On Mon, May 31, 2021 at 5:36 PM Andrew Morton [off-list ref] wrote:

On Mon, 31 May 2021 17:11:52 -0700 Mina Almasry [off-list ref] wrote:

quoted

On Mon, May 31, 2021 at 4:25 PM Andrew Morton [off-list ref] wrote:

quoted

On Thu, 27 May 2021 17:50:29 -0700 Mina Almasry [off-list ref] wrote:

quoted

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.

Awesome, thanks! And sorry for the confusion!