Re: [PATCH v5 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.

From: Baoquan He <bhe@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Masayoshi Mizuma <msys.mizuma@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.
Date: Sat, 14 Dec 2019 07:29:28 +0800	[thread overview]
Message-ID: <20191213232928.GI28917@MiWiFi-R3L-srv> (raw)
In-Reply-To: <20191213163818.GB25899@zn.tnic>

On 12/13/19 at 05:38pm, Borislav Petkov wrote:
> On Fri, Dec 13, 2019 at 10:54:48PM +0800, Baoquan He wrote:
> > On 12/13/19 at 03:15pm, Borislav Petkov wrote:
> > > On Fri, Dec 13, 2019 at 09:28:50PM +0800, Baoquan He wrote:
> > > > In Documentation/x86/x86_64/mm.rst, the physical memory regions mapping
> > > > with page_offset is called as the direct mapping of physical memory.
> > > 
> > > The fact that it happens to compute the *first* region's size, which
> > > *happens* to be the direct mapping of all physical memory is immaterial
> > > here.
> > > 
> > > It is actually causing more confusion in an already complex piece of
> > > code. You can call this function just as well
> > > 
> > >   calc_region_size()
> > > 
> > > which won't confuse readers. Because all you care about here is the
> > > region's size - not which region it is.
> > 
> > Won't calc_region_size be too generic? We also have vmalloc and vmemmap,
> > and here we are specifically calculating the direct mapping of physical
> > memory.
> 
> It sounds like you didn't read what I wrote above so read it again pls.

Got it, I believe people won't be confused with calc_region_size().
It's fine to me.

> 
> > If not knowing the max address to cover all the possible hotplugged
> > memory, later memory hotplug will fail.
> 
> You don't have to state the obvious - I can see that in the code.
> 
> So let me ask you differently: can the parsing of the SRAT table happen
> shortly before kernel_randomize_memory() *without* adding all that gunk
> to the compressed stage, and without adding the boot_params member and
> done only for memory hot_add machines?

OK, you mean parsing SRAT again before kernel_randomize_memory(). I
think this is what Masa made this patchset to avoid. Then we will have
three times SRAT parsing. Passing the max addr from boot to
kernel_randomize_memory() was raised when review Chao Fan's patchset, I
vaguely remember. Chao didn't take the SRAT parsing way in boot code
firstly, later someone suggested to parse, and said the issue in
kernel_randomize_memory() can be fixed with the parsed value.

Surely, parsing SRAT here is also good. Maybe Masa can make a draft
patch, let people see what it looks like.

Thanks
Baoquan