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

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Wed, Mar 22, 2023 at 06:17:25PM +0800, Baoquan He wrote:
> 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:

The reason for this is that:-

1. kmap_atomic() explicitly states that it is now deprecated and must no longer
   be used, and kmap_local_page() should be used instead:-

 * kmap_atomic - Atomically map a page for temporary usage - Deprecated!

 * Do not use in new code. Use kmap_local_page() instead.

2. kmap_local_page() explicitly states that it can be used in any context:-

 * Can be invoked from any context, including interrupts.

I wanted follow this advice as strictly as I could, hence the change. However,
we do need preemption/pagefaults explicitly disabled in this context (we are
happy to fail if the faulted in pages are unmapped in meantime), and I didn't
check the internals to make sure.

So I think for safety it is better to use k[un]map_atomic() here, I'll respin
and put that back in, good catch!

>
> 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
> >
>