Re: [PATCH 01/17] mm/highmem: Lift memcpy_[to|from]_page and memset_page to core

[Matthew Wilcox]

On Fri, Nov 27, 2020 at 03:06:24PM +0200, Joonas Lahtinen wrote:
> Quoting ira.weiny@xxxxxxxxx (2020-11-24 08:07:39)
> > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > 
> > Working through a conversion to a call such as kmap_thread() revealed
> > many places where the pattern kmap/memcpