linux-kernel - Re: [PATCH v3] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO

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Date:   Sat, 22 May 2021 14:19:46 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm, hugetlb: fix resv_huge_pages underflow on
 UFFDIO_COPY

On Fri, 21 May 2021 00:44:33 -0700 Mina Almasry <almasrymina@...gle.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.