linux-kernel - Re: [PATCH 1/1] alpha: fix user-space corruption during memory compaction

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Message-ID: <aVkcmDm1z5PGZNg6@minute>
Date: Sat, 3 Jan 2026 14:41:44 +0100
From: Ivan Kokshaysky <ink@...een.parts>
To: Magnus Lindholm <linmag7@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
	hch@...radead.org, macro@...am.me.uk, glaubitz@...sik.fu-berlin.de,
	mattst88@...il.com, richard.henderson@...aro.org
Subject: Re: [PATCH 1/1] alpha: fix user-space corruption during memory
 compaction

On Fri, Jan 02, 2026 at 06:30:43PM +0100, Magnus Lindholm wrote:
> Alpha systems can suffer sporadic user-space crashes and heap
> corruption when memory compaction is enabled.
> 
> Symptoms include SIGSEGV, glibc allocator failures (e.g. "unaligned
> tcache chunk"), and compiler internal errors. The failures disappear
> when compaction is disabled or when using global TLB invalidation.
> 
> The root cause is insufficient TLB shootdown during page migration.
> Alpha relies on ASN-based MM context rollover for instruction cache
> coherency, but this alone is not sufficient to prevent stale data or
> instruction translations from surviving migration.
> 
> Fix this by introducing a migration-specific helper that combines:
>   - MM context invalidation (ASN rollover),
>   - immediate per-CPU TLB invalidation (TBI),
>   - synchronous cross-CPU shootdown when required.
> 
> The helper is used only by migration/compaction paths to avoid changing
> global TLB semantics.
> 
> Additionally, update flush_tlb_other(), pte_clear(), to use
> READ_ONCE()/WRITE_ONCE() for correct SMP memory ordering.
> 
> This fixes observed crashes on both UP and SMP Alpha systems.
> 
> Signed-off-by: Magnus Lindholm <linmag7@...il.com>

Impressive work! Spot on, I guess.

Reviewed-by: Ivan Kokshaysky <ink@...een.parts>

Ivan.