linux-kernel - Re: [PATCH] riscv: Don't use hugepage mappings for vmemmap if it's not supported

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Message-ID: <20240524205146.ZaCva-qC@linutronix.de>
Date: Fri, 24 May 2024 22:51:46 +0200
From: Nam Cao <namcao@...utronix.de>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Don't use hugepage mappings for vmemmap if it's
 not supported

On Fri, May 24, 2024 at 10:07:40PM +0200, Alexandre Ghiti wrote:
> Hi Nam,
Hi Alex,

> On 08/05/2024 20:46, Nam Cao wrote:
> > On Wed, May 08, 2024 at 08:22:43PM +0200, Alexandre Ghiti wrote:
> > > > Commit ff172d4818ad ("riscv: Use hugepage mappings for vmemmap") broke XIP
> > > > kernel, because huge pages are not supported on XIP kernel.
> > > I don't understand why XIP kernels can't support huge vmalloc mappings,
> > Me neither.
> > 
> > > so I
> > > think the right fix would be to enable such mappings on XIP. WDYT?
> > I agree that is the ideal solution. But I don't want to send any new
> > feature to the stable trees (stable folks may even reject such patch).
> > So I intend that to be in another patch.
> 
> 
> I have been thinking about that, and I actually think that the real fix is
> enabling huge vmalloc mappings for XIP as it was an oversight/mistake in the
> first place. On 64-bit kernels, there are no reasons we can't use PMD
> mappings for vmalloc. So I'd rather not go with this fix.

If you have strong preference for that approach, then we can go that way.

I don't like this patch that much anyway. The only upside is that it is
safer to backport, because it is obvious that it won't break anything else.

I will do some testing with PMD mapping on XIP, and come back later.

Best regards,
Nam