Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings

[Dave Hansen <dave.hansen@intel.com>]

On 1/17/22 6:46 PM, Nicholas Piggin wrote:
>> This all sounds very fragile to me.  Every time a new architecture would
>> get added for huge vmalloc() support, the developer needs to know to go
>> find that architecture's module_alloc() and add this flag.
> This is documented in the Kconfig.
> 
>  #
>  #  Archs that select this would be capable of PMD-sized vmaps (i.e.,
>  #  arch_vmap_pmd_supported() returns true), and they must make no assumptions
>  #  that vmalloc memory is mapped with PAGE_SIZE ptes. The VM_NO_HUGE_VMAP flag
>  #  can be used to prohibit arch-specific allocations from using hugepages to
>  #  help with this (e.g., modules may require it).
>  #
>  config HAVE_ARCH_HUGE_VMALLOC
>          depends on HAVE_ARCH_HUGE_VMAP
>          bool
> 
> Is it really fair to say it's *very* fragile? Surely it's reasonable to 
> read the (not very long) documentation ad understand the consequences for
> the arch code before enabling it.

Very fragile or not, I think folks are likely to get it wrong.  It would
be nice to have it default *everyone* to safe and slow and make *sure*
they go look at the architecture modules code itself before enabling
this for modules.

Just from that Kconfig text, I don't think I'd know off the top of my
head what do do for x86, or what code I needed to go touch.