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

[Song Liu <songliubraving@fb.com>]

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?

Thanks,
Song


> 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
>