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IPa1T1vwxbxlVLeHFQf6/C5T++uh7j13NRnyOFL30T1cufXOgdmQVff6zOsNSq/z22D9xmH7RapaUBSXuWUKj/9mNrCbPp1rdzrPomcFI5JsRkVbokas5uS9H+HjtZKEceIl6/IPlQiFqSc13YBr5lWMDaQ== 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: List-Subscribe: List-Unsubscribe: On Wed, Dec 24, 2025 at 08:51:14PM +0800, Hao Li wrote: > 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 Also update an outdated comment in calculate_sizes().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 > Signed-off-by: Hao Li > --- > Hi Harry, this patch adds more detailed object layout documentation. Let > me know if you have any comments. Hi Hao, thanks for improving it! It looks much clearer now. few nits below. > + * Object field layout: > + * > + * [Left redzone padding] (if SLAB_RED_ZONE) > + * - Field size: s->red_left_pad > + * - Filled with 0xbb (SLUB_RED_INACTIVE) for inactive objects and > + * 0xcc (SLUB_RED_ACTIVE) for objects in use when SLAB_RED_ZONE. nit: although it becomes clear after reading the Notes: section, I would like to make it clear that object address starts here (after the left redzone) and the left redzone is right before each object. > + * [Object bytes] > + * - 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_POISON. > + * - 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 slabcaches are merged then the object_size and inuse boundaries are > + * mostly ignored. Therefore no slab options that rely on these boundaries > * may be used with merged slabcaches. For the last paragraph, perhaps it'll be clearer to say: "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 merging. Otherwise, the cache would misbehave as s->object_size and s->inuse are adjusted during cache merging" Otherwise looks great to me, so please feel free to add: Acked-by: Harry Yoo -- Cheers, Harry / Hyeonggon