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

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Date:	Thu, 19 Aug 2010 09:57:52 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Jun'ichi Nomura <j-nomura@...jp.nec.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 9/9] hugetlb: add corrupted hugepage counter

> +void increment_corrupted_huge_page(struct page *page);
> +void decrement_corrupted_huge_page(struct page *page);

nitpick: increment/decrement are not verbs.

> +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.
 */

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.

Thanks,
Fengguang

> +	h->corrupted_huge_pages--;
> +	spin_unlock(&hugetlb_lock);
> +}
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