Re: [PATCHv6 32/36] thp: reintroduce split_huge_page()

[Kirill A. Shutemov]

On Wed, Jun 10, 2015 at 05:44:30PM +0200, Vlastimil Babka wrote:
> On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> >+static int __split_huge_page_tail(struct page *head, int tail,
> >+		struct lruvec *lruvec, struct list_head *list)
> >+{
> >+	int mapcount;
> >+	struct page *page_tail = head + tail;
> >+
> >+	mapcount = page_mapcount(page_tail);
> 
> Isn't page_mapcount() unnecessarily heavyweight here? When you are splitting
> a page, it already should have zero compound_mapcount() and shouldn't be
> PageDoubleMap(), no? So you should care about page->_mapcount only? Sure,
> splitting THP is not a hotpath, but when done 512 times per split, it could
> make some difference in the split's latency.

Okay, replaced with direct atomic_read().

> >+	VM_BUG_ON_PAGE(atomic_read(&page_tail->_count) != 0, page_tail);
> >+
> >+	/*
> >+	 * tail_page->_count is zero and not changing from under us. But
> >+	 * get_page_unless_zero() may be running from under us on the
> >+	 * tail_page. If we used atomic_set() below instead of atomic_add(), we
> >+	 * would then run atomic_set() concurrently with
> >+	 * get_page_unless_zero(), and atomic_set() is implemented in C not
> >+	 * using locked ops. spin_unlock on x86 sometime uses locked ops
> >+	 * because of PPro errata 66, 92, so unless somebody can guarantee
> >+	 * atomic_set() here would be safe on all archs (and not only on x86),
> >+	 * it's safer to use atomic_add().
> 
> I would be surprised if this was the first place to use atomic_set() with
> potential concurrent atomic_add(). Shouldn't atomic_*() API guarantee that
> this works?

I don't have much insight on the issue. This part is carried over from
pre-rework split_huge_page().

> 
> >+	 */
> >+	atomic_add(page_mapcount(page_tail) + 1, &page_tail->_count);
> 
> You already have the value in mapcount variable, so why read it again.

Fixed.

-- 
 Kirill A. Shutemov
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