linux-kernel - Re: [PATCH] mm: huge_memory: don't force huge page alignment on 32 bit

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Date: Thu, 18 Jan 2024 20:14:43 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Yang Shi <shy828301@...il.com>
Cc: jirislaby@...nel.org, surenb@...gle.com, riel@...riel.com, cl@...ux.com,
	akpm@...ux-foundation.org, yang@...amperecomputing.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: huge_memory: don't force huge page alignment on 32
 bit

On Thu, Jan 18, 2024 at 05:35:04AM -0800, Yang Shi wrote:
> It doesn't make too much sense to force huge page alignment on 32 bit
> system due to the constrained virtual address space.
> 
> [1] https://lore.kernel.org/linux-mm/CAHbLzkqa1SCBA10yjWTtA2mKCsoK5+M1BthSDL8ROvUq2XxZMw@mail.gmail.com/T/#mf211643a0427f8d6495b5b53f8132f453d60ab95
> [2] https://lore.kernel.org/linux-mm/CAHbLzkqa1SCBA10yjWTtA2mKCsoK5+M1BthSDL8ROvUq2XxZMw@mail.gmail.com/T/#me93dff2ccbd9902c3e395e1c022fb454e48ecb1d

I feel sure there are shorter URLs for those messages ...

> @@ -811,6 +812,14 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>  	loff_t off_align = round_up(off, size);
>  	unsigned long len_pad, ret;
>  
> +	/*
> +	 * It doesn't make too much sense to froce huge page alignment on
> +	 * 32 bit system or compat userspace due to the contrained virtual
> +	 * address space and address entropy.
> +	 */

I honestly wouldn't even comment this.  But if you must,

	/* Using THP alignment is not as important as address randomisation */

> +	if (IS_ENABLED(CONFIG_32BIT) || in_compat_syscall())
> +		return 0;
> +
>  	if (off_end <= off_align || (off_end - off_align) < size)
>  		return 0;
>  
> -- 
> 2.41.0
>