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SubjectRe: [PATCHv4 12/24] thp: PMD splitting without splitting compound page
"Kirill A. Shutemov" <> writes:

> Current split_huge_page() combines two operations: splitting PMDs into
> tables of PTEs and splitting underlying compound page. This patch
> changes split_huge_pmd() implementation to split the given PMD without
> splitting other PMDs this page mapped with or underlying compound page.
> In order to do this we have to get rid of tail page refcounting, which
> uses _mapcount of tail pages. Tail page refcounting is needed to be able
> to split THP page at any point: we always know which of tail pages is
> pinned (i.e. by get_user_pages()) and can distribute page count
> correctly.
> We can avoid this by allowing split_huge_page() to fail if the compound
> page is pinned. This patch removes all infrastructure for tail page
> refcounting and make split_huge_page() to always return -EBUSY. All
> split_huge_page() users already know how to handle its fail. Proper
> implementation will be added later.
> Without tail page refcounting, implementation of split_huge_pmd() is
> pretty straight-forward.

mm/gup.c: In function ‘gup_huge_pgd’:
mm/gup.c:1183:2: error: ‘tail’ undeclared (first use in this function)
tail = page;
diff --git a/mm/gup.c b/mm/gup.c
index 2f776850108c..141b5a81cf8a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1180,7 +1180,6 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
refs = 0;
head = pgd_page(orig);
page = head + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
- tail = page;
do {
VM_BUG_ON_PAGE(compound_head(page) != head, page);
pages[*nr] = page;

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