On Wed, Jan 27, 2021 at 11:59 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Wed 27-01-21 10:42:13, Roman Gushchin wrote: > > On Tue, Jan 26, 2021 at 04:05:55PM +0100, Michal Hocko wrote: > > > On Tue 26-01-21 14:48:38, Matthew Wilcox wrote: > > > > On Mon, Jan 25, 2021 at 11:38:17PM +0200, Mike Rapoport wrote: > > > > > I cannot use __GFP_ACCOUNT because cma_alloc() does not use gfp. > > > > > Besides, kmem accounting with __GFP_ACCOUNT does not seem > > > > > to update stats and there was an explicit request for statistics: > > > > > > > > > > https://lore.kernel.org/lkml/CALo0P13aq3GsONnZrksZNU9RtfhMsZXGWhK1n=xYJWQizCd4Zw@xxxxxxxxxxxxxx/ > > > > > > > > > > As for (ab)using NR_SLAB_UNRECLAIMABLE_B, as it was already discussed here: > > > > > > > > > > https://lore.kernel.org/lkml/20201129172625.GD557259@xxxxxxxxxx/ > > > > > > > > > > I think that a dedicated stats counter would be too much at the moment and > > > > > NR_SLAB_UNRECLAIMABLE_B is the only explicit stat for unreclaimable memory. > > > > > > > > That's not true -- Mlocked is also unreclaimable. And doesn't this > > > > feel more like mlocked memory than unreclaimable slab? It's also > > > > Unevictable, so could be counted there instead. > > > > > > yes, that is indeed true, except the unreclaimable counter is tracking > > > the unevictable LRUs. These pages are not on any LRU and that can cause > > > some confusion. Maybe they shouldn't be so special and they should live > > > on unevistable LRU and get their stats automagically. > > > > > > I definitely do agree that this would be a better fit than NR_SLAB > > > abuse. But considering that this is somehow even more special than mlock > > > then a dedicated counter sounds as even better fit. > > > > I think it depends on how large these areas will be in practice. > > If they will be measured in single or double digits MBs, a separate entry > > is hardly a good choice: because of the batching the displayed value > > will be in the noise range, plus every new vmstat item adds to the > > struct mem_cgroup size. > > > > If it will be measured in GBs, of course, a separate counter is preferred. > > So I'd suggest to go with NR_SLAB (which should have been named NR_KMEM) > > as now and conditionally switch to a separate counter later. > > I really do not think the overall usage matters when it comes to abusing > other counters. Changing this in future will be always tricky and there > always be our favorite "Can this break userspace" question. Yes we dared > to change meaning of some counters but this is not generally possible. > Just have a look how accounting shmem as a page cache has turned out > being much more tricky than many like. > > Really if a separate counter is a big deal, for which I do not see any > big reason, then this should be accounted as unevictable (as suggested > by Matthew) and ideally pages of those mappings should be sitting in the > unevictable LRU as well unless there is a strong reason against. > Why not decide based on the movability of these pages? If movable then unevictable LRU seems like the right way otherwise NR_SLAB.
