Re: [PATCH v3] mm, hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY

[Mina Almasry]

On Sat, May 22, 2021 at 2:19 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, 21 May 2021 00:44:33 -0700 Mina Almasry <almasrymina@xxxxxxxxxx> 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?
>

Umm I'll yield to Mike. This is a transient underflow; not actually
THAT serious of an issue. Sorry, I'll clarify that in the commit
message for the next version.

> Are we able to identify a Fixes: for this?
>

No, this issue has been there latent for some time. It repros as far
back as 5.11 at least, which is why maybe it's not that serious to
require in -stable.

> 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?
>

Yes. If we only do the hugetlbfs_pagecache_present() check then that
gets us some 90% of the way there, the rest of the patch addresses an
unlikely race.

> If you don't think -stable needs this then this version will be OK as-is.