Re: [Part1 PATCH v4 15/17] percpu: introduce DEFINE_PER_CPU_UNENCRYPTED

[Borislav Petkov <bp@suse.de>]

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.

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    Boris.

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