linux-kernel - Re: [PATCH 1/2] riscv: force PAGE_SIZE linear mapping if debug

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Date: Wed, 22 May 2024 13:16:56 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Nam Cao <namcao@...utronix.de>, 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
Cc: stable@...r.kernel.org
Subject: Re: [PATCH 1/2] riscv: force PAGE_SIZE linear mapping if
 debug_pagealloc is enabled

Hi Nam,

On 15/05/2024 07:50, Nam Cao wrote:
> debug_pagealloc is a debug feature which clears the valid bit in page table
> entry for freed pages to detect illegal accesses to freed memory.
>
> For this feature to work, virtual mapping must have PAGE_SIZE resolution.
> (No, we cannot map with huge pages and split them only when needed; because
> pages can be allocated/freed in atomic context and page splitting cannot be
> done in atomic context)
>
> Force linear mapping to use small pages if debug_pagealloc is enabled.
>
> Note that it is not necessary to force the entire linear mapping, but only
> those that are given to memory allocator. Some parts of memory can keep
> using huge page mapping (for example, kernel's executable code). But these
> parts are minority, so keep it simple. This is just a debug feature, some
> extra overhead should be acceptable.
>
> Fixes: 5fde3db5eb02 ("riscv: add ARCH_SUPPORTS_DEBUG_PAGEALLOC support")
> Signed-off-by: Nam Cao <namcao@...utronix.de>
> Cc: stable@...r.kernel.org
> ---
> Interestingly this feature somehow still worked when first introduced.
> My guess is that back then only 2MB page size is used. When a 4KB page is
> freed, the entire 2MB will be (incorrectly) invalidated by this feature.
> But 2MB is quite small, so no one else happen to use other 4KB pages in
> this 2MB area. In other words, it used to work by luck.
>
> Now larger page sizes are used, so this feature invalidate large chunk of
> memory, and the probability that someone else access this chunk and
> trigger a page fault is much higher.
>
>   arch/riscv/mm/init.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 2574f6a3b0e7..73914afa3aba 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -682,6 +682,9 @@ void __init create_pgd_mapping(pgd_t *pgdp,
>   static uintptr_t __init best_map_size(phys_addr_t pa, uintptr_t va,
>   				      phys_addr_t size)
>   {
> +	if (debug_pagealloc_enabled())
> +		return PAGE_SIZE;
> +
>   	if (pgtable_l5_enabled &&
>   	    !(pa & (P4D_SIZE - 1)) && !(va & (P4D_SIZE - 1)) && size >= P4D_SIZE)
>   		return P4D_SIZE;


You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex