linux-kernel - Re: [PATCHv3 3/8] mm: Pass down mm_struct to untagged

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Date:   Sat, 18 Jun 2022 01:38:13 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.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 Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv3 3/8] mm: Pass down mm_struct to untagged_addr()

On Fri, Jun 17, 2022 at 05:27:46PM +0200, Alexander Potapenko wrote:
> On Fri, Jun 10, 2022 at 4:35 PM Kirill A. Shutemov
> <kirill.shutemov@...ux.intel.com> wrote:
> >
> > Intel Linear Address Masking (LAM) brings per-mm untagging rules. Pass
> > down mm_struct to the untagging helper. It will help to apply untagging
> > policy correctly.
> >
> > In most cases, current->mm is the one to use, but there are some
> > exceptions, such as get_user_page_remote().
> 
> Wouldn't it be easier to keep using current->mm in untagged_addr(addr)
> by default, and introduce a separate macro for the exceptions?

I don't think it is a good idea. Explicit mm forces writer to consider
what mm she wants to use in the particular case.

> > +/*
> > + * Architectures that support memory tagging (assigning tags to memory regions,
> > + * embedding these tags into addresses that point to these memory regions, and
> > + * checking that the memory and the pointer tags match on memory accesses)
> > + * redefine this macro to strip tags from pointers.
> > + * It's defined as noop for architectures that don't support memory tagging.
> > + */
> > +#ifndef untagged_addr
> > +#define untagged_addr(mm, addr) (addr)
> > +#endif
> The comment above should probably be extended to explain the effect of `mm`.

Sure, will update.

-- 
 Kirill A. Shutemov