Re: [PATCH v4 6/8] x86: bump ZO_z_extra_bytes margin for zstd

From: Sedat Dilek <sedat.dilek@gmail.com>
To: Nick Terrell <terrelln@fb.com>
Cc: Borislav Petkov <bp@alien8.de>,
	Nick Terrell <nickrterrell@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Chris Mason <clm@fb.com>,
	"linux-kbuild@vger.kernel.org" <linux-kbuild@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	Petr Malat <oss@malat.biz>, Kees Cook <keescook@chromium.org>,
	Kernel Team <Kernel-team@fb.com>,
	Adam Borowski <kilobyte@angband.pl>,
	Patrick Williams <patrickw3@fb.com>,
	Michael van der Westhuizen <rmikey@fb.com>,
	"mingo@kernel.org" <mingo@kernel.org>,
	Patrick Williams <patrick@stwcx.xyz>
Subject: Re: [PATCH v4 6/8] x86: bump ZO_z_extra_bytes margin for zstd
Date: Mon, 6 Apr 2020 12:47:30 +0200	[thread overview]
Message-ID: <CA+icZUWP0HR5ZiWq438bpcyu71cxmkv2HPqeuR2EwUAz5CjwBg@mail.gmail.com> (raw)
In-Reply-To: <2370741E-FE5A-44C1-8BF3-24A03E321F4E@fb.com>

On Thu, Apr 2, 2020 at 10:26 PM Nick Terrell <terrelln@fb.com> wrote:
>
>
>
> > On Apr 2, 2020, at 8:58 AM, Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Wed, Apr 01, 2020 at 05:33:03PM +0000, Nick Terrell wrote:
> >> The code is currently written so that all the compression algorithms use the
> >> same ZO_z_extra_bytes. It is taken to be the maximum of the growth rate
> >> plus the maximum fixed overhead. Just a few lines above is the comment:
> >>
> >> # … Hence safety
> >> # margin should be updated to cover all decompressors so that we don't
> >> # need to deal with each of them separately. Please check
> >> # the description in lib/decompressor_xxx.c for specific information.
> >>
> >> So I was been following the guidance in the comments.
> >
> > Please state that in the commit message when you send your next
> > revision.
>
> Will do.
>
> >> Does it matter? I’m not an expert here,
> >
> > Huh, you're only sending the code then? Or what do you mean with not
> > being an expert?
>
> I mean that while I’ve read and understood this piece of the code, have tested
> the patches, have followed the template of other compression methods
> added, and am confident in the correctness of the code, I’m not a regular
> contributor to the pre-boot x86 kernel code. So it is possible that there is a
> use case for kernel compression that I’m not aware of where RAM is extremely
> tight and within 64 KB of the current limits.
>
> It seems to me that adding 64KB to the memory requirement for kernel
> decompression is not going to break anyone. If it did the kernel image is taking
> up nearly all available RAM, which doesn’t seem likely. But, I don’t know all
> use cases. If it does break someone, we can put up a separate patch that
> switches all the compression methods over a per-method ZO_z_extra_bytes.
>

Hi Nick,

are you planning a zstd-v5?
If yes, please CC me, thanks.

Regards,
- Sedat -