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

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Message-ID: <ad920491-9d73-4512-8996-badace520699@leemhuis.info>
Date: Sat, 3 Feb 2024 10:24:20 +0100
From: Thorsten Leemhuis <regressions@...mhuis.info>
To: Yang Shi <shy828301@...il.com>, jirislaby@...nel.org, surenb@...gle.com,
 riel@...riel.com, willy@...radead.org, cl@...ux.com,
 akpm@...ux-foundation.org,
 Linux kernel regressions list <regressions@...ts.linux.dev>
Cc: 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 18.01.24 14:35, Yang Shi wrote:
> From: Yang Shi <yang@...amperecomputing.com>
> 
> The commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
> boundaries") caused two issues [1] [2] reported on 32 bit system or compat
> userspace.
> 
> 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

[FWIW, this is now 4ef9ad19e17676 ("mm: huge_memory: don't force huge
page alignment on 32 bit") in mainline]

Quick question: it it okay to ask Greg to pick this up for linux-6.7.y
series?

I'm wondering because Jiri's report ([1] in above quote) sounded like
this is something that will affect and annoy quite a few people with the
linux-6.7.y.

Ciao, Thorsten

> Fixes: efa7df3e3bb5 ("mm: align larger anonymous mappings on THP boundaries")
> Reported-by: Jiri Slaby <jirislaby@...nel.org>
> Reported-by: Suren Baghdasaryan <surenb@...gle.com>
> Tested-by: Jiri Slaby <jirislaby@...nel.org>
> Tested-by: Suren Baghdasaryan <surenb@...gle.com>
> Cc: Rik van Riel <riel@...riel.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Christopher Lameter <cl@...ux.com>
> Signed-off-by: Yang Shi <yang@...amperecomputing.com>
> ---
>  mm/huge_memory.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 94ef5c02b459..e9fbaccbe0c0 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -37,6 +37,7 @@
>  #include <linux/page_owner.h>
>  #include <linux/sched/sysctl.h>
>  #include <linux/memory-tiers.h>
> +#include <linux/compat.h>
>  
>  #include <asm/tlb.h>
>  #include <asm/pgalloc.h>
> @@ -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.
> +	 */
> +	if (IS_ENABLED(CONFIG_32BIT) || in_compat_syscall())
> +		return 0;
> +
>  	if (off_end <= off_align || (off_end - off_align) < size)
>  		return 0;
>