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

[HATAYAMA Daisuke]

From: Eugene Surovegin <surovegin@xxxxxxxxxx>
Subject: Re: [PATCH] kdump: force page alignment for per-CPU crash notes.
Date: Wed, 29 Feb 2012 17:39:55 -0800

> On Wed, Feb 29, 2012 at 5:32 PM, Simon Horman <horms@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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.
> 

This is elf_prstatus and I guess it's mostly equal to registers.

crash> p sizeof(struct elf_prstatus)
$3 = 336
crash> ptype struct elf_prstatus
type = struct elf_prstatus {
    struct elf_siginfo pr_info;
    short int pr_cursig;
    long unsigned int pr_sigpend;
    long unsigned int pr_sighold;
    pid_t pr_pid;
    pid_t pr_ppid;
    pid_t pr_pgrp;
    pid_t pr_sid;
    struct timeval pr_utime;
    struct timeval pr_stime;
    struct timeval pr_cutime;
    struct timeval pr_cstime;
    elf_gregset_t pr_reg; <-- this
    int pr_fpvalid;
}

What kinds of architecture does have so many registers? It's just my
interest. Or possibly other kinds of notes is written here?

Thanks.
HATAYAMA, Daisuke

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