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From: ebiederm@xmission.com (Eric W. Biederman)
To: Vivek Goyal <vgoyal@redhat.com>
Cc: kexec@lists.infradead.org, heiko.carstens@de.ibm.com,
	linux-kernel@vger.kernel.org, lisa.mitchell@hp.com,
	HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>,
	kumagai-atsushi@mxc.nes.nec.co.jp, zhangyanfei@cn.fujitsu.com,
	akpm@linux-foundation.org, cpw@sgi.com
Subject: Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement
Date: Wed, 20 Mar 2013 13:55:55 -0700	[thread overview]
Message-ID: <87txo5bxk4.fsf@xmission.com> (raw)
In-Reply-To: <20130320135716.GE17274@redhat.com> (Vivek Goyal's message of "Wed, 20 Mar 2013 09:57:16 -0400")

Vivek Goyal <vgoyal@redhat.com> writes:

> On Tue, Mar 19, 2013 at 03:38:45PM -0700, Eric W. Biederman wrote:
>> HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> writes:
>> 
>> > If there's some vmcore object that doesn't satisfy page-size boundary
>> > requirement, remap_pfn_range() fails to remap it to user-space.
>> >
>> > Objects that posisbly don't satisfy the requirement are ELF note
>> > segments only. The memory chunks corresponding to PT_LOAD entries are
>> > guaranteed to satisfy page-size boundary requirement by the copy from
>> > old memory to buffer in 2nd kernel done in later patch.
>> >
>> > This patch doesn't copy each note segment into the 2nd kernel since
>> > they amount to so large in total if there are multiple CPUs. For
>> > example, current maximum number of CPUs in x86_64 is 5120, where note
>> > segments exceed 1MB with NT_PRSTATUS only.
>> 
>> So you require the first kernel to reserve an additional 20MB, instead
>> of just 1.6MB.  336 bytes versus 4096 bytes.
>> 
>> That seems like completely the wrong tradeoff in memory consumption,
>> filesize, and backwards compatibility.
>
> Agreed. 
>
> So we already copy ELF headers in second kernel's memory. If we start
> copying notes too, then both headers and notes will support mmap().

The only real is it could be a bit tricky to allocate all of the memory
for the notes section on high cpu count systems in a single allocation.

> For mmap() of memory regions which are not page aligned, we can map
> extra bytes (as you suggested in one of the mails). Given the fact
> that we have one ELF header for every memory range, we can always modify
> the file offset where phdr data is starting to make space for mapping
> of extra bytes.

Agreed ELF file offset % PAGE_SIZE should == physical address % PAGE_SIZE to
make mmap work.

> That way whole of vmcore should be mmappable and user does not have
> to worry about reading part of the file and mmaping the rest.

That sounds simplest.

If core counts on the high end do more than double every 2 years we
might have a problem.  Otherwise making everything mmapable seems easy
and sound.

Eric

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  reply	other threads:[~2013-03-20 20:56 UTC|newest]

