linux-ext4 - Re: riscv32 EXT4 splat, 6.8 regression?

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Date: Tue, 16 Apr 2024 17:31:51 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Nam Cao <namcao@...utronix.de>,
	Björn Töpel <bjorn@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-riscv@...ts.infradead.org, Theodore Ts'o <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Conor Dooley <conor@...nel.org>,
	Anders Roxell <anders.roxell@...aro.org>,
	Alexandre Ghiti <alex@...ti.fr>
Subject: Re: riscv32 EXT4 splat, 6.8 regression?

On Tue, Apr 16, 2024 at 07:19:27PM +0300, Mike Rapoport wrote:
> > "last page of the first gigabyte" - why first gigabyte? Do you mean
> > last page of *last* gigabyte?
>  
> With 3G-1G split linear map can map only 1G from 0xc0000000 to 0xffffffff
> (or 0x00000000 with 32-bit overflow):
> 
> [    0.000000]       lowmem : 0xc0000000 - 0x00000000   (1024 MB)

... but you can't map that much.  You need to reserve space for (may not
be exhaustive):

 - PCI BARs (or other MMIO)
 - vmap
 - kmap
 - percpu
 - ioremap
 - modules
 - fixmap
 - Maybe EFI runtime services?

You'll be lucky to get 800MB of ZONE_NORMAL.