linux-kernel - Re: [PATCH 2/2 v2] mm: print out information of file affected by memory error

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Date:	Tue, 6 Nov 2012 12:12:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Tony Luck <tony.luck@...el.com>, Andi Kleen <andi.kleen@...el.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	"Jun'ichi Nomura" <j-nomura@...jp.nec.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 2/2 v2] mm: print out information of file affected by
 memory error

On Tue,  6 Nov 2012 00:07:53 -0500
Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:

> On Mon, Nov 05, 2012 at 02:01:54PM -0800, Andrew Morton wrote:
> > On Fri,  2 Nov 2012 12:33:13 -0400
> > Naoya Horiguchi <n-horiguchi@...jp.nec.com> wrote:
> > 
> > > Printing out the information about which file can be affected by a
> > > memory error in generic_error_remove_page() is helpful for user to
> > > estimate the impact of the error.
> > > 
> > > Changelog v2:
> > >   - dereference mapping->host after if (!mapping) check for robustness
> > > 
> > > ...
> > >
> > > --- v3.7-rc3.orig/mm/truncate.c
> > > +++ v3.7-rc3/mm/truncate.c
> > > @@ -151,14 +151,20 @@ int truncate_inode_page(struct address_space *mapping, struct page *page)
> > >   */
> > >  int generic_error_remove_page(struct address_space *mapping, struct page *page)
> > >  {
> > > +	struct inode *inode;
> > > +
> > >  	if (!mapping)
> > >  		return -EINVAL;
> > > +	inode = mapping->host;
> > >  	/*
> > >  	 * Only punch for normal data pages for now.
> > >  	 * Handling other types like directories would need more auditing.
> > >  	 */
> > > -	if (!S_ISREG(mapping->host->i_mode))
> > > +	if (!S_ISREG(inode->i_mode))
> > >  		return -EIO;
> > > +	pr_info("MCE %#lx: file info pgoff:%lu, inode:%lu, dev:%s\n",
> > > +		page_to_pfn(page), page_index(page),
> > > +		inode->i_ino, inode->i_sb->s_id);
> > >  	return truncate_inode_page(mapping, page);
> > >  }
> > >  EXPORT_SYMBOL(generic_error_remove_page);
> > 
> > A couple of things.
> > 
> > - I worry that if a hardware error occurs, it might affect a large
> >   amount of memory all at the same time.  For example, if a 4G memory
> >   block goes bad, this message will be printed a million times?
> 
> If the error on 4G memory block triggered by SRAO MCE and these 1M pages
> are all pagecache pages, the answer is yes.

Well that's bad.

> But I think that if it's a whole DIMM error, it should be reported by
> another type of MCE than SRAO, so printing a million times seems to be
> unlikely to happen.

"should be" and "unlikely" aren't very reassuring things to hear! 
Emitting a million lines into syslog is pretty poor behaviour and
should be reliably avoided.

> > - hard-wiring "MCE" in here seems a bit of a layering violation? 
> >   What right does the generic, core .error_remove_page() implementation
> >   have to assume that it was called because of an MCE?
> 
> OK, we need not assume that. I change "MCE " prefix to more specific
> one like "Memory error ".
> 
> > Many CPU types don't eveh have such a thing?
> 
> No. At least currently, only SRAO MCE triggers memory_failure() and
> it's defined only on some newest highend models of Intel CPUs.

Again, your reply is full of assumptions about one particualar
implementation on one particular CPU.  But this is generic,
cross-architecture code!

Now, it's pretty harmless to make these assumptions at this time.  But
this new code will need to redone if/when other CPU types come along,
and because there's a printk in there, that rework will cause
user-visible changes in kernel behaviour.  It would be best if we can
just avoid the problem on day one.

Maybe move the printk into x86-specific code?  And just one printk
please - not a million!

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