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

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Date:   Wed, 22 Jul 2020 09:11:02 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Alex Ghiti <alex@...ti.fr>, 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

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 ?

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.

Cheers,
Ben.