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

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Message-Id: <1F07FFF3-663B-43D4-A9DA-C89856F2962A@dilger.ca>
Date: Wed, 17 Apr 2024 16:09:23 -0600
From: Andreas Dilger <adilger@...ger.ca>
To: Nam Cao <namcao@...utronix.de>
Cc: Mike Rapoport <rppt@...nel.org>,
 Matthew Wilcox <willy@...radead.org>,
 Björn Töpel <bjorn@...nel.org>,
 Christian Brauner <brauner@...nel.org>,
 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 Apr 16, 2024, at 4:36 PM, Nam Cao <namcao@...utronix.de> wrote:
> 
> 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.

Is that one page of memory really worthwhile to preserve?  Better to
have a simple solution that works, maybe even mapping that page
read-only so that any code which tries to dereference an ERR_PTR
address immediately gets a fault?

Cheers, Andreas

> 
> 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));


Cheers, Andreas






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