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Message-ID: <CAHS8izMVBZ2W+4ug6mKtU3B0QhcLz7SyPePxOgqAD8Ou3xx52Q@mail.gmail.com>
Date: Sat, 22 May 2021 12:41:21 -0700
From: Mina Almasry <almasrymina@...gle.com>
To: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Linux-MM <linux-mm@...ck.org>,
open list <linux-kernel@...r.kernel.org>,
Oscar Salvador <osalvador@...e.de>,
Michal Hocko <mhocko@...e.com>,
Muchun Song <songmuchun@...edance.com>,
Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
David Hildenbrand <david@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Miaohe Lin <linmiaohe@...wei.com>,
Andrew Morton <akpm@...ux-foundation.org>,
stable@...r.kernel.org
Subject: Re: [PATCH] userfaultfd: hugetlbfs: fix new flag usage in error path
On Fri, May 21, 2021 at 4:40 PM Mike Kravetz <mike.kravetz@...cle.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@...r.kernel.org>
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> ---
> fs/hugetlbfs/inode.c | 2 +-
> mm/userfaultfd.c | 28 ++++++++++++++--------------
> 2 files changed, 15 insertions(+), 15 deletions(-)
>
Reviewed-by: Mina Almasry <almasry.mina@...gle.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
>
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