[10/13] mm, hugetlb: fix simple resv_huge_pages underflow on UFFDIO_COPY

Commit Message

Andrew Morton June 5, 2021, 3:01 a.m. UTC

From: Mina Almasry <[e-mail]>
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.

[[e-mail]: changelog fixes]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 8fb5debc5fcd ("userfaultfd: hugetlbfs: add hugetlb_mcopy_atomic_pte for userfaultfd support")
Signed-off-by: Mina Almasry <[e-mail]>
Reviewed-by: Mike Kravetz <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

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

Patch

--- 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,