[PATCH V3 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

[ira.weiny at intel.com]

From: Ira Weiny <ira.weiny at intel.com>

DAX pages were previously unprotected from longterm pins when users
called get_user_pages_fast().

Use the new FOLL_LONGTERM flag to check for DEVMAP pages and fall
back to regular GUP processing if a DEVMAP page is encountered.

Signed-off-by: Ira Weiny <ira.weiny at intel.com>

---
Changes from V2:
	Add comment on special use case of FOLL_LONGTERM

 mm/gup.c | 40 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 0fa7244d6f19..567bd1b295f0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1613,6 +1613,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
 			goto pte_unmap;
 
 		if (pte_devmap(pte)) {
+			if (unlikely(flags & FOLL_LONGTERM))
+				goto pte_unmap;
+
 			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
 			if (unlikely(!pgmap)) {
 				undo_dev_pagemap(nr, nr_start, pages);
@@ -1752,8 +1755,11 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
 	if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
 		return 0;
 
-	if (pmd_devmap(orig))
+	if (pmd_devmap(orig)) {
+		if (unlikely(flags & FOLL_LONGTERM))
+			return 0;
 		return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
+	}
 
 	refs = 0;
 	page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
@@ -1790,8 +1796,11 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
 	if (!pud_access_permitted(orig, flags & FOLL_WRITE))
 		return 0;
 
-	if (pud_devmap(orig))
+	if (pud_devmap(orig)) {
+		if (unlikely(flags & FOLL_LONGTERM))
+			return 0;
 		return __gup_device_huge_pud(orig, pudp, addr, end, pages, nr);
+	}
 
 	refs = 0;
 	page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
@@ -2034,6 +2043,29 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	return nr;
 }
 
+static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
+				   unsigned int gup_flags, struct page **pages)
+{
+	int ret;
+
+	/*
+	 * FIXME: FOLL_LONGTERM does not work with
+	 * get_user_pages_unlocked() (see comments in that function)
+	 */
+	if (gup_flags & FOLL_LONGTERM) {
+		down_read(&current->mm->mmap_sem);
+		ret = __gup_longterm_locked(current, current->mm,
+					    start, nr_pages,
+					    pages, NULL, gup_flags);
+		up_read(&current->mm->mmap_sem);
+	} else {
+		ret = get_user_pages_unlocked(start, nr_pages,
+					      pages, gup_flags);
+	}
+
+	return ret;
+}
+
 /**
  * get_user_pages_fast() - pin user pages in memory
  * @start:	starting user address
@@ -2079,8 +2111,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 		start += nr << PAGE_SHIFT;
 		pages += nr;
 
-		ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
-					      gup_flags);
+		ret = __gup_longterm_unlocked(start, nr_pages - nr,
+					      gup_flags, pages);
 
 		/* Have to be a bit careful with return values */
 		if (nr > 0) {
-- 
2.20.1