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

From: Song Liu <songliubraving@fb.com>
To: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	"rick.p.edgecombe@intel.com" <rick.p.edgecombe@intel.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: Fri, 8 Apr 2022 21:22:47 +0000	[thread overview]
Message-ID: <3D4CC64A-1852-45B3-BA35-17EAD2504447@fb.com> (raw)
In-Reply-To: <20220408120831.69b80310@p-imbrenda>

> On Apr 8, 2022, at 3:08 AM, Claudio Imbrenda <imbrenda@linux.ibm.com> wrote:
> 
> On Thu, 7 Apr 2022 19:57:25 +0000
> Song Liu <songliubraving@fb.com> wrote:
> 
>> Hi Nicholas and Claudio, 
>> 
>>> On Apr 5, 2022, at 4:54 PM, Song Liu <songliubraving@fb.com> wrote:
>>> 
>>>> 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?   
>> 
>> Could you please share your comments on this? Specifically, does it make 
>> sense to replace VM_NO_HUGE_VMAP with an opt-in flag? If we think current
>> opt-out flag is better approach, what would be the best practice to find 
>> all the cases to opt-out?
> 
> An opt in flag would surely make sense, and it would be more backwards
> compatible with existing code. That way each user can decide whether to
> fix the code to allow for hugepages, if possible at all. For example,
> the case you mentioned for s390 (kvm_s390_pv_alloc_vm) would not be
> fixable, because of a hardware limitation (the whole area _must_ be
> mapped with 4k pages)
> 
> If the consensus were to keep the current opt-put, then I guess each
> user would have to check each usage of vmalloc and similar, and see if
> anything breaks. To be honest, I think an opt-out would only be
> possible after having the opt-in for a (long) while, when most users
> would have fixed their code.
> 
> In short, I fully support opt-in.

Thanks Claudio!

I will prepare patches to replace VM_NO_HUGE_VMAP with an opt-in flag, 
and use the new flag in BPF. Please let me know any comments/suggestions
ont this direction. 

Song