linux-kernel - Re: [PATCH v4] mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO

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Message-Id: <20210531173652.c21404a16a8f8542ce40afa8@linux-foundation.org>
Date:   Mon, 31 May 2021 17:36:52 -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 <linux-mm@...ck.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] mm, hugetlb: fix racy resv_huge_pages underflow on
 UFFDIO_COPY

On Mon, 31 May 2021 17:11:52 -0700 Mina Almasry <almasrymina@...gle.com> wrote:

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

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.