Re: [PATCH, RFC 00/10] THP refcounting redesign

[Andrea Arcangeli]

On Tue, Jun 10, 2014 at 11:46:40PM +0300, Kirill A. Shutemov wrote:
> Agreed. The patchset drops tail page refcounting.

Very possibly I misread something or a later patch fixes this up, I
just did a basic code review, but from the new code of split_huge_page
it looks like it returns -EBUSY after checking the individual tail
page refcounts, so it's not clear how that defines as "dropped".

+       for (i = 0; i < HPAGE_PMD_NR; i++)
+               tail_count += page_mapcount(page + i);
+       if (tail_count != page_count(page) - 1) {
+               BUG_ON(tail_count > page_count(page) - 1);
+               compound_unlock(page);
+               spin_unlock_irq(&zone->lru_lock);
+               return -EBUSY;
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