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Date:   Wed, 11 May 2022 09:23:10 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFCv2 07/10] x86/mm: Handle tagged memory accesses from kernel
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References: <20220511022751.65540-1-kirill.shutemov@linux.intel.com>
 <20220511022751.65540-9-kirill.shutemov@linux.intel.com>
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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.