Re: /proc/vmcore mmap() failure issue

["chaowang@redhat.com" <chaowang@redhat.com>]

On 11/25/13 at 08:09am, Atsushi Kumagai wrote:
> Hello WANG,
> 
> On 2013/11/21 16:15:22, kexec <kexec-bounces@lists.infradead.org> wrote:
> > > How about this fail back structure instead of such an extra option ?
> > > 
> > > Thanks
> > > Atsushi Kumagai
> > > 
> > > From: Atsushi Kumagai <kumagai-atsushi@mxc.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@mxc.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.
> 
> Thanks for your testing !
> 
> > However, I have a question:
> > Why not switch to mmap() back after read()?
> 
> I made this patch as a general safety net, not only for the partial page
> issue.
> When facing unknown issues related mmap(), the kernel may have some bugs
> and mmap() can fail for every pages. In the worst case, most all mmap()
> will fail and try read() with error messages after every fail, but this
> patch will prevent the chattering of the switch and so many error messages.

Thanks for you explanation. I agree with you. Since mmap() is error
prone after first mmap failure, use read() instead as a fail safe makes
much sense to me.

WANG Chao
> 
> 
> Thanks
> Atsushi Kumagai
> 
> > Thanks
> > WANG Chao
> > 
> > >  		}
> > >  	} else {
> > >  		if (lseek(info->fd_memory, offset, SEEK_SET) == failed) {
> > > -- 
> > > 1.8.0.2
> > 
> > _______________________________________________
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> > kexec@lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/kexec
> >