linux-kernel - Re: [PATCH] Put each per-cpu kdump ELF notes into a single page

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Message-ID: <20140912115230.GA29945@redhat.com>
Date:	Fri, 12 Sep 2014 07:52:30 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Petr Tesarik <ptesarik@...e.cz>
Cc:	kexec@...ts.infradead.org, Eric Biederman <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Put each per-cpu kdump ELF notes into a single page

On Fri, Sep 12, 2014 at 12:15:37AM +0200, Petr Tesarik wrote:
> On Thu, 11 Sep 2014 17:16:37 -0400
> Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > On Thu, Sep 11, 2014 at 10:43:30PM +0200, Petr Tesarik wrote:
> > > On Thu, 11 Sep 2014 16:01:10 -0400
> > > Vivek Goyal <vgoyal@...hat.com> wrote:
> > > 
> > > > On Fri, Sep 05, 2014 at 06:33:14PM +0200, Petr Tesarik wrote:
> > > > > On architectures that use percpu-vm, the percpu region is not guaranteed
> > > > > to be contiguous in physical space.
> > > > 
> > > > Petr,
> > > > 
> > > > Which are those arches?
> > > 
> > > All except nommu. Actually, percpu-km will be used instead even on MMU
> > > if SMP is disabled, but since SMP is pretty standard now, I guess the
> > > vast majority of all kernels out there is affected. ;-)
> > 
> > Hi Petr,
> > 
> > To make sure I understand it correctly I will just summarize what you
> > said.
> > 
> > alloc_percpu() code does not guarantee that an object will be on physically
> > contiguous pages if object crosses page boundary. That's why we are forcing
> > allocation of object aligned to nearest higher power of two boundary of
> > object size and that way object will always be on same page (as long as object
> > is not bigger than a page).
> > 
> > Is that a fair summary?
> 
> Yes. I might add a note why physically contiguous memory is needed
> here, but maybe it's obvious to anyone dealing with kdump.

I think adding couple of lines to explain why physically contiguous notes
are needed is a good idea. It will not be ovious to anybody new to kdump.

Thanks
Vivek
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