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From: Hao Li To: Vlastimil Babka Cc: Harry Yoo , akpm@linux-foundation.org, 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: References: <20260105080230.13171-1-harry.yoo@oracle.com> <20260105080230.13171-9-harry.yoo@oracle.com> <7uiizca4ejiqw6zegjwmou5va4kw7na7wivy4kxebrju7dsdwo@5brr7vhwf5oh> <30ecc144-ea2c-4259-afbf-3d96849aded2@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <30ecc144-ea2c-4259-afbf-3d96849aded2@suse.cz> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 1D36420011 X-Rspamd-Server: rspam04 X-Stat-Signature: sp8yx77w6j97669uiu4w9cst5czo7a4h X-HE-Tag: 1767876542-317965 X-HE-Meta: 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 GMEQBg05 tLuCf7+iTbOXyJayBfmYZeuWQuGkLZvC55oYDdOB9+JwhRHBGF2C4w9qcR469Sdn1nAqyOKzEd2Mlj5kVybcnnOgqHRTyWHqxXbg9nxc+PGAKKj7WVpqVyhdqdrS6ABZXRW94zVvFrIr06gd6ieI8IppZEJ0SbIYbN/ItaVzfg2Cbg43E+64xObi24igqnU8Kb/47q3xdLlBEZS7LxPH9j9LEHXxlSukPx1j0cil0xCiNhgZ5RnY5GPU48WkGOf/iwwQUgwkZZ9elYShV1wPY4CElyrkxnwonTyMNVUG/XPkQdagxoyntnAWCX2uMY3Hixvr/PO3pzkTXyvw= 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 11:52:36AM +0100, Vlastimil Babka wrote: > On 1/8/26 11:44, Harry Yoo wrote: > > On Thu, Jan 08, 2026 at 05:52:27PM +0800, Hao Li wrote: > >> 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. > > > > Waaaait, SLAB_TYPESAFE_BY_RCU isn't the only case where we put freelist > > pointer after the object. > > > > What about caches with constructor? > > We do place it after object, but slab_ksize() may return s->size? > > I think the freelist pointer is fine because it's not used by allocated objects? > Also ksize() should no longer be used to fill more of the object than that > was requested in the first place. Yes - being conservative here seems safest. Exposing extra bytes that callers don't expect could easily break assumptions and lead to subtle bugs. -- Thanks, Hao > > >