linux-kernel - Re: [PATCH] mm: hugetlb: avoid soft lockup when mprotect with PROT

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Message-ID: <aNbFtaVr8f2klqdD@arm.com>
Date: Fri, 26 Sep 2025 17:56:21 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Yang Shi <yang@...amperecomputing.com>, muchun.song@...ux.dev,
	osalvador@...e.de, david@...hat.com, akpm@...ux-foundation.org,
	will@...nel.org, carl@...amperecomputing.com, linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: hugetlb: avoid soft lockup when mprotect with
 PROT_MTE

On Fri, Sep 26, 2025 at 09:29:54AM -0700, Christoph Lameter (Ampere) wrote:
> On Fri, 26 Sep 2025, Yang Shi wrote:
> > When calling mprotect() with PROT_MTE, kernel will initialize MTE tags
> > for every single page in the affected area. Soft lockup was observed
> > when doing this for large HugeTLB memory area in our customer's workload
> > (~300GB memory):
> 
> AFAICT this is a bug fix. The hugetlb path should be doing a
> cond_resched() like the base page code does.
> 
> It is not MTE specific. If other processing takes a long time in the loop
> (setting up terabyte size mappings for hugetlb for example) then the
> softlockup could also be triggered on non MTE workloads.

Yeah, with MTE set_huge_pte_at() isn't just setting a pte but also
clearing the tags. So it can take considerable time.

The fix is indeed not related to MTE, so I don't think the Fixes tag
should mention MTE (but I'm fine with a cc stable). Let's say we change
a hugetlb from RW to RX and have to do cache maintenance, we'd trigger a
similar soft lockup, depending on how fast the system is.

Reviewed-by: Catalin Marinas <catalin.marinas@....com>