linux-kernel - [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL

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Message-Id: <20230619231044.112894-3-peterx@redhat.com>
Date:   Mon, 19 Jun 2023 19:10:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        John Hubbard <jhubbard@...dia.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        James Houghton <jthoughton@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>, peterx@...hat.com,
        Jason Gunthorpe <jgg@...dia.com>
Subject: [PATCH v2 2/8] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL_PIN

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 CoR 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
do CoR".  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.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/hugetlb.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f75f5e78ff0b..9a6918c4250a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6463,13 +6463,6 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	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;
-
 	hugetlb_vma_lock_read(vma);
 	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
 	if (!pte)
@@ -6478,8 +6471,21 @@ 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 (gup_must_unshare(vma, flags, page)) {
+			/* Tell the caller to do Copy-On-Read */
+			page = ERR_PTR(-EMLINK);
+			goto out;
+		}
+
+		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
+			page = NULL;
+			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.
-- 
2.40.1