/proc/vmcore mmap() failure issue
d.hatayama at jp.fujitsu.com
Thu Nov 21 03:31:46 EST 2013
(2013/11/21 14:00), Atsushi Kumagai wrote:
> Hello Vivek,
> On 2013/11/21 0:00:01, kexec <kexec-bounces at lists.infradead.org> wrote:
>>>>>> Is there any chance that you could look into fixing this. I
>>>>>> have no experience writing code for makedumpfile.
>>>>> I'll send a patch to fix this soon.
>>>> Thanks Atsushi.
>>> Vivek, could you test this patch ?
>>> Atsushi Kumagai
>>> From: Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp>
>>> Date: Wed, 20 Nov 2013 10:05:03 +0900
>>> Subject: [PATCH] Disable mmap() for reading fractional pages.
>>> Since mmap() was introduced on /proc/vmcore, it fails for fractional
>>> pages which don't start or end at page boundary due to kernel issue.
>>> This patch disables mmap() temporarily for fractional pages to avoid
>>> this issue, so mmap() will be used only for aligned pages.
>>> Signed-off-by: Atsushi Kumagai <kumagai-atsushi at mxc.nes.nec.co.jp>
>> Hi Atsushi,
>> Even with this patch applied I see mmap() failure.
>> mem_map (39)
>> mem_map : ffffea0004e00000
>> pfn_start : 138000
>> pfn_end : 140000
>> read /proc/vmcore with mmap()
>> Excluding unnecessary pages : [100.0 %] |STEP [Excluding
>> unnecessary pages] : 0.035925 seconds
>> Excluding unnecessary pages : [100.0 %] \STEP [Excluding
>> unnecessary pages] : 0.035774 seconds
>> Excluding unnecessary pages : [100.0 %] -STEP [Excluding
>> unnecessary pages] : 0.035229 seconds
>> Copying data : [ 40.9 %] -Can't map
>> [b98fd000-b9cfd000] with mmap()
>> read_from_vmcore: Can't read the dump memory(/proc/vmcore) with mmap().
>> readpage_elf: Can't read the dump memory(/proc/vmcore).
>> readmem: type_addr: 1, addr:bffba000, size:4096
>> read_pfn: Can't get the page data.
>> Resource temporarily unavailable
>> makedumpfile Failed.
>> kdump: saving vmcore failed
>> Following is part of /proc/iomem on my system.
>> 00100000-bffc283f : System RAM
>> 01000000-018c551d : Kernel code
>> 018c551e-01ef3f3f : Kernel data
>> 0204a000-02984fff : Kernel bss
>> 2e000000-35ffffff : Crash kernel
>> bffc2840-bfffffff : reserved
>> This is a different system than what I used last time. So I am not sure if this is same error or something else. But one thing is clear that System RAM last page is partial and we should face mmap() failure.
> Thanks for your testing, I've found my mistake.
> My patch tries to disable mmap() when a partial page is found, but
> actually mmap() has already been called because update_mmap_range()
> calls mmap() for every 4MB region in advance.
> If we try to keep using mmap() as much as possible, update_mmap_range()
> has to check whether the target region of mmap() includes the partial
> pages before calling mmap(), but it's too tough as workaround.
> So I think the patch I sent is enough, the policy will be simpler as
> "Don't use mmap() for buggy kernels".
> [PATCH] Fall back to read() when mmap() fails.
I think logic becomes not so complex. For example, if input vmcore
format is ELF, then:
o in update_mmap_range():
- first calculate a range of the corresponding PT_LOAD entry truncated with
- Then, truncate range of mmap() by the truncated range of the corresponding
PT_LOAD entry, i.e., exlucde partial pages from mmap() target range.
- Then determine offsets of two partial pages; the number of partial pages
are always at most two. The offsets can easily be calculated from the
original range of the corresponding PT_LOAD entry
o in read_from_vmcore(), if a given offset belongs to either of two partial
pages, then go to read() path; if not, go to mmap() path.
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