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@xxxxxxxxx>
> ---
> 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
>
