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

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Date:   Tue, 18 Jan 2022 09:28:45 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>, x86@...nel.org
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] x86: Support huge vmalloc mappings

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.