Thread overview: 76+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-03-16  4:00 [PATCH v3 00/21] kdump, vmcore: support mmap() on /proc/vmcore HATAYAMA Daisuke
2013-03-16  4:00 ` [PATCH v3 01/21] vmcore: reference e_phoff member explicitly to get position of program header table HATAYAMA Daisuke
2013-03-19 21:44   ` Eric W. Biederman
2013-03-21  2:50     ` HATAYAMA Daisuke
2013-03-21  6:11       ` Eric W. Biederman
2013-03-21 14:12       ` Vivek Goyal
2013-03-22  0:25         ` HATAYAMA Daisuke
2013-03-16  4:00 ` [PATCH v3 02/21] vmcore: clean up by removing unnecessary variable HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 03/21] vmcore: rearrange program headers without assuming consequtive PT_NOTE entries HATAYAMA Daisuke
2013-03-19 21:59   ` Eric W. Biederman
2013-03-16  4:01 ` [PATCH v3 04/21] vmcore, sysfs: export ELF note segment size instead of vmcoreinfo data size HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 05/21] vmcore: allocate buffer for ELF headers on page-size alignment HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 06/21] vmcore: round up buffer size of ELF headers by PAGE_SIZE HATAYAMA Daisuke
2013-03-19 22:07   ` Eric W. Biederman
2013-03-16  4:01 ` [PATCH v3 07/21] vmcore, procfs: introduce a flag to distinguish objects copied in 2nd kernel HATAYAMA Daisuke
2013-03-19 19:35   ` Andrew Morton
2013-03-16  4:01 ` [PATCH v3 08/21] vmcore: copy non page-size aligned head and tail pages " HATAYAMA Daisuke
2013-03-19 19:37   ` Andrew Morton
2013-03-19 20:59   ` Eric W. Biederman
2013-03-19 21:22     ` Vivek Goyal
2013-03-19 23:35       ` Eric W. Biederman
2013-03-16  4:01 ` [PATCH v3 09/21] vmcore: modify vmcore clean-up function to free buffer on " HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 10/21] vmcore: clean up read_vmcore() HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 11/21] vmcore: read buffers for vmcore objects copied from old memory HATAYAMA Daisuke
2013-03-16  4:01 ` [PATCH v3 12/21] vmcore: allocate per-cpu crash_notes objects on page-size boundary HATAYAMA Daisuke
2013-03-19 21:06   ` Eric W. Biederman
2013-03-19 22:12   ` Eric W. Biederman
2013-03-20 13:48     ` Vivek Goyal
2013-03-20 20:48       ` Eric W. Biederman
2013-03-16  4:02 ` [PATCH v3 13/21] kexec: allocate vmcoreinfo note buffer " HATAYAMA Daisuke
2013-03-19 21:07   ` Eric W. Biederman
2013-03-19 22:12   ` Eric W. Biederman
2013-03-16  4:02 ` [PATCH v3 14/21] kexec, elf: introduce NT_VMCORE_DEBUGINFO note type HATAYAMA Daisuke
2013-03-16  4:02 ` [PATCH v3 15/21] elf: introduce NT_VMCORE_PAD type HATAYAMA Daisuke
2013-03-16  4:02 ` [PATCH v3 16/21] kexec: fill note buffers by NT_VMCORE_PAD notes in page-size boundary HATAYAMA Daisuke
2013-03-19 22:17   ` Eric W. Biederman
2013-03-16  4:02 ` [PATCH v3 17/21] vmcore: check NT_VMCORE_PAD as a mark indicating the end of ELF note buffer HATAYAMA Daisuke
2013-03-19 21:11   ` Eric W. Biederman
2013-03-21  2:59     ` HATAYAMA Daisuke
2013-03-21  3:54       ` Eric W. Biederman
2013-03-21 14:36         ` Vivek Goyal
2013-03-22  0:30           ` HATAYAMA Daisuke
2013-03-22  0:41             ` Eric W. Biederman
2013-03-19 22:20   ` Eric W. Biederman
2013-03-16  4:02 ` [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement HATAYAMA Daisuke
2013-03-19 20:02   ` Andrew Morton
2013-03-19 21:22     ` Eric W. Biederman
2013-03-20 13:51     ` Vivek Goyal
2013-03-19 22:38   ` Eric W. Biederman
2013-03-20 13:57     ` Vivek Goyal
2013-03-20 20:55       ` Eric W. Biederman [this message]
2013-03-21  3:25         ` HATAYAMA Daisuke
2013-03-21  4:18           ` Eric W. Biederman
2013-03-21  6:14             ` HATAYAMA Daisuke
2013-03-21  6:29               ` Eric W. Biederman
2013-03-21  6:46                 ` HATAYAMA Daisuke
2013-03-21  7:07                   ` Eric W. Biederman
2013-03-21 15:21                     ` Vivek Goyal
2013-03-21 15:27                       ` Vivek Goyal
2013-03-22  0:43                         ` HATAYAMA Daisuke
2013-03-22  0:54                         ` Eric W. Biederman
2013-03-22  2:30                           ` HATAYAMA Daisuke
2013-03-21 14:57                 ` Vivek Goyal
2013-03-21  7:22               ` Eric W. Biederman
2013-03-21 14:49                 ` Vivek Goyal
2013-03-22  7:11                   ` HATAYAMA Daisuke
2013-03-21 13:50         ` Vivek Goyal
2013-03-16  4:02 ` [PATCH v3 19/21] vmcore: round-up offset of vmcore object in page-size boundary HATAYAMA Daisuke
2013-03-16  4:02 ` [PATCH v3 20/21] vmcore: count holes generated by round-up operation for vmcore size HATAYAMA Daisuke
2013-03-16  4:02 ` [PATCH v3 21/21] vmcore: introduce mmap_vmcore() HATAYAMA Daisuke
2013-03-19 19:30 ` [PATCH v3 00/21] kdump, vmcore: support mmap() on /proc/vmcore Andrew Morton
2013-03-21  3:52   ` HATAYAMA Daisuke
2013-03-21  6:16     ` Eric W. Biederman
2013-03-21  6:35       ` HATAYAMA Daisuke
2013-03-21  7:14         ` Eric W. Biederman
2013-03-19 23:16 ` Eric W. Biederman

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