Re: [RESEND 1/7] mm/gup: Replace get_user_pages_longterm() with FOLL_LONGTERM

[Dan Williams <dan.j.williams@intel.com>]

On Sun, Mar 17, 2019 at 7:36 PM <ira.weiny@intel.com> wrote:
>
> From: Ira Weiny <ira.weiny@intel.com>
>
> Rather than have a separate get_user_pages_longterm() call,
> introduce FOLL_LONGTERM and change the longterm callers to use
> it.
>
> This patch does not change any functionality.
>
> FOLL_LONGTERM can only be supported with get_user_pages() as it
> requires vmas to determine if DAX is in use.
>
> CC: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> CC: Andrew Morton <akpm@linux-foundation.org>
> CC: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
[..]
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2d483dbdffc0..6831077d126c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
[..]
> @@ -2609,6 +2596,7 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>  #define FOLL_REMOTE    0x2000  /* we are working on non-current tsk/mm */
>  #define FOLL_COW       0x4000  /* internal GUP flag */
>  #define FOLL_ANON      0x8000  /* don't do file mappings */
> +#define FOLL_LONGTERM  0x10000 /* mapping is intended for a long term pin */

Let's change this comment to say something like /* mapping lifetime is
indefinite / at the discretion of userspace */, since "longterm is not
well defined.

I think it should also include a /* FIXME: */ to say something about
the havoc a long term pin might wreak on fs and mm code paths.

>  static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags)
>  {
> diff --git a/mm/gup.c b/mm/gup.c
> index f84e22685aaa..8cb4cff067bc 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1112,26 +1112,7 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
>  }
>  EXPORT_SYMBOL(get_user_pages_remote);
>
> -/*
> - * This is the same as get_user_pages_remote(), just with a
> - * less-flexible calling convention where we assume that the task
> - * and mm being operated on are the current task's and don't allow
> - * passing of a locked parameter.  We also obviously don't pass
> - * FOLL_REMOTE in here.
> - */
> -long get_user_pages(unsigned long start, unsigned long nr_pages,
> -               unsigned int gup_flags, struct page **pages,
> -               struct vm_area_struct **vmas)
> -{
> -       return __get_user_pages_locked(current, current->mm, start, nr_pages,
> -                                      pages, vmas, NULL,
> -                                      gup_flags | FOLL_TOUCH);
> -}
> -EXPORT_SYMBOL(get_user_pages);
> -
>  #if defined(CONFIG_FS_DAX) || defined (CONFIG_CMA)
> -
> -#ifdef CONFIG_FS_DAX
>  static bool check_dax_vmas(struct vm_area_struct **vmas, long nr_pages)
>  {
>         long i;
> @@ -1150,12 +1131,6 @@ static bool check_dax_vmas(struct vm_area_struct **vmas, long nr_pages)
>         }
>         return false;
>  }
> -#else
> -static inline bool check_dax_vmas(struct vm_area_struct **vmas, long nr_pages)
> -{
> -       return false;
> -}
> -#endif
>
>  #ifdef CONFIG_CMA
>  static struct page *new_non_cma_page(struct page *page, unsigned long private)
> @@ -1209,10 +1184,13 @@ static struct page *new_non_cma_page(struct page *page, unsigned long private)
>         return __alloc_pages_node(nid, gfp_mask, 0);
>  }
>
> -static long check_and_migrate_cma_pages(unsigned long start, long nr_pages,
> -                                       unsigned int gup_flags,
> +static long check_and_migrate_cma_pages(struct task_struct *tsk,
> +                                       struct mm_struct *mm,
> +                                       unsigned long start,
> +                                       unsigned long nr_pages,
>                                         struct page **pages,
> -                                       struct vm_area_struct **vmas)
> +                                       struct vm_area_struct **vmas,
> +                                       unsigned int gup_flags)
>  {
>         long i;
>         bool drain_allow = true;
> @@ -1268,10 +1246,14 @@ static long check_and_migrate_cma_pages(unsigned long start, long nr_pages,
>                                 putback_movable_pages(&cma_page_list);
>                 }
>                 /*
> -                * We did migrate all the pages, Try to get the page references again
> -                * migrating any new CMA pages which we failed to isolate earlier.
> +                * We did migrate all the pages, Try to get the page references
> +                * again migrating any new CMA pages which we failed to isolate
> +                * earlier.
>                  */
> -               nr_pages = get_user_pages(start, nr_pages, gup_flags, pages, vmas);
> +               nr_pages = __get_user_pages_locked(tsk, mm, start, nr_pages,
> +                                                  pages, vmas, NULL,
> +                                                  gup_flags);
> +

Why did this need to change to __get_user_pages_locked?

>                 if ((nr_pages > 0) && migrate_allow) {
>                         drain_allow = true;
>                         goto check_again;
> @@ -1281,66 +1263,115 @@ static long check_and_migrate_cma_pages(unsigned long start, long nr_pages,
>         return nr_pages;
>  }
>  #else
> -static inline long check_and_migrate_cma_pages(unsigned long start, long nr_pages,
> -                                              unsigned int gup_flags,
> -                                              struct page **pages,
> -                                              struct vm_area_struct **vmas)
> +static long check_and_migrate_cma_pages(struct task_struct *tsk,
> +                                       struct mm_struct *mm,
> +                                       unsigned long start,
> +                                       unsigned long nr_pages,
> +                                       struct page **pages,
> +                                       struct vm_area_struct **vmas,
> +                                       unsigned int gup_flags)
>  {
>         return nr_pages;
>  }
>  #endif
>
>  /*
> - * This is the same as get_user_pages() in that it assumes we are
> - * operating on the current task's mm, but it goes further to validate
> - * that the vmas associated with the address range are suitable for
> - * longterm elevated page reference counts. For example, filesystem-dax
> - * mappings are subject to the lifetime enforced by the filesystem and
> - * we need guarantees that longterm users like RDMA and V4L2 only
> - * establish mappings that have a kernel enforced revocation mechanism.
> + * __gup_longterm_locked() is a wrapper for __get_uer_pages_locked which

s/uer/user/

> + * allows us to process the FOLL_LONGTERM flag if present.
> + *
> + * FOLL_LONGTERM Checks for either DAX VMAs or PPC CMA regions and either fails
> + * the pin or attempts to migrate the page as appropriate.
> + *
> + * In the filesystem-dax case mappings are subject to the lifetime enforced by
> + * the filesystem and we need guarantees that longterm users like RDMA and V4L2
> + * only establish mappings that have a kernel enforced revocation mechanism.
> + *
> + * In the CMA case pages can't be pinned in a CMA region as this would
> + * unnecessarily fragment that region.  So CMA attempts to migrate the page
> + * before pinning.
>   *
>   * "longterm" == userspace controlled elevated page count lifetime.
>   * Contrast this to iov_iter_get_pages() usages which are transient.

Ah, here's the longterm documentation, but if I was a developer
considering whether to use FOLL_LONGTERM or not I would expect to find
the documentation at the flag definition site.

I think it has become more clear since get_user_pages_longterm() was
initially merged that we need to warn people not to use it, or at
least seriously reconsider whether they want an interface to support
indefinite pins.

>   */
> -long get_user_pages_longterm(unsigned long start, unsigned long nr_pages,
> -                            unsigned int gup_flags, struct page **pages,
> -                            struct vm_area_struct **vmas_arg)
> +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,

...why the __always_inline?