Re: RFC: get_user_pages_locked|unlocked to leverage VM_FAULT_RETRY

[Peter Zijlstra]

On Fri, Sep 26, 2014 at 07:25:35PM +0200, Andrea Arcangeli wrote:
> diff --git a/drivers/dma/iovlock.c b/drivers/dma/iovlock.c
> index bb48a57..12ea7c3 100644
> --- a/drivers/dma/iovlock.c
> +++ b/drivers/dma/iovlock.c
> @@ -95,17 +95,11 @@ struct dma_pinned_list *dma_pin_iovec_pages(struct iovec 
> *iov, size_t len)
>               pages += page_list->nr_pages;
>  
>               /* pin pages down */
> -             down_read(&current->mm->mmap_sem);
> -             ret = get_user_pages(
> -                     current,
> -                     current->mm,
> +             ret = get_user_pages_fast(
>                       (unsigned long) iov[i].iov_base,
>                       page_list->nr_pages,
>                       1,      /* write */
> -                     0,      /* force */
> -                     page_list->pages,
> -                     NULL);
> -             up_read(&current->mm->mmap_sem);
> +                     page_list->pages);
>  
>               if (ret != page_list->nr_pages)
>                       goto unpin;

> --- a/drivers/misc/sgi-gru/grufault.c
> +++ b/drivers/misc/sgi-gru/grufault.c
> @@ -198,8 +198,7 @@ static int non_atomic_pte_lookup(struct vm_area_struct 
> *vma,
>  #else
>       *pageshift = PAGE_SHIFT;
>  #endif
> -     if (get_user_pages
> -         (current, current->mm, vaddr, 1, write, 0, &page, NULL) <= 0)
> +     if (get_user_pages_fast(vaddr, 1, write, &page) <= 0)
>               return -EFAULT;
>       *paddr = page_to_phys(page);
>       put_page(page);

> diff --git a/drivers/scsi/st.c b/drivers/scsi/st.c
> index aff9689..c89dcfa 100644
> --- a/drivers/scsi/st.c
> +++ b/drivers/scsi/st.c
> @@ -4536,18 +4536,12 @@ static int sgl_map_user_pages(struct st_buffer *STbp,
>               return -ENOMEM;
>  
>          /* Try to fault in all of the necessary pages */
> -     down_read(&current->mm->mmap_sem);
>          /* rw==READ means read from drive, write into memory area */
> -     res = get_user_pages(
> -             current,
> -             current->mm,
> +     res = get_user_pages_fast(
>               uaddr,
>               nr_pages,
>               rw == READ,
> -             0, /* don't force */
> -             pages,
> -             NULL);
> -     up_read(&current->mm->mmap_sem);
> +             pages);
>  
>       /* Errors and no page mapped should return here */
>       if (res < nr_pages)


For all these and the other _fast() users, is there an actual limit to
the nr_pages passed in? Because we used to have the 64 pages limit from
DIO, but without that we get rather long IRQ-off latencies.
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