5O4yP/8oOparXbGzP/kiyvJj3n+rkQ80q6DRQ0czKHQum0ktAOn1fbj9b8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=fgRPatP5; arc=none smtp.client-ip=91.218.175.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="fgRPatP5" Date: Mon, 29 Dec 2025 20:24:39 +0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1767011103; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type; bh=rgD96VXLpwPca73tk8XcXo8AuLjoIr0us/dYXAVju8I=; b=fgRPatP5VKCbyIM9xkP9bbGPhYmZYLjlEp2Dl61XOrRtgEixli4mCxGdHcTCzbFgnGtU8h mWo4UghuiuNUo1Y40HSQUB5dxgNFNW13tWBTkoT1a+T7eYZhEXJ5XoTyaDrhlgHvp8G3/C GMqEFZSXcws4CC2UkxGZhG5tbAdYCIY= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Li To: vbabka@suse.cz, akpm@linux-foundation.org, harry.yoo@oracle.com, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, hao.li@linux.dev Cc: cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hao Li Subject: [PATCH v2] slub: clarify object field layout comments Message-ID: <20251229122415.192377-1-hao.li@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline X-Migadu-Flow: FLOW_OUT Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The comments above check_pad_bytes() document the field layout of a single object. Rewrite them to improve clarity and precision. Also update an outdated comment in calculate_sizes(). Suggested-by: Harry Yoo Acked-by: Harry Yoo Signed-off-by: Hao Li --- Changes from v1: [Left redzone padding]: Clarify that the left redzone immediately precedes each object. [Object bytes]: Clarify that the object starts at the address immediately after the left redzone. mm/slub.c | 96 ++++++++++++++++++++++++++++++++----------------------- 1 file changed, 56 insertions(+), 40 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index a94c64f56504..fa08c932db70 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1211,44 +1211,60 @@ check_bytes_and_report(struct kmem_cache *s, struct= slab *slab, } =20 /* - * Object layout: - * - * object address - * Bytes of the object to be managed. - * If the freepointer may overlay the object then the free - * pointer is at the middle of the object. - * - * Poisoning uses 0x6b (POISON_FREE) and the last byte is - * 0xa5 (POISON_END) - * - * object + s->object_size - * Padding to reach word boundary. This is also used for Redzoning. - * Padding is extended by another word if Redzoning is enabled and - * object_size =3D=3D inuse. - * - * We fill with 0xbb (SLUB_RED_INACTIVE) for inactive objects and with - * 0xcc (SLUB_RED_ACTIVE) for objects in use. - * - * object + s->inuse - * Meta data starts here. - * - * A. Free pointer (if we cannot overwrite object on free) - * B. Tracking data for SLAB_STORE_USER - * C. Original request size for kmalloc object (SLAB_STORE_USER enabled) - * D. Padding to reach required alignment boundary or at minimum - * one word if debugging is on to be able to detect writes - * before the word boundary. - * - * Padding is done using 0x5a (POISON_INUSE) - * - * object + s->size - * Nothing is used beyond s->size. - * - * If slabcaches are merged then the object_size and inuse boundaries are = mostly - * ignored. And therefore no slab options that rely on these boundaries - * may be used with merged slabcaches. + * Object field layout: + * + * [Left redzone padding] (if SLAB_RED_ZONE) + * - Field size: s->red_left_pad + * - Immediately precedes each object when SLAB_RED_ZONE is set. + * - Filled with 0xbb (SLUB_RED_INACTIVE) for inactive objects and + * 0xcc (SLUB_RED_ACTIVE) for objects in use when SLAB_RED_ZONE. + * + * [Object bytes] (object address starts here) + * - Field size: s->object_size + * - Object payload bytes. + * - If the freepointer may overlap the object, it is stored inside + * the object (typically near the middle). + * - Poisoning uses 0x6b (POISON_FREE) and the last byte is + * 0xa5 (POISON_END) when __OBJECT_POISON is enabled. + * + * [Word-align padding] (right redzone when SLAB_RED_ZONE is set) + * - Field size: s->inuse - s->object_size + * - If redzoning is enabled and ALIGN(size, sizeof(void *)) adds no + * padding, explicitly extend by one word so the right redzone is + * non-empty. + * - Filled with 0xbb (SLUB_RED_INACTIVE) for inactive objects and + * 0xcc (SLUB_RED_ACTIVE) for objects in use when SLAB_RED_ZONE. + * + * [Metadata starts at object + s->inuse] + * - A. freelist pointer (if freeptr_outside_object) + * - B. alloc tracking (SLAB_STORE_USER) + * - C. free tracking (SLAB_STORE_USER) + * - D. original request size (SLAB_KMALLOC && SLAB_STORE_USER) + * - E. KASAN metadata (if enabled) + * + * [Mandatory padding] (if CONFIG_SLUB_DEBUG && SLAB_RED_ZONE) + * - One mandatory debug word to guarantee a minimum poisoned gap + * between metadata and the next object, independent of alignment. + * - Filled with 0x5a (POISON_INUSE) when SLAB_POISON is set. + * [Final alignment padding] + * - Any bytes added by ALIGN(size, s->align) to reach s->size. + * - Filled with 0x5a (POISON_INUSE) when SLAB_POISON is set. + * + * Notes: + * - Redzones are filled by init_object() with SLUB_RED_ACTIVE/INACTIVE. + * - Object contents are poisoned with POISON_FREE/END when __OBJECT_POISO= N. + * - The trailing padding is pre-filled with POISON_INUSE by + * setup_slab_debug() when SLAB_POISON is set, and is validated by + * check_pad_bytes(). + * - The first object pointer is slab_address(slab) + + * (s->red_left_pad if redzoning); subsequent objects are reached by + * adding s->size each time. + * + * If a slab cache flag relies on specific metadata to exist at a fixed + * offset, the flag must be included in SLAB_NEVER_MERGE to prevent mergin= g. + * Otherwise, the cache would misbehave as s->object_size and s->inuse are + * adjusted during cache merging (see __kmem_cache_alias()). */ - static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p) { unsigned long off =3D get_info_end(s); /* The end of info */ @@ -7103,9 +7119,9 @@ static int calculate_sizes(struct kmem_cache_args *ar= gs, struct kmem_cache *s) =20 =20 /* - * If we are Redzoning then check if there is some space between the - * end of the object and the free pointer. If not then add an - * additional word to have some bytes to store Redzone information. + * If we are Redzoning and there is no space between the end of the + * object and the following fields, add one word so the right Redzone + * is non-empty. */ if ((flags & SLAB_RED_ZONE) && size =3D=3D s->object_size) size +=3D sizeof(void *); --=20 2.50.1