Re: [PATCH] userfaultfd: hugetlbfs: fix new flag usage in error path

[Mina Almasry <almasrymina@google.com>]

On Fri, May 21, 2021 at 4:40 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> In commit d6995da31122 ("hugetlb: use page.private for hugetlb specific
> page flags") the use of PagePrivate to indicate a reservation count
> should be restored at free time was changed to the hugetlb specific flag
> HPageRestoreReserve.  Changes to a userfaultfd error path as well as a
> VM_BUG_ON() in remove_inode_hugepages() were overlooked.
>
> Users could see incorrect hugetlb reserve counts if they experience an
> error with a UFFDIO_COPY operation.  Specifically, this would be the
> result of an unlikely copy_huge_page_from_user error.  There is not an
> increased chance of hitting the VM_BUG_ON.
>
> Fixes: d6995da31122 ("hugetlb: use page.private for hugetlb specific page flags")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  fs/hugetlbfs/inode.c |  2 +-
>  mm/userfaultfd.c     | 28 ++++++++++++++--------------
>  2 files changed, 15 insertions(+), 15 deletions(-)
>

Reviewed-by: Mina Almasry <almasry.mina@google.com>

I'm guessing this may interact with my patch in review "[PATCH v3] mm,
hugetlb: fix resv_huge_pages underflow on UFFDIO_COPY". I'll rebase my
patch and re-test.

> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 9d9e0097c1d3..55efd3dd04f6 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -529,7 +529,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>                          * the subpool and global reserve usage count can need
>                          * to be adjusted.
>                          */
> -                       VM_BUG_ON(PagePrivate(page));
> +                       VM_BUG_ON(HPageRestoreReserve(page));
>                         remove_huge_page(page);
>                         freed++;
>                         if (!truncate_op) {
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 5508f7d9e2dc..9ce5a3793ad4 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -432,38 +432,38 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
>                  * If a reservation for the page existed in the reservation
>                  * map of a private mapping, the map was modified to indicate
>                  * the reservation was consumed when the page was allocated.
> -                * We clear the PagePrivate flag now so that the global
> +                * We clear the HPageRestoreReserve flag now so that the global
>                  * reserve count will not be incremented in free_huge_page.
>                  * The reservation map will still indicate the reservation
>                  * was consumed and possibly prevent later page allocation.
>                  * This is better than leaking a global reservation.  If no
> -                * reservation existed, it is still safe to clear PagePrivate
> -                * as no adjustments to reservation counts were made during
> -                * allocation.
> +                * reservation existed, it is still safe to clear
> +                * HPageRestoreReserve as no adjustments to reservation counts
> +                * were made during allocation.
>                  *
>                  * The reservation map for shared mappings indicates which
>                  * pages have reservations.  When a huge page is allocated
>                  * for an address with a reservation, no change is made to
> -                * the reserve map.  In this case PagePrivate will be set
> -                * to indicate that the global reservation count should be
> +                * the reserve map.  In this case HPageRestoreReserve will be
> +                * set to indicate that the global reservation count should be
>                  * incremented when the page is freed.  This is the desired
>                  * behavior.  However, when a huge page is allocated for an
>                  * address without a reservation a reservation entry is added
> -                * to the reservation map, and PagePrivate will not be set.
> -                * When the page is freed, the global reserve count will NOT
> -                * be incremented and it will appear as though we have leaked
> -                * reserved page.  In this case, set PagePrivate so that the
> -                * global reserve count will be incremented to match the
> -                * reservation map entry which was created.
> +                * to the reservation map, and HPageRestoreReserve will not be
> +                * set. When the page is freed, the global reserve count will
> +                * NOT be incremented and it will appear as though we have
> +                * leaked reserved page.  In this case, set HPageRestoreReserve
> +                * so that the global reserve count will be incremented to
> +                * match the reservation map entry which was created.
>                  *
>                  * Note that vm_alloc_shared is based on the flags of the vma
>                  * for which the page was originally allocated.  dst_vma could
>                  * be different or NULL on error.
>                  */
>                 if (vm_alloc_shared)
> -                       SetPagePrivate(page);
> +                       SetHPageRestoreReserve(page);
>                 else
> -                       ClearPagePrivate(page);
> +                       ClearHPageRestoreReserve(page);
>                 put_page(page);
>         }
>         BUG_ON(copied < 0);
> --
> 2.31.1
>