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

[Baoquan He <bhe@redhat.com>]

On 03/22/23 at 10:32am, Lorenzo Stoakes wrote:
> 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.

Yeah, I saw that stated in document too. With my understanding, it's the
page mapping itself will be guaranteed and can be used in any context
when kmap_local_page() is taken. However, here kmap_local_page() is used
to make the code block atomic, it could be not achieved.

> 
> 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
> > >
> >
>