Re: [PATCH] x86/e820: discard high memory that can't be addressed by 32-bit systems

[Mike Rapoport <rppt@kernel.org>]

On Fri, Apr 18, 2025 at 08:33:02AM +0200, Ingo Molnar wrote:
> 
> * Nathan Chancellor <nathan@kernel.org> wrote:
> 
> > Hi Mike,
> > 
> > On Sun, Apr 13, 2025 at 11:08:58AM +0300, Mike Rapoport wrote:
> > ...
> > >  arch/x86/kernel/e820.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > index 57120f0749cc..5f673bd6c7d7 100644
> > > --- a/arch/x86/kernel/e820.c
> > > +++ b/arch/x86/kernel/e820.c
> > > @@ -1300,6 +1300,14 @@ void __init e820__memblock_setup(void)
> > >  		memblock_add(entry->addr, entry->size);
> > >  	}
> > >  
> > > +	/*
> > > +	 * 32-bit systems are limited to 4BG of memory even with HIGHMEM and
> > > +	 * to even less without it.
> > > +	 * Discard memory after max_pfn - the actual limit detected at runtime.
> > > +	 */
> > > +	if (IS_ENABLED(CONFIG_X86_32))
> > > +		memblock_remove(PFN_PHYS(max_pfn), -1);
> > > +
> > >  	/* Throw away partial pages: */
> > >  	memblock_trim_memory(PAGE_SIZE);
> > 
> > Our CI noticed a boot failure after this change as commit 1e07b9fad022
> > ("x86/e820: Discard high memory that can't be addressed by 32-bit
> > systems") in -tip when booting i386_defconfig with a simple buildroot
> > initrd.
> 
> I've zapped this commit from tip:x86/urgent for the time being:
> 
>   1e07b9fad022 ("x86/e820: Discard high memory that can't be addressed by 32-bit systems")
> 
> until these bugs are better understood.

With X86_PAE disabled phys_addr_t is 32 bit, PFN_PHYS(MAX_NONPAE_PFN)
overflows and we get memblock_remove(0, -1) :(

Using max_pfn instead of MAX_NONPAE_PFN would work because there's a hole
under 4G and max_pfn should never overflow.

Another option is to skip e820 entries above 4G and not add them to
memblock at the first place, e.g.

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 57120f0749cc..2b617f36f11a 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1297,6 +1297,17 @@ void __init e820__memblock_setup(void)
 		if (entry->type != E820_TYPE_RAM)
 			continue;
 
+#ifdef CONFIG_X86_32
+		/*
+		 * Discard memory above 4GB because 32-bit systems are limited
+		 * to 4GB of memory even with HIGHMEM.
+		 */
+		if (entry->addr > SZ_4G)
+			continue;
+		if (entry->addr + entry->size > SZ_4G)
+			entry->size = SZ_4G - entry->addr;
+#endif
+
 		memblock_add(entry->addr, entry->size);
 	}
 
 
> Thanks,
> 
> 	Ingo

-- 
Sincerely yours,
Mike.