netdev - Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for bpf_prog

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Date:   Tue, 5 Apr 2022 00:07:34 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "rick.p.edgecombe@...el.com" <rick.p.edgecombe@...el.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Song Liu <song@...nel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "pmenzel@...gen.mpg.de" <pmenzel@...gen.mpg.de>
Subject: Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for
 bpf_prog_pack

On Fri, Apr 01, 2022 at 10:22:00PM +0000, Song Liu wrote:
> >> Please fix the underlying issues instead of papering over them and
> >> creating a huge maintainance burden for others.
> 
> After reading the code a little more, I wonder what would be best strategy. 
> IIUC, most of the kernel is not ready for huge page backed vmalloc memory.
> For example, all the module_alloc cannot work with huge pages at the moment.
> And the error Paul Menzel reported in drm_fb_helper.c will probably hit 
> powerpc with 5.17 kernel as-is? (trace attached below) 
> 
> Right now, we have VM_NO_HUGE_VMAP to let a user to opt out of huge pages. 
> However, given there are so many users of vmalloc, vzalloc, etc., we 
> probably do need a flag for the user to opt-in? 
> 
> Does this make sense? Any recommendations are really appreciated. 

I think there is multiple aspects here:

 - if we think that the kernel is not ready for hugepage backed vmalloc
   in general we need to disable it in powerpc for now.
 - if we think even in the longer run only some users can cope with
   hugepage backed vmalloc we need to turn it into an opt-in in
   general and not just for x86
 - there still to appear various unresolved underlying x86 specific
   issues that need to be fixed either way