On Mon, 31 May 2021 17:11:52 -0700 Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > On Mon, May 31, 2021 at 4:25 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, 27 May 2021 17:50:29 -0700 Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > > > > > On UFFDIO_COPY, 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. > > > > Many conflicts here with material that is queued for 5.14-rc1. > > > > How serious is this problem? Is a -stable backport warranted? > > > > I've sent 2 similar patches to the list: > > 1. "[PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on UFFDIO_COPY" > > This one is sent to -stable and linux-mm and is a fairly simple fix. > > 2. "[PATCH v4] mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO_COPY" Ah, OK, the title of the first patch was changed, which threw me off. I'd skipped "[PATCH v4] mm, hugetlb: Fix simple resv_huge_pages underflow on UFFDIO_COPY" because Mike's comments appeared to require a v5. I applied it and made Mike's changelog suggestions. Queued for 5.13 and -stable. And I queued "[PATCH v4] mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO_COPY" for 5.14.
