linux-kernel - Re: [PATCH v2 4/4] x86/mm: Reset global ASID space when ASID rollover

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Message-ID: <d1197b5e2799879570fe3400ddc40c4cce8c9e56.camel@surriel.com>
Date: Tue, 20 Jan 2026 12:24:49 -0500
From: Rik van Riel <riel@...riel.com>
To: Hou Wenlong <houwenlong.hwl@...group.com>, linux-kernel@...r.kernel.org
Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>,  Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>,  Borislav Petkov	
 <bp@...en8.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/mm: Reset global ASID space when ASID
 rollover

On Tue, 2026-01-20 at 21:44 +0800, Hou Wenlong wrote:
> The global ASID allocation always searches for free ASIDs starting
> from
> the last allocated ASID and resets the ASID space when the last
> allocated ASID reaches the edge of the available ASID space. However,
> this approach fails in scenarios where the maximum ASID is allocated
> first and a smaller ASID is freed later. In such cases, the
> 'last_global_asid' may never reach the edge of the available ASID
> space,
> leading to allocation failures even when global ASIDs are available.
> 
> For example, if all ASIDs are allocated in the first round and then a
> smaller ASID is freed, the ASID space can be reset because
> 'last_global_asid' equals 'MAX_ASID_AVAILABLE - 1'. The smaller ASID
> can
> then be allocated, and 'last_global_asid' is updated accordingly.
> However, when this smaller ASID is freed again, the allocation will
> fail
> since 'last_global_asid' no longer equals 'MAX_ASID_AVAILABLE - 1',
> preventing the ASID space from being reset. To address this issue,
> the
> global ASID space should be reset based on ASID rollover rather than
> depending on the value of 'last_global_asid'.
> 
> Fixes: d504d1247e36 ("x86/mm: Add global ASID allocation helper
> functions")
> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>

Reviewed-by: Rik van Riel <riel@...riel.com>

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