Re: [PATCH v4] mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO_COPY

[Mina Almasry <almasrymina@google.com>]

On Mon, May 31, 2021 at 4:25 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 27 May 2021 17:50:29 -0700 Mina Almasry <almasrymina@google.com> 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"

Which is this patch. It's a more complicated and not critical fix, so
not targeted for -stable. It's only sent to linux-mm.

> If we decide to get this into 5.13 (and perhaps -stable) then I can
> take a look at reworking all the 5.14 material on top.  If not very
> serious then we could rework this on top of the already queued
> material.
>

I assume given the above we want to rework this on top of the already
queued material. I can upload a v5 that is rebased on top of your
branch. Note that you have an earlier version of this fix in your
branch, so really this patch will turn into a fix for that patch if I
rebase it (I assume that's fine).