linux-kernel - Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

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Date:   Sat, 22 May 2021 00:46:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Len Brown <lenb@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen via Libc-alpha <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Bae\, Chang Seok" <chang.seok.bae@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>, Borislav Petkov <bp@...en8.de>,
        Keno Fischer <keno@...iacomputing.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Willy Tarreau <w@....eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Fri, May 21 2021 at 18:07, Len Brown wrote:
> On Fri, May 21, 2021 at 3:10 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> Regarding pre-allocation vs on-demand allocation, consider two scenarios:
>
> 1. Synchronous.  At process or thread start up time, prctl()
> synchronously allocates 8K context switch buffers.  Return code is 0
> -- good go go!  10 seconds later the program decides to create
> additional threads.  Woops. vmalloc failed, and the process
> synchronously dies.  bug filed.

No. pthread_create() will fail with -ENOMEM. A return value of
-ENOMEM is not a bug. 

If the application fails to check the error code then it's not the
kernels problem and not a kernel bug either.

> 2. On demand.  Same scenario, except vmalloc failure upon creation of
> those additional threads sends a SIGSEGV at the instruction where AMX
> is touched.  bug filed.
>
> Why ignore the 2nd bug and not ignore the 1st bug?

See above.

> My concern about synchronous allocation is that it will be very easy
> to abuse.  programs and threads can ask for buffers they will never
> use.  With on-demand allocation, we allocate buffers only if they are
> actually needed.

Programs ask for memory in various ways. The buffer is not any different
than any other memory allocation of the application/thread. It's
accounted for and when the limits are reached the allocation fails.

But it fails in a way which can be acted upon at the application level
and not in a way where the kernel has no other choice than killing the
whole process.

So where is the problem? 

Thanks,

        tglx