Re: [PATCH v4 3/4] iov_iter: add copy_page_to_iter_atomic()

[Baoquan He <bhe@redhat.com>]

On 03/21/23 at 08:54pm, Lorenzo Stoakes wrote:
> Provide an atomic context equivalent for copy_page_to_iter(). This eschews
> the might_fault() check copies memory in the same way that
> copy_page_from_iter_atomic() does.
> 
> This functions assumes a non-compound page, however this mimics the
> existing behaviour of copy_page_from_iter_atomic(). I am keeping the
> behaviour consistent between the two, deferring any such change to an
> explicit folio-fication effort.
> 
> This is being added in order that an iteratable form of vread() can be
> implemented with known prefaulted pages to avoid the need for mutex
> locking.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>  include/linux/uio.h |  2 ++
>  lib/iov_iter.c      | 28 ++++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 27e3fd942960..fab07103090f 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -154,6 +154,8 @@ static inline struct iovec iov_iter_iovec(const struct iov_iter *iter)
>  
>  size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
>  				  size_t bytes, struct iov_iter *i);
> +size_t copy_page_to_iter_atomic(struct page *page, unsigned offset,
> +				size_t bytes, struct iov_iter *i);
>  void iov_iter_advance(struct iov_iter *i, size_t bytes);
>  void iov_iter_revert(struct iov_iter *i, size_t bytes);
>  size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t bytes);
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 274014e4eafe..48ca1c5dfc04 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -821,6 +821,34 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset, size_t byt
>  }
>  EXPORT_SYMBOL(copy_page_from_iter_atomic);
>  
> +size_t copy_page_to_iter_atomic(struct page *page, unsigned offset, size_t bytes,
> +				struct iov_iter *i)
> +{
> +	char *kaddr = kmap_local_page(page);

I am a little confused about the name of this new function. In its
conterpart, copy_page_from_iter_atomic(), kmap_atomic()/kunmpa_atomic()
are used. With them, if CONFIG_HIGHMEM=n, it's like below:

static inline void *kmap_atomic(struct page *page)
{
        if (IS_ENABLED(CONFIG_PREEMPT_RT))
                migrate_disable();
        else
                preempt_disable();
        pagefault_disable();
        return page_address(page);
}

But kmap_local_page() is only having page_address(), the code block
between kmap_local_page() and kunmap_local() is also atomic, it's a
little messy in my mind.

static inline void *kmap_local_page(struct page *page)
{
        return page_address(page);
}

> +	char *p = kaddr + offset;
> +	size_t copied = 0;
> +
> +	if (!page_copy_sane(page, offset, bytes) ||
> +	    WARN_ON_ONCE(i->data_source))
> +		goto out;
> +
> +	if (unlikely(iov_iter_is_pipe(i))) {
> +		copied = copy_page_to_iter_pipe(page, offset, bytes, i);
> +		goto out;
> +	}
> +
> +	iterate_and_advance(i, bytes, base, len, off,
> +		copyout(base, p + off, len),
> +		memcpy(base, p + off, len)
> +	)
> +	copied = bytes;
> +
> +out:
> +	kunmap_local(kaddr);
> +	return copied;
> +}
> +EXPORT_SYMBOL(copy_page_to_iter_atomic);
> +
>  static void pipe_advance(struct iov_iter *i, size_t size)
>  {
>  	struct pipe_inode_info *pipe = i->pipe;
> -- 
> 2.39.2
>