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

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Date:	Wed, 29 Feb 2012 17:39:55 -0800
From:	Eugene Surovegin <surovegin@...gle.com>
To:	Simon Horman <horms@...ge.net.au>
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 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.

--
Eugene
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