On Fri 18-10-24 09:04:24, Suren Baghdasaryan wrote: > On Fri, Oct 18, 2024 at 6:03 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Tue 15-10-24 08:58:59, Suren Baghdasaryan wrote: > > > On Tue, Oct 15, 2024 at 8:42 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > [...] > > > > Right, I think what John is concerned about (and me as well) is that > > > > once a new feature really needs a page flag, there will be objection > > > > like "no you can't, we need them for allocation tags otherwise that > > > > feature will be degraded". > > > > > > I do understand your concern but IMHO the possibility of degrading a > > > feature should not be a reason to always operate at degraded capacity > > > (which is what we have today). If one is really concerned about > > > possible future regression they can set > > > CONFIG_PGALLOC_TAG_USE_PAGEFLAGS=n and keep what we have today. That's > > > why I'm strongly advocating that we do need > > > CONFIG_PGALLOC_TAG_USE_PAGEFLAGS so that the user has control over how > > > this scarce resource is used. > > > > I really do not think users will know how/why to setup this and I wouldn't > > even bother them thinking about that at all TBH. > > > > This is an implementation detail. It is fine to reuse unused flags space > > as a storage as a performance optimization but why do you want users to > > bother with that? Why would they ever want to say N here? > > In this patch you can find a couple of warnings that look like this: > > pr_warn("With module %s there are too many tags to fit in %d page flag > bits. Memory profiling is disabled!\n", mod->name, > NR_UNUSED_PAGEFLAG_BITS); > emitted when we run out of page flag bits during a module loading, > > pr_err("%s: alignment %lu is incompatible with allocation tag > indexing, disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS", mod->name, > align); > emitted when the arch-specific section alignment is incompatible with > alloc_tag indexing. You are asking users to workaround implementation issue by configuration which sounds like a really bad idea. Why cannot you make the fallback automatic? -- Michal Hocko SUSE Labs