Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for bpf_prog_pack

From: Song Liu <songliubraving@fb.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: "rick.p.edgecombe@intel.com" <rick.p.edgecombe@intel.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>,
	Song Liu <song@kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
	X86 ML <x86@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"andrii@kernel.org" <andrii@kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"pmenzel@molgen.mpg.de" <pmenzel@molgen.mpg.de>
Subject: Re: [PATCH bpf 0/4] introduce HAVE_ARCH_HUGE_VMALLOC_FLAG for bpf_prog_pack
Date: Tue, 5 Apr 2022 23:54:37 +0000	[thread overview]
Message-ID: <482D450C-9006-4979-8736-A9F1B47246E4@fb.com> (raw)
In-Reply-To: <YkvqtvNFtzDNkEhy@infradead.org>

> On Apr 5, 2022, at 12:07 AM, Christoph Hellwig <hch@infradead.org> wrote:
> 
> 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.

Nicholas and Claudio, 

What do you think about the status of hugepage backed vmalloc on powerpc? 
I found module_alloc and kvm_s390_pv_alloc_vm() opt-out of huge pages.
But I am not aware of users that benefit from huge pages (except vfs hash,
which was mentioned in 8abddd968a30). Does an opt-in flag (instead of 
current opt-out flag, VM_NO_HUGE_VMAP) make sense to you? 

Thanks,
Song

> - 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