[PATCH 02/19] mm/gup: factor out duplicate code from four routines

[John Hubbard <jhubbard@nvidia.com>]

There are four locations in gup.c that have a fair amount of code
duplication. This means that changing one requires making the same
changes in four places, not to mention reading the same code four
times, and wondering if there are subtle differences.

Factor out the common code into static functions, thus reducing the
overall line count and the code's complexity.

Also, take the opportunity to slightly improve the efficiency of the
error cases, by doing a mass subtraction of the refcount, surrounded
by get_page()/put_page().

Also, further simplify (slightly), by waiting until the the successful
end of each routine, to increment *nr.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 113 ++++++++++++++++++++++---------------------------------
 1 file changed, 46 insertions(+), 67 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 85caf76b3012..8fb0d9cdfaf5 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1969,6 +1969,35 @@ static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
 }
 #endif
 
+static int __record_subpages(struct page *page, unsigned long addr,
+			     unsigned long end, struct page **pages, int nr)
+{
+	int nr_recorded_pages = 0;
+
+	do {
+		pages[nr] = page;
+		nr++;
+		page++;
+		nr_recorded_pages++;
+	} while (addr += PAGE_SIZE, addr != end);
+	return nr_recorded_pages;
+}
+
+static void __remove_refs_from_head(struct page *page, int refs)
+{
+	/* Do a get_page() first, in case refs == page->_refcount */
+	get_page(page);
+	page_ref_sub(page, refs);
+	put_page(page);
+}
+
+static int __huge_pt_done(struct page *head, int nr_recorded_pages, int *nr)
+{
+	*nr += nr_recorded_pages;
+	SetPageReferenced(head);
+	return 1;
+}
+
 #ifdef CONFIG_ARCH_HAS_HUGEPD
 static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
 				      unsigned long sz)
@@ -1998,34 +2027,19 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	/* hugepages are never "special" */
 	VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 
-	refs = 0;
 	head = pte_page(pte);
-
 	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
-	do {
-		VM_BUG_ON(compound_head(page) != head);
-		pages[*nr] = page;
-		(*nr)++;
-		page++;
-		refs++;
-	} while (addr += PAGE_SIZE, addr != end);
+	refs = __record_subpages(page, addr, end, pages, *nr);
 
 	head = try_get_compound_head(head, refs);
-	if (!head) {
-		*nr -= refs;
+	if (!head)
 		return 0;
-	}
 
 	if (unlikely(pte_val(pte) != pte_val(*ptep))) {
-		/* Could be optimized better */
-		*nr -= refs;
-		while (refs--)
-			put_page(head);
+		__remove_refs_from_head(head, refs);
 		return 0;
 	}
-
-	SetPageReferenced(head);
-	return 1;
+	return __huge_pt_done(head, refs, nr);
 }
 
 static int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
@@ -2071,30 +2085,18 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 					     pages, nr);
 	}
 
-	refs = 0;
 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
-	do {
-		pages[*nr] = page;
-		(*nr)++;
-		page++;
-		refs++;
-	} while (addr += PAGE_SIZE, addr != end);
+	refs = __record_subpages(page, addr, end, pages, *nr);
 
 	head = try_get_compound_head(pmd_page(orig), refs);
-	if (!head) {
-		*nr -= refs;
+	if (!head)
 		return 0;
-	}
 
 	if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
-		*nr -= refs;
-		while (refs--)
-			put_page(head);
+		__remove_refs_from_head(head, refs);
 		return 0;
 	}
-
-	SetPageReferenced(head);
-	return 1;
+	return __huge_pt_done(head, refs, nr);
 }
 
 static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
@@ -2114,30 +2116,18 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 					     pages, nr);
 	}
 
-	refs = 0;
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
-	do {
-		pages[*nr] = page;
-		(*nr)++;
-		page++;
-		refs++;
-	} while (addr += PAGE_SIZE, addr != end);
+	refs = __record_subpages(page, addr, end, pages, *nr);
 
 	head = try_get_compound_head(pud_page(orig), refs);
-	if (!head) {
-		*nr -= refs;
+	if (!head)
 		return 0;
-	}
 
 	if (unlikely(pud_val(orig) != pud_val(*pudp))) {
-		*nr -= refs;
-		while (refs--)
-			put_page(head);
+		__remove_refs_from_head(head, refs);
 		return 0;
 	}
-
-	SetPageReferenced(head);
-	return 1;
+	return __huge_pt_done(head, refs, nr);
 }
 
 static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
@@ -2151,30 +2141,19 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 		return 0;
 
 	BUILD_BUG_ON(pgd_devmap(orig));
-	refs = 0;
+
 	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
-	do {
-		pages[*nr] = page;
-		(*nr)++;
-		page++;
-		refs++;
-	} while (addr += PAGE_SIZE, addr != end);
+	refs = __record_subpages(page, addr, end, pages, *nr);
 
 	head = try_get_compound_head(pgd_page(orig), refs);
-	if (!head) {
-		*nr -= refs;
+	if (!head)
 		return 0;
-	}
 
 	if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
-		*nr -= refs;
-		while (refs--)
-			put_page(head);
+		__remove_refs_from_head(head, refs);
 		return 0;
 	}
-
-	SetPageReferenced(head);
-	return 1;
+	return __huge_pt_done(head, refs, nr);
 }
 
 static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
-- 
2.23.0