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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? Ah, right. Nevermind. I was just confused while reading the code/comment. > Also ksize() should no longer be used to fill more of the object than that > was requested in the first place. since v6.1, yeah. -- Cheers, Harry / Hyeonggon