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
Fixes: 8fb5debc5fcd ("userfaultfd: hugetlbfs: add hugetlb_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 470f7b5b437e6d53bb374c3e7c51bd3fd567c2b0..5560b50876fb7ad0a108f216320a3e0f560f625e 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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,