On Fri, 15 Sept 2023 at 23:50, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > I have the feeling folks just grabbed the first big-ish chunk they saw > free in the memory map and stole that one. Not a horrible approach, > mind you, but I have the feeling it didn't go through the most rigorous > sizing procedure. :) > > My laptop memory is ~6% consumed by slab, 90% of which is reclaimable. > If a 64TB system had the same ratio, it would bump into this 512GB > limit. But it _should_ just reclaim thing earlier rather than falling over. > > That said, we still have gobs of actual vmalloc() space. It's ~30TiB in > size and I'm not aware of anyone consuming anywhere near that much. If > the 512GB fills up somehow, there are other places to steal the space. > > One minor concern is that the virtual area is the same size on 4 and > 5-level paging systems. It might be a good idea to pick one of the > holes that actually gets bigger on 5-level systems. One of the other ideas that we had was to use the KASAN shadow memory instead of a dedicated area. As far as I know the KASAN region is not used by anything else when KASAN is disabled, and I don't think it makes sense to have both KASAN and SLAB_VIRTUAL enabled at the same time (see the patch which introduces the Kconfig option for why). The KASAN region is 16 TiB on 4-level systems and 8 PiB on 5-level, in both cases 1/16th the size of the address space. Could that work? -- Matteo
