+ mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy.patch added to -mm tree

The patch titled
     Subject: mm, hugetlb: fix simple resv_huge_pages underflow on UFFDIO_COPY
has been added to the -mm tree.  Its filename is
     mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy.patch

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------------------------------------------------------
From: Mina Almasry <redacted>
Subject: 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.

[mike.kravetz@oracle.com: changelog fixes]
Link: https://lkml.kernel.org/r/20210528004649.85298-1-almasrymina@google.com
Signed-off-by: Mina Almasry <redacted>
Reviewed-by: Mike Kravetz <redacted>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy
+++ a/mm/hugetlb.c

@@ -4889,10 +4889,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_s
 		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,

_

Patches currently in -mm which might be from almasrymina@google.com are

mm-hugetlb-fix-simple-resv_huge_pages-underflow-on-uffdio_copy.patch
mm-hugetlb-fix-racy-resv_huge_pages-underflow-on-uffdio_copy.patch