Re: [PATCH V2] MCE: fix an error of mce_bad_pages statistics

[Simon Jeons]

On Mon, 2012-12-10 at 17:06 +0800, Xishi Qiu wrote:
> On 2012/12/10 16:33, Wanpeng Li wrote:
> 
> > On Fri, Dec 07, 2012 at 02:11:02PM -0800, Andrew Morton wrote:
> >> On Fri, 7 Dec 2012 16:48:45 +0800
> >> Xishi Qiu <qiuxishi@xxxxxxxxxx> wrote:
> >>
> >>> On x86 platform, if we use "/sys/devices/system/memory/soft_offline_page" to offline a
> >>> free page twice, the value of mce_bad_pages will be added twice. So this is an error,
> >>> since the page was already marked HWPoison, we should skip the page and don't add the
> >>> value of mce_bad_pages.
> >>>
> >>> $ cat /proc/meminfo | grep HardwareCorrupted
> >>>
> >>> soft_offline_page()
> >>> 	get_any_page()
> >>> 		atomic_long_add(1, &mce_bad_pages)
> >>>
> >>> ...
> >>>
> >>> --- a/mm/memory-failure.c
> >>> +++ b/mm/memory-failure.c
> >>> @@ -1582,8 +1582,11 @@ int soft_offline_page(struct page *page, int flags)
> >>>  		return ret;
> >>>
> >>>  done:
> >>> -	atomic_long_add(1, &mce_bad_pages);
> >>> -	SetPageHWPoison(page);
> >>>  	/* keep elevated page count for bad page */
> >>> +	if (!PageHWPoison(page)) {
> >>> +		atomic_long_add(1, &mce_bad_pages);
> >>> +		SetPageHWPoison(page);
> >>> +	}
> >>> +
> >>>  	return ret;
> >>>  }
> >>
> >> A few things:
> >>
> >> - soft_offline_page() already checks for this case:
> >>
> >> 	if (PageHWPoison(page)) {
> >> 		unlock_page(page);
> >> 		put_page(page);
> >> 		pr_info("soft offline: %#lx page already poisoned\n", pfn);
> >> 		return -EBUSY;
> >> 	}
> >>
> >>  so why didn't this check work for you?
> >>
> >>  Presumably because one of the earlier "goto done" branches was
> >>  taken.  Which one, any why?
> >>
> >>  This function is an utter mess.  It contains six return points
> >>  randomly intermingled with three "goto done" return points.
> >>
> >>  This mess is probably the cause of the bug you have observed.  Can
> >>  we please fix it up somehow?  It *seems* that the design (lol) of
> >>  this function is "for errors, return immediately.  For success, goto
> >>  done".  In which case "done" should have been called "success".  But
> >>  if you just look at the function you'll see that this approach didn't
> >>  work.  I suggest it be converted to have two return points - one for
> >>  the success path, one for the failure path.  Or something.
> >>
> >> - soft_offline_huge_page() is a miniature copy of soft_offline_page()
> >>  and might suffer the same bug.
> >>
> >> - A cleaner, shorter and possibly faster implementation is
> >>
> >> 	if (!TestSetPageHWPoison(page))
> >> 		atomic_long_add(1, &mce_bad_pages);
> >>
> > 
> > Hi Andrew,
> > 
> > Since hwpoison bit for free buddy page has already be set in get_any_page, 
> > !TestSetPageHWPoison(page) will not increase mce_bad_pages count even for 
> > the first time.
> > 
> > Regards,
> > Wanpeng Li
> > 
> 
> The poisoned page is isolated in bad_page(), I wonder whether it could be isolated
> immediately in soft_offline_page() and memory_failure()?
> 
> buffered_rmqueue()
> 	prep_new_page()
> 		check_new_page()
> 			bad_page()

Do you mean else if(is_free_buddy_page(p)) branch is redundancy?

> 
> Thanks
> Xishi Qiu
> 
> >> - We have atomic_long_inc().  Use it?
> >>
> >> - Why do we have a variable called "mce_bad_pages"?  MCE is an x86
> >>  concept, and this code is in mm/.  Lights are flashing, bells are
> >>  ringing and a loudspeaker is blaring "layering violation" at us!
> >>
> >> --
> >> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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> >> see: http://www.linux-mm.org/ .
> >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
> > 
> > 
> > .
> > 
> 
> 
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>


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