[PATCH v4 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL_PIN

[Peter Xu]

follow_page() doesn't use FOLL_PIN, meanwhile hugetlb seems to not be the
target of FOLL_WRITE either.  However add the checks.

Namely, either the need to CoW due to missing write bit, or proper
unsharing on !AnonExclusive pages over R/O pins to reject the follow page.
That brings this function closer to follow_hugetlb_page().

So we don't care before, and also for now.  But we'll care if we switch
over slow-gup to use hugetlb_follow_page_mask().  We'll also care when to
return -EMLINK properly, as that's the gup internal api to mean "we should
unshare".  Not really needed for follow page path, though.

When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
clear that it just should never fail.  When error happens, instead of
setting page==NULL, capture the errno instead.

Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 mm/hugetlb.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d04ba5782fdd..4410139cf890 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6462,13 +6462,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	struct page *page = NULL;
 	spinlock_t *ptl;
 	pte_t *pte, entry;
-
-	/*
-	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
-	 * follow_hugetlb_page().
-	 */
-	if (WARN_ON_ONCE(flags & FOLL_PIN))
-		return NULL;
+	int ret;
 
 	hugetlb_vma_lock_read(vma);
 	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
@@ -6478,8 +6472,23 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	ptl = huge_pte_lock(h, mm, pte);
 	entry = huge_ptep_get(pte);
 	if (pte_present(entry)) {
-		page = pte_page(entry) +
-				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+		page = pte_page(entry);
+
+		if (!huge_pte_write(entry)) {
+			if (flags & FOLL_WRITE) {
+				page = NULL;
+				goto out;
+			}
+
+			if (gup_must_unshare(vma, flags, page)) {
+				/* Tell the caller to do unsharing */
+				page = ERR_PTR(-EMLINK);
+				goto out;
+			}
+		}
+
+		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
+
 		/*
 		 * Note that page may be a sub-page, and with vmemmap
 		 * optimizations the page struct may be read only.
@@ -6489,8 +6498,10 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 		 * try_grab_page() should always be able to get the page here,
 		 * because we hold the ptl lock and have verified pte_present().
 		 */
-		if (try_grab_page(page, flags)) {
-			page = NULL;
+		ret = try_grab_page(page, flags);
+
+		if (WARN_ON_ONCE(ret)) {
+			page = ERR_PTR(ret);
 			goto out;
 		}
 	}
-- 
2.41.0