FAILED: patch "[PATCH] mm, hugetlb: fix simple resv_huge_pages underflow on" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From d84cf06e3dd8c5c5b547b5d8931015fc536678e5 Mon Sep 17 00:00:00 2001
From: Mina Almasry <[email protected]>
Date: Fri, 4 Jun 2021 20:01:36 -0700
Subject: [PATCH] mm, hugetlb: fix simple resv_huge_pages underflow on
UFFDIO_COPY

The userfaultfd hugetlb tests cause a resv_huge_pages underflow. This happens when hugetlb_mcopy_atomic_pte() is called with !is_continue on an index for which we already have a page in the cache. When this happens, we allocate a second page, double consuming the reservation, and then fail to insert the page into the cache and return -EEXIST.

To fix this, we first check if there is a page in the cache which already consumed the reservation, and return -EEXIST immediately if so.

There is still a rare condition where we fail to copy the page contents AND race with a call for hugetlb_no_page() for this index and again we will underflow resv_huge_pages. That is fixed in a more complicated patch not targeted for -stable.

Test:

Hacked the code locally such that resv_huge_pages underflows produce a warning, then:

./tools/testing/selftests/vm/userfaultfd hugetlb_shared 10

	2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success
  ./tools/testing/selftests/vm/userfaultfd hugetlb 10
	2 /tmp/kokonut_test/huge/userfaultfd_test && echo test success

Both tests succeed and produce no warnings. After the test runs number of free/resv hugepages is correct.

[[email protected]: changelog fixes]

index 470f7b5b437e..5560b50876fb 100644
@@ -4889,10 +4889,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 		if (!page)
 			goto out;
 	} else if (!*pagep) {
-		ret = -ENOMEM;
+		/* If a page already exists, then it's UFFDIO_COPY for
+		 * a non-missing case. Return -EEXIST.
+		 */
+		if (vm_shared &&
+		    hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
+			ret = -EEXIST;
+			goto out;
+		}
+
 		page = alloc_huge_page(dst_vma, dst_addr, 0);
-		if (IS_ERR(page))
+		if (IS_ERR(page)) {
+			ret = -ENOMEM;
 			goto out;
+		}
 
 		ret = copy_huge_page_from_user(page,
 						(const void __user *) src_addr,