Re: [PATCH 9/9] hugetlb: add corrupted hugepage counter

[Wu Fengguang]

On Tue, Aug 24, 2010 at 11:01:33AM +0800, Naoya Horiguchi wrote:
> On Thu, Aug 19, 2010 at 09:57:52AM +0800, Wu Fengguang wrote:
> > > +void increment_corrupted_huge_page(struct page *page);
> > > +void decrement_corrupted_huge_page(struct page *page);
> >
> > nitpick: increment/decrement are not verbs.
> 
> OK, increase/decrease are correct.
> 
> 
> > > +void increment_corrupted_huge_page(struct page *hpage)
> > > +{
> > > +   struct hstate *h = page_hstate(hpage);
> > > +   spin_lock(&hugetlb_lock);
> > > +   h->corrupted_huge_pages++;
> > > +   spin_unlock(&hugetlb_lock);
> > > +}
> > > +
> > > +void decrement_corrupted_huge_page(struct page *hpage)
> > > +{
> > > +   struct hstate *h = page_hstate(hpage);
> > > +   spin_lock(&hugetlb_lock);
> > > +   BUG_ON(!h->corrupted_huge_pages);
> >
> > There is no point to have BUG_ON() here:
> >
> > /*
> >  * Don't use BUG() or BUG_ON() unless there's really no way out; one
> >  * example might be detecting data structure corruption in the middle
> >  * of an operation that can't be backed out of.  If the (sub)system
> >  * can somehow continue operating, perhaps with reduced functionality,
> >  * it's probably not BUG-worthy.
> >  *
> >  * If you're tempted to BUG(), think again:  is completely giving up
> >  * really the *only* solution?  There are usually better options, where
> >  * users don't need to reboot ASAP and can mostly shut down cleanly.
> >  */
> 
> OK. I understand.
> BUG_ON() is too severe for just a counter.
> 
> >
> > And there is a race case that (corrupted_huge_pages==0)!
> > Suppose the user space calls unpoison_memory() on a good pfn, and the page
> > happen to be hwpoisoned between lock_page() and TestClearPageHWPoison(),
> > corrupted_huge_pages will go negative.
> 
> I see.
> When this race happens, unpoison runs and decreases HugePages_Crpt,
> but racing memory failure returns without increasing it.
> Yes, this is a problem we need to fix.
> 
> Moreover for hugepage we should pay attention to the possiblity of
> mce_bad_pages mismatch which can occur by race between unpoison and
> multiple memory failures, where each failure increases mce_bad_pages
> by the number of pages in a hugepage.

Yup.

> I think counting corrupted hugepages is not directly related to
> hugepage migration, and this problem only affects the counter,
> not other behaviors, so I'll separate hugepage counter fix patch
> from this patch set and post as another patch series. Is this OK?

That would be better, thanks.

Thanks,
Fengguang
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