linux-kernel - Re: [PATCH] x86/mm: Handle alloc failure in phys_*

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Message-ID: <aEE6_S2a-1tk1dtI@gmail.com>
Date: Thu, 5 Jun 2025 08:36:45 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Em Sharnoff <sharnoff@...n.tech>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	Oleg Vasilev <oleg@...n.tech>,
	Arthur Petukhovsky <arthur@...n.tech>,
	Stefan Radig <stefan@...n.tech>, Misha Sakhnov <misha@...n.tech>
Subject: Re: [PATCH] x86/mm: Handle alloc failure in phys_*_init()

* Em Sharnoff <sharnoff@...n.tech> wrote:

> tl;dr:
> 
> * When setting up page table mappings for physical addresses after boot,
>   alloc_low_page() uses GFP_ATOMIC, which is allowed to fail.
> * This isn't currently handled, and results in a null pointer
>   dereference when it occurs.
> * This allocation failure can happen during memory hotplug.
> 
> To handle failure, change phys_pud_init() and similar functions to
> return zero if allocation failed (either directly or transitively), and
> convert that to -ENOMEM in arch_add_memory().

> +		/*
> +		 * Bail only after updating pgd/p4d to keep progress from p4d across retries.
> +		 */
> +		if (!paddr_last)
> +			return 0;
> +
>  		pgd_changed = true;

> -	init_memory_mapping(start, start + size, params->pgprot);
> +	if (!init_memory_mapping(start, start + size, params->pgprot))
> +		return -ENOMEM;

I agree that it makes total sense to fix all this (especially since you 
are actively triggering it), but have you tried also changing it away 
from GFP_ATOMIC? There's no real reason why it should be GFP_ATOMIC 
AFAICS, other than some historic inertia that nobody bothered to fix.

Plus, could you please change the return flow from this zero 
special-case over to something like ERR_PTR(-ENOMEM) and IS_ERR()?

*Technically* zero is a valid physical address, although we 
intentionally never use it in the kernel AFAIK and wouldn't ever put a 
page table there either. ERR_PTR()/IS_ERR() is much easier on the eyes 
than the zero special-case.

Finally, could you make this a 2-patch fix series: first one to fix the 
error return path to not crash, and the second one to change it away 
from GFP_ATOMIC?

Thanks,

	Ingo