linux-kernel - Re: [PATCH] kdump: force page alignment for per-CPU crash notes.

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Message-ID: <20120301025328.GF10053@verge.net.au>
Date:	Thu, 1 Mar 2012 11:53:29 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Eugene Surovegin <surovegin@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	kexec-list <kexec@...ts.infradead.org>,
	Eric Biederman <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH] kdump: force page alignment for per-CPU crash notes.

On Wed, Feb 29, 2012 at 05:39:55PM -0800, Eugene Surovegin wrote:
> On Wed, Feb 29, 2012 at 5:32 PM, Simon Horman <horms@...ge.net.au> wrote:
> > On Wed, Feb 29, 2012 at 05:23:10PM -0800, Eugene Surovegin wrote:
> >> On Wed, Feb 29, 2012 at 5:18 PM, Simon Horman <horms@...ge.net.au> wrote:
> >>
> >> > On Wed, Feb 29, 2012 at 09:21:23AM -0800, Eugene Surovegin wrote:
> >> > > Per-CPU allocations are not guaranteed to be physically contiguous.
> >> > > However, kdump kernel and user-space code assumes that per-CPU
> >> > > memory, used for saving CPU registers on crash, is.
> >> > > This can cause corrupted /proc/vmcore in some cases - the main
> >> > > symptom being huge ELF note section.
> >> > >
> >> > > Force page alignment for note_buf_t to ensure that this assumption holds.
> >> >
> >> > Ouch. I'm surprised there is an allocation on crash, perhaps
> >> > it could at least be done earlier? And am I right in thinking
> >> > that this change increases the likely hood that the allocation
> >> > could fail?
> >> >
> >>
> >> I'm not following. This allocation is done on start-up, not on crash.
> >> If you cannot allocate this much memory on system boot, I'm not sure what
> >> else you can do on this system....
> >
> > Sorry, my eyes deceived me. You are correct and I agree.
> >
> > Is it the case that note_buf_t is never larger than PAGE_SIZE?
> > If so I your patch looks good to me.
> 
> Currently, maximum note size is hardcoded in kexec-tools to 1024
> (MAX_NOTE_BYTES).
> Usually it's way less. IIRC on x86_64 it's 336 bytes.

Ok, understood.

Reviewed-by: Simon Horman <horms@...ge.net.au>
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