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From: Hao Li To: Harry Yoo Cc: akpm@linux-foundation.org, vbabka@suse.cz, andreyknvl@gmail.com, cl@gentwo.org, dvyukov@google.com, glider@google.com, hannes@cmpxchg.org, linux-mm@kvack.org, mhocko@kernel.org, muchun.song@linux.dev, rientjes@google.com, roman.gushchin@linux.dev, ryabinin.a.a@gmail.com, shakeel.butt@linux.dev, surenb@google.com, vincenzo.frascino@arm.com, yeoreum.yun@arm.com, tytso@mit.edu, adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org Subject: Re: [PATCH V5 8/8] mm/slab: place slabobj_ext metadata in unused space within s->size Message-ID: <7uiizca4ejiqw6zegjwmou5va4kw7na7wivy4kxebrju7dsdwo@5brr7vhwf5oh> References: <20260105080230.13171-1-harry.yoo@oracle.com> <20260105080230.13171-9-harry.yoo@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: EA5661C000A X-Stat-Signature: qfe1qcr7eomoo881igxg77dbc4uwsxhe X-Rspam-User: X-HE-Tag: 1767865969-946128 X-HE-Meta: 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 ZjhzNiL3 9JFu9Nrno90sQToADfGwOOap6z4nNCKk9RB/zm+dhsuUgW9P8YSGFARxoK94njuJZp7sue6g9c6UA4c3hCp65KY26MdgSx8eNOFLaJFQx2e2bMLVNZZpUp8nI7aLPFFRVzsmO6XlCZTc/5sHXHPXGp1jIoMHxK644SrfBAUA0DMym7wVaLF/oOYqr/e2r0B6NTkS9Ilm0LN8ndOJpVyYxfQLTWQXpX/X5g9FpdQHp6KVXRdM0O1aUy1Zt/QEQ1TIQIiWeKGgNDnXhMf1db8gBZgq/0cK54kSrQNACtDvmU2iueyflVOVGYPxl8lRh7GR9oxFyv0pSdb83A4c= 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 Thu, Jan 08, 2026 at 05:41:00PM +0900, Harry Yoo wrote: > On Thu, Jan 08, 2026 at 01:52:09PM +0800, Hao Li wrote: > > On Mon, Jan 05, 2026 at 05:02:30PM +0900, Harry Yoo wrote: > > > When a cache has high s->align value and s->object_size is not aligned > > > to it, each object ends up with some unused space because of alignment. > > > If this wasted space is big enough, we can use it to store the > > > slabobj_ext metadata instead of wasting it. > > > > Hi, Harry, > > Hi Hao, > > > When we save obj_ext in s->size space, it seems that slab_ksize() might > > be missing the corresponding handling. > > Oops. > > > It still returns s->size, which could cause callers of slab_ksize() > > to see unexpected data (i.e. obj_ext), or even overwrite the obj_ext data. > > Yes indeed. > Great point, thanks! > > I'll fix it by checking if the slab has obj_exts within the object > layout and returning s->object_size if so. Makes sense - I think there's one more nuance worth capturing. slab_ksize() seems to compute the maximum safe size by applying layout constraints from most-restrictive to least-restrictive: redzones/poison/KASAN clamp it to object_size, tail metadata (SLAB_TYPESAFE_BY_RCU / SLAB_STORE_USER) clamps it to inuse, and only when nothing metadata lives does it return s->size. With that ordering in mind, SLAB_OBJ_EXT_IN_OBJ should behave like another "tail metadata" cap: put the check right before `return s->size`, and if it's set, return s->inuse instead. Curious what you think. -- Thanks, Hao > > -- > Cheers, > Harry / Hyeonggon