linux-kernel - Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

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Date:   Thu, 23 Jul 2020 01:21:50 -0400
From:   Alex Ghiti <alex@...ti.fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     mpe@...erman.id.au, paulus@...ba.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Anup Patel <Anup.Patel@....com>,
        Atish Patra <Atish.Patra@....com>, zong.li@...ive.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 1/4] riscv: Move kernel mapping to vmalloc zone

Hi Benjamin,

Le 7/21/20 à 7:11 PM, Benjamin Herrenschmidt a écrit :
> On Tue, 2020-07-21 at 14:36 -0400, Alex Ghiti wrote:
>>>> I guess I don't understand why this is necessary at all.
>>>> Specifically: why
>>>> can't we just relocate the kernel within the linear map?  That would
>>>> let the
>>>> bootloader put the kernel wherever it wants, modulo the physical
>>>> memory size we
>>>> support.  We'd need to handle the regions that are coupled to the
>>>> kernel's
>>>> execution address, but we could just put them in an explicit memory
>>>> region
>>>> which is what we should probably be doing anyway.
>>>
>>> Virtual relocation in the linear mapping requires to move the kernel
>>> physically too. Zong implemented this physical move in its KASLR RFC
>>> patchset, which is cumbersome since finding an available physical spot
>>> is harder than just selecting a virtual range in the vmalloc range.
>>>
>>> In addition, having the kernel mapping in the linear mapping prevents
>>> the use of hugepage for the linear mapping resulting in performance loss
>>> (at least for the GB that encompasses the kernel).
>>>
>>> Why do you find this "ugly" ? The vmalloc region is just a bunch of
>>> available virtual addresses to whatever purpose we want, and as noted by
>>> Zong, arm64 uses the same scheme.
> 
> I don't get it :-)
> 
> At least on powerpc we move the kernel in the linear mapping and it
> works fine with huge pages, what is your problem there ? You rely on
> punching small-page size holes in there ?
> 

ARCH_HAS_STRICT_KERNEL_RWX prevents the use of a hugepage for the kernel 
mapping in the direct mapping as it sets different permissions to 
different part of the kernel (data, text..etc).


> At least in the old days, there were a number of assumptions that
> the kernel text/data/bss resides in the linear mapping.
> 
> If you change that you need to ensure that it's still physically
> contiguous and you'll have to tweak __va and __pa, which might induce
> extra overhead.
> 

Yes that's done in this patch and indeed there is an overhead to those 
functions.

> Cheers,
> Ben.
>   
> 

Thanks,

Alex