Re: riscv32 EXT4 splat, 6.8 regression?

[Nam Cao]

On 2024-04-16 Mike Rapoport wrote:
> On Tue, Apr 16, 2024 at 06:00:29PM +0100, Matthew Wilcox wrote:
> > On Tue, Apr 16, 2024 at 07:31:54PM +0300, Mike Rapoport wrote:
> > > > -	if (!IS_ENABLED(CONFIG_64BIT)) {
> > > > -		max_mapped_addr = __pa(~(ulong)0);
> > > > -		if (max_mapped_addr == (phys_ram_end - 1))
> > > > -			memblock_set_current_limit(max_mapped_addr - 4096);
> > > > -	}
> > > > +	memblock_reserve(__pa(-PAGE_SIZE), PAGE_SIZE);
> > > 
> > > Ack.
> > 
> > Can this go to generic code instead of letting architecture maintainers
> > fall over it?
> 
> Yes, it's just have to happen before setup_arch() where most architectures
> enable memblock allocations.

This also works, the reported problem disappears.

However, I am confused about one thing: doesn't this make one page of
physical memory inaccessible?

Is it better to solve this by setting max_low_pfn instead? Then at
least the page is still accessible as high memory.

Best regards,
Nam

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index fa34cf55037b..6e3130cae675 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -197,7 +197,6 @@ early_param("mem", early_mem);
 static void __init setup_bootmem(void)
 {
 	phys_addr_t vmlinux_end = __pa_symbol(&_end);
-	phys_addr_t max_mapped_addr;
 	phys_addr_t phys_ram_end, vmlinux_start;
 
 	if (IS_ENABLED(CONFIG_XIP_KERNEL))
@@ -235,23 +234,9 @@ static void __init setup_bootmem(void)
 	if (IS_ENABLED(CONFIG_64BIT))
 		kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
 
-	/*
-	 * memblock allocator is not aware of the fact that last 4K bytes of
-	 * the addressable memory can not be mapped because of IS_ERR_VALUE
-	 * macro. Make sure that last 4k bytes are not usable by memblock
-	 * if end of dram is equal to maximum addressable memory.  For 64-bit
-	 * kernel, this problem can't happen here as the end of the virtual
-	 * address space is occupied by the kernel mapping then this check must
-	 * be done as soon as the kernel mapping base address is determined.
-	 */
-	if (!IS_ENABLED(CONFIG_64BIT)) {
-		max_mapped_addr = __pa(~(ulong)0);
-		if (max_mapped_addr == (phys_ram_end - 1))
-			memblock_set_current_limit(max_mapped_addr - 4096);
-	}
-
 	min_low_pfn = PFN_UP(phys_ram_base);
-	max_low_pfn = max_pfn = PFN_DOWN(phys_ram_end);
+	max_pfn = PFN_DOWN(phys_ram_end);
+	max_low_pfn = min(max_pfn, PFN_DOWN(__pa(-PAGE_SIZE)));
 	high_memory = (void *)(__va(PFN_PHYS(max_low_pfn)));
 
 	dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));