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From:   Peter Xu <peterx@redhat.com>
To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Mike Rapoport <rppt@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>, peterx@redhat.com,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        James Houghton <jthoughton@google.com>,
        Hugh Dickins <hughd@google.com>
Subject: [PATCH 1/7] mm/hugetlb: Handle FOLL_DUMP well in follow_page_mask()
Date:   Tue, 13 Jun 2023 17:53:40 -0400
Message-Id: <20230613215346.1022773-2-peterx@redhat.com>
In-Reply-To: <20230613215346.1022773-1-peterx@redhat.com>
References: <20230613215346.1022773-1-peterx@redhat.com>
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Firstly, the no_page_table() is meaningless for hugetlb which is a no-op
there, because a hugetlb page always satisfies:

  - vma_is_anonymous() == false
  - vma->vm_ops->fault != NULL

So we can already safely remove it in hugetlb_follow_page_mask(), alongside
with the page* variable.

Meanwhile, what we do in follow_hugetlb_page() actually makes sense for a
dump: we try to fault in the page only if the page cache is already
allocated.  Let's do the same here for follow_page_mask() on hugetlb.

It should so far has zero effect on real dumps, because that still goes
into follow_hugetlb_page().  But this may start to influence a bit on
follow_page() users who mimics a "dump page" scenario, but hopefully in a
good way.  This also paves way for unifying the hugetlb gup-slow.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/gup.c     | 9 ++-------
 mm/hugetlb.c | 9 +++++++++
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index dbe96d266670..aa0668505d61 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -781,7 +781,6 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 			      struct follow_page_context *ctx)
 {
 	pgd_t *pgd;
-	struct page *page;
 	struct mm_struct *mm = vma->vm_mm;
 
 	ctx->page_mask = 0;
@@ -794,12 +793,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 	 * hugetlb_follow_page_mask is only for follow_page() handling here.
 	 * Ordinary GUP uses follow_hugetlb_page for hugetlb processing.
 	 */
-	if (is_vm_hugetlb_page(vma)) {
-		page = hugetlb_follow_page_mask(vma, address, flags);
-		if (!page)
-			page = no_page_table(vma, flags);
-		return page;
-	}
+	if (is_vm_hugetlb_page(vma))
+		return hugetlb_follow_page_mask(vma, address, flags);
 
 	pgd = pgd_offset(mm, address);
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 270ec0ecd5a1..82dfdd96db4c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6501,6 +6501,15 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
 	spin_unlock(ptl);
 out_unlock:
 	hugetlb_vma_unlock_read(vma);
+
+	/*
+	 * Fixup retval for dump requests: if pagecache doesn't exist,
+	 * don't try to allocate a new page but just skip it.
+	 */
+	if (!page && (flags & FOLL_DUMP) &&
+	    !hugetlbfs_pagecache_present(h, vma, address))
+		page = ERR_PTR(-EFAULT);
+
 	return page;
 }
 
-- 
2.40.1