linux-kernel - Re: [Part1 PATCH v4 15/17] percpu: introduce DEFINE_PER_CPU

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Date:   Wed, 20 Sep 2017 09:34:26 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>, linux-arch@...r.kernel.org
Subject: Re: [Part1 PATCH v4 15/17] percpu: introduce
 DEFINE_PER_CPU_UNENCRYPTED

On Tue, Sep 19, 2017 at 08:50:20AM -0500, Brijesh Singh wrote:
> "..shared_aligned" section does not start and end with page-size alignment.

Nowhere in the code there's a comment saying: "This percpu section really must
be page-size aligned because <reasons>." You need to be more verbose
with requirements like that.

Also, you're ending up needing a whole page per-CPU for those variables.
And now with the alignment before and after, you have worst-case two
pages fragmentation of percpu memory and percpu memory is a rather
limited resource AFAIR.

If only there were a alloc_percpu_page()...

> Since the C-bit works on PAGE_SIZE alignment hence the "..unencrypted" section

Btw, call that section "..decrypted" and everywhere do
s/unencrypted/decrypted/g.

> starts and ends with page-size alignment. The closest I can find is
> "..page_aligned" but again it does not end with page-size alignment.
> 
> Additionally, since we clear the C-bit from unencrypted section hence we
> should avoid overloading the existing section -- we don't want to expose more
> than we wish.

Add that to the comment too.

Thx.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--