Messages in this thread | | | Date | Sat, 22 May 2021 14:19:46 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH v3] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY |
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On Fri, 21 May 2021 00:44:33 -0700 Mina Almasry <almasrymina@google.com> wrote:
> The userfaultfd hugetlb tests detect 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 if there exists a page in the cache which already
^ check
> consumed the reservation, and return -EEXIST immediately if so. > > Secondly, if we fail to copy the page contents while holding the > hugetlb_fault_mutex, we will drop the mutex and return to the caller > after allocating a page that consumed a reservation. In this case there > may be a fault that double consumes the reservation. To handle this, we > free the allocated page, fix the reservations, and allocate a temporary > hugetlb page and return that to the caller. When the caller does the > copy outside of the lock, we again check the cache, and allocate a page > consuming the reservation, and copy over the contents. > > Test: > Hacked the code locally such that resv_huge_pages underflows produce > a warning and the copy_huge_page_from_user() always fails, 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. > > ... > > include/linux/hugetlb.h | 4 ++ > mm/hugetlb.c | 103 ++++++++++++++++++++++++++++++++++++---- > mm/migrate.c | 39 +++------------ > 3 files changed, 103 insertions(+), 43 deletions(-)
I'm assuming we want this in -stable?
Are we able to identify a Fixes: for this?
It's a large change. Can we come up with some smaller and easier to review and integrate version which we can feed into 5.13 and -stable and do the fancier version for 5.14?
If you don't think -stable needs this then this version will be OK as-is.
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