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Date: Sat, 21 May 2022 03:20:28 +0000 From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com> To: "song@...nel.org" <song@...nel.org>, "arnd@...db.de" <arnd@...db.de>, "mcgrof@...nel.org" <mcgrof@...nel.org> CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "daniel@...earbox.net" <daniel@...earbox.net>, "peterz@...radead.org" <peterz@...radead.org>, "ast@...nel.org" <ast@...nel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>, "kernel-team@...com" <kernel-team@...com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "Torvalds, Linus" <torvalds@...ux-foundation.org> Subject: Re: [PATCH v3 bpf-next 5/8] bpf: use module_alloc_huge for bpf_prog_pack On Fri, 2022-05-20 at 18:00 -0700, Luis Chamberlain wrote: > although VM_FLUSH_RESET_PERMS is rather new my concern here is we're > essentially enabling sloppy users to grow without also addressing > what if we have to take the leash back to support > VM_FLUSH_RESET_PERMS > properly? If the hack to support this on other architectures other > than > x86 is as simple as the one you in vm_remove_mappings() today: > > if (flush_reset && > !IS_ENABLED(CONFIG_ARCH_HAS_SET_DIRECT_MAP)) { > set_memory_nx(addr, area->nr_pages); > set_memory_rw(addr, area->nr_pages); > } > > then I suppose this isn't a big deal. I'm just concerned here this > being > a slippery slope of sloppiness leading to something which we will > regret later. > > My intution tells me this shouldn't be a big issue, but I just want > to > confirm. Yea, I commented the same concern on the last thread: https://lore.kernel.org/lkml/83a69976cb93e69c5ad7a9511b5e57c402eee19d.camel@intel.com/ Song said he plans to make kprobes and ftrace work with this new allocator. If that happens VM_FLUSH_RESET_PERMS would only have one user - modules. Care to chime in with your plans for modules? If there are actual near term plans to keep working on this, VM_FLUSH_RESET_PERMS might be changed again or turn into something else. Like if we are about to re-think everything, then it doesn't matter as much to fix what would then be old. Besides not fixing VM_FLUSH_RESET_PERMS/hibernate though, I think this allocator still feels a little rough. For example I don't think we actually know how much the huge mappings are helping. It is also allocating memory in a big chunk from a single node and reusing it, where before we were allocating based on numa node for each jit. Would some user's suffer from that? Maybe it's obvious to others, but I would have expected to see more discussion of MM things like that. But I like general direction of caching and using text_poke() to write the jits a lot. However it works, it seems to make a big impact in at least some workloads. So yea, seems sloppy, but probably (...I guess?) more good for users then sloppy for us.
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