On Tuesday, November 22, 2016 9:17 AM Mike Kravetz wrote:
> I am not sure if you are convinced ClearPagePrivate is an acceptable
> solution to this issue. If you do, here is the simple patch to add
> it and an appropriate comment.
>
Hi Mike and Andrea
Sorry for my jumping in.
In commit 07443a85ad
("mm, hugetlb: return a reserved page to a reserved pool if failed")
newly allocated huge page gets cleared for a successful COW.
I'm wondering if we can handle our error path along that way?
Obvious I could miss the points you are concerning.
thanks
Hillf
>
> If __mcopy_atomic_hugetlb exits with an error, put_page will be called
> if a huge page was allocated and needs to be freed. If a reservation
> was associated with the huge page, the PagePrivate flag will be set.
> Clear PagePrivate before calling put_page/free_huge_page so that the
> global reservation count is not incremented.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
> mm/userfaultfd.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index b565481..d56ba83 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -303,8 +303,23 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> out_unlock:
> up_read(&dst_mm->mmap_sem);
> out:
> - if (page)
> + if (page) {
> + /*
> + * We encountered an error and are about to free a newly
> + * allocated huge page. It is possible that there was a
> + * reservation associated with the page that has been
> + * consumed. See the routine restore_reserve_on_error
> + * for details. Unfortunately, we can not call
> + * restore_reserve_on_error now as it would require holding
> + * mmap_sem. Clear the PagePrivate flag 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.
> + */
> + ClearPagePrivate(page);
> put_page(page);
> + }
> BUG_ON(copied < 0);
> BUG_ON(err > 0);
> BUG_ON(!copied && !err);
> --
> 2.7.4
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