linux-kernel - Re: [RFCv2 07/10] x86/mm: Handle tagged memory accesses from kernel threads

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Message-ID: <20220511072310.GU76023@worktop.programming.kicks-ass.net>
Date:   Wed, 11 May 2022 09:23:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 07/10] x86/mm: Handle tagged memory accesses from kernel
 threads

On Wed, May 11, 2022 at 05:27:48AM +0300, Kirill A. Shutemov wrote:
> When a kernel thread performs memory access on behalf of a process (like
> in async I/O, io_uring, etc.) it has to respect tagging setup of the
> process as user addresses can include tags.
> 
> Normally, LAM setup is per-thread and recorded in thread features, but
> for this use case kernel also tracks LAM setup per-mm. mm->context.lam
> would record LAM that allows the most tag bits among the threads of
> the mm.

Then why does it *ever* make sense to track it per thread? It's not like
it makes heaps of sense to allow one thread in a process to use LAM but
not the others.