linux-kernel - Re: /proc/vmcore mmap() failure issue

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Date:	Thu, 21 Nov 2013 15:14:42 +0800
From:	"chaowang@...hat.com" <chaowang@...hat.com>
To:	Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
Cc:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	"bhe@...hat.com" <bhe@...hat.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"dyoung@...hat.com" <dyoung@...hat.com>,
	"vgoyal@...hat.com" <vgoyal@...hat.com>
Subject: Re: /proc/vmcore mmap() failure issue

On 11/20/13 at 05:27am, Atsushi Kumagai wrote:
> On 2013/11/19 18:56:21, kexec <kexec-bounces@...ts.infradead.org> wrote:
> > (2013/11/18 9:51), Atsushi Kumagai wrote:
> > > (2013/11/15 23:26), Vivek Goyal wrote:
> > >> On Fri, Nov 15, 2013 at 06:41:52PM +0900, HATAYAMA Daisuke wrote:
> > >>
> > >> [..]
> > >>>> Given the fact that hpa does not like fixing it in kernel. We are 
> > >>>> left with option of fixing it in following places.
> > >>>>
> > >>>> - Drop partial pages in kexec-tools
> > >>>> - Drop partial pages in makeudmpfile.
> > >>>> - Read partial pages using read() interface in makedumpfile
> > >>>> - Modify /proc/vmcore to copy partial pages in second kernel's memory.
> > >>>>
> > >>>> It is not clear to me that partial pages are really useful.  So I 
> > >>>> want to avoid modifying /proc/vmcore to deal with partial pages and 
> > >>>> increase complexity.
> > >>>>
> > >>>> So fixing makedumpfile (either option2 or option 3) seems least 
> > >>>> risky to me. In fact I would say let us keep it simple and truncate 
> > >>>> partial pages in makedumpfile to keep it simple. And look at option 
> > >>>> 3 once we have a strong use case for partial pages.
> > >>>>
> > >>>> What do you think?
> > >>>>
> > >>>
> > >>> As you say, it's not clear that partial pages are really useful, but 
> > >>> on the other hand, it seems to me not clear that they are really useless.
> > >>> I think we should get them as long as we have access to them.
> > >>>
> > >>> It seems best to me the option 3). Switching between read and mmap 
> > >>> would be not so complex and also it's by far flexible in 
> > >>> makedumpfile than in kernel.
> > >>
> > >> Ok, I am fine with option 3. It is more complicated option but safe 
> > >> option.
> > > 
> > > It sounds reasonable also to me.
> > > 
> > >> 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.
> > 
> > BTW, now the following patch has been applied on top of makedumpfile in kexec-tools package on fedora in order to avoid the issue.
> > 
> > https://lists.fedoraproject.org/pipermail/kexec/2013-November/000254.html
> > 
> > I remember prototype version of mmap patch implemented a kind of --no-mmap option and we could use it to disable mmap() use and use read() instead, I think which is useful when we face this kind of issue.
> 
> How about this fail back structure instead of such an extra option ?
> 
> Thanks
> Atsushi Kumagai
> 
> From: Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
> Date: Wed, 20 Nov 2013 14:10:19 +0900
> Subject: [PATCH] Fall back to read() when mmap() fails.
> 
> Signed-off-by: Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
> ---
>  makedumpfile.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/makedumpfile.c b/makedumpfile.c
> index ca03440..f583602 100644
> --- a/makedumpfile.c
> +++ b/makedumpfile.c
> @@ -324,7 +324,15 @@ read_from_vmcore(off_t offset, void *bufptr, unsigned long size)
>  		if (!read_with_mmap(offset, bufptr, size)) {
>  			ERRMSG("Can't read the dump memory(%s) with mmap().\n",
>  			       info->name_memory);
> -			return FALSE;
> +
> +			ERRMSG("This kernel might have some problems about mmap().\n");
> +			ERRMSG("read() will be used instead of mmap() from now.\n");
> +
> +			/*
> +			 * Fall back to read().
> +			 */
> +			info->flag_usemmap = FALSE;
> +			read_from_vmcore(offset, bufptr, size);

Hi, Atsushi

I've got such a workstation too. And I confirm this patch works for me.

However, I have a question:
Why not switch to mmap() back after read()?

Thanks
WANG Chao

>  		}
>  	} else {
>  		if (lseek(info->fd_memory, offset, SEEK_SET) == failed) {
> -- 
> 1.8.0.2
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