linux-kernel - Re: [PATCH] mm: Fix access_remote_vm() regression on tagged addresses

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Message-ID: <CAHk-=whS19C=y32vNMRp7UfQMVw38HOfzhs9v5rjLayEFjMNPA@mail.gmail.com>
Date:   Thu, 10 Aug 2023 11:39:12 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Schimpe, Christina" <christina.schimpe@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] mm: Fix access_remote_vm() regression on tagged addresses

On Thu, 10 Aug 2023 at 05:42, Schimpe, Christina
<christina.schimpe@...el.com> wrote:
>
> We don't have any LAM support in GDB yet, we are just working on it.
> We currently rely on that feature, but could still change it. We don't
> necessarily require /proc/PID/mem to support tagged addresses.
>
> ARM's TBI support in GDB does not rely on /proc/PID/mem to support tagged
> addresses AFAIK.

Ahh. That would explain why nobody noticed.

I do wonder if perhaps /proc/<pid>/mem should just match the real
addresses (ie the ones you would see in /proc/<pid>/maps).

The main reason GUP does the untagging is that obviously people will
pass in their own virtual addresses when doing direct-IO etc.

So /proc/<pid>/mem is a bit different.

That said, untagging does make some things easier, so I think it's
probably the right thing to do.

             Linus