Re: [RESEND 4/7] mm/gup: Add FOLL_LONGTERM capability to GUP fast

[Dan Williams <dan.j.williams@xxxxxxxxx>]

On Sun, Mar 17, 2019 at 7:36 PM <ira.weiny@xxxxxxxxx> wrote:
>
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
>
> DAX pages were previously unprotected from longterm pins when users
> called get_user_pages_fast().
>
> Use the new FOLL_LONGTERM flag to check for DEVMAP pages and fall
> back to regular GUP processing if a DEVMAP page is encountered.
>
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> ---
>  mm/gup.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 0684a9536207..173db0c44678 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1600,6 +1600,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>                         goto pte_unmap;
>
>                 if (pte_devmap(pte)) {
> +                       if (unlikely(flags & FOLL_LONGTERM))
> +                               goto pte_unmap;
> +
>                         pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
>                         if (unlikely(!pgmap)) {
>                                 undo_dev_pagemap(nr, nr_start, pages);
> @@ -1739,8 +1742,11 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>         if (!pmd_access_permitted(orig, flags & FOLL_WRITE))
>                 return 0;
>
> -       if (pmd_devmap(orig))
> +       if (pmd_devmap(orig)) {
> +               if (unlikely(flags & FOLL_LONGTERM))
> +                       return 0;
>                 return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
> +       }
>
>         refs = 0;
>         page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> @@ -1777,8 +1783,11 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
>         if (!pud_access_permitted(orig, flags & FOLL_WRITE))
>                 return 0;
>
> -       if (pud_devmap(orig))
> +       if (pud_devmap(orig)) {
> +               if (unlikely(flags & FOLL_LONGTERM))
> +                       return 0;
>                 return __gup_device_huge_pud(orig, pudp, addr, end, pages, nr);
> +       }
>
>         refs = 0;
>         page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> @@ -2066,8 +2075,20 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>                 start += nr << PAGE_SHIFT;
>                 pages += nr;
>
> -               ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
> -                                             gup_flags);
> +               if (gup_flags & FOLL_LONGTERM) {
> +                       down_read(&current->mm->mmap_sem);
> +                       ret = __gup_longterm_locked(current, current->mm,
> +                                                   start, nr_pages - nr,
> +                                                   pages, NULL, gup_flags);
> +                       up_read(&current->mm->mmap_sem);
> +               } else {
> +                       /*
> +                        * retain FAULT_FOLL_ALLOW_RETRY optimization if
> +                        * possible
> +                        */
> +                       ret = get_user_pages_unlocked(start, nr_pages - nr,
> +                                                     pages, gup_flags);

I couldn't immediately grok why this path needs to branch on
FOLL_LONGTERM? Won't get_user_pages_unlocked(..., FOLL_LONGTERM) do
the right thing?