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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: 8fb5debc5f ("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>
This commit is contained in:
Mina Almasry 2021-06-04 20:01:36 -07:00 committed by Linus Torvalds
parent 7b6889f54a
commit d84cf06e3d
1 changed files with 13 additions and 3 deletions
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16
mm/hugetlb.c
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@ -4889,10 +4889,20 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
if (!page)
goto out;
} else if (!*pagep) {
ret = -ENOMEM;
page = alloc_huge_page(dst_vma, dst_addr, 0);
if (IS_ERR(page))
/* 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)) {
ret = -ENOMEM;
goto out;
}
ret = copy_huge_page_from_user(page,
(const void __user *) src_addr,