[PATCHv5 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu <kernelfans@xxxxxxxxx>]

FOLL_LONGTERM suggests a pin which is going to be given to hardware and
can't move. It would truncate CMA permanently and should be excluded.

FOLL_LONGTERM has already been checked in the slow path, but not checked in
the fast path, which means a possible leak of CMA page to longterm pinned
requirement through this crack.

Place a check in try_get_compound_head() in the fast path.

Some note about the check:
Huge page's subpages have the same migrate type due to either
allocation from a free_list[] or alloc_contig_range() with param
MIGRATE_MOVABLE. So it is enough to check on a single subpage
by is_migrate_cma_page(subpage)

Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx>
Cc: Keith Busch <keith.busch@xxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
To: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 mm/gup.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index cd8075e..f0d6804 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -33,9 +33,21 @@ struct follow_page_context {
  * Return the compound head page with ref appropriately incremented,
  * or NULL if that failed.
  */
-static inline struct page *try_get_compound_head(struct page *page, int refs)
+static inline struct page *try_get_compound_head(struct page *page, int refs,
+	unsigned int flags)
 {
-	struct page *head = compound_head(page);
+	struct page *head;
+
+	/*
+	 * Huge page's subpages have the same migrate type due to either
+	 * allocation from a free_list[] or alloc_contig_range() with param
+	 * MIGRATE_MOVABLE. So it is enough to check on a single subpage.
+	 */
+	if (unlikely(flags & FOLL_LONGTERM) &&
+		is_migrate_cma_page(page))
+		return NULL;
+
+	head = compound_head(page);
 
 	if (WARN_ON_ONCE(page_ref_count(head) < 0))
 		return NULL;
@@ -1908,7 +1920,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
 		page = pte_page(pte);
 
-		head = try_get_compound_head(page, 1);
+		head = try_get_compound_head(page, 1, flags);
 		if (!head)
 			goto pte_unmap;
 
@@ -2083,7 +2095,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
 	page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(head, refs);
+	head = try_get_compound_head(head, refs, flags);
 	if (!head)
 		return 0;
 
@@ -2142,7 +2154,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pmd_page(orig), refs);
+	head = try_get_compound_head(pmd_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
@@ -2174,7 +2186,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pud_page(orig), refs);
+	head = try_get_compound_head(pud_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
@@ -2203,7 +2215,7 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
 	page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
 	refs = record_subpages(page, addr, end, pages + *nr);
 
-	head = try_get_compound_head(pgd_page(orig), refs);
+	head = try_get_compound_head(pgd_page(orig), refs, flags);
 	if (!head)
 		return 0;
 
-- 
2.7.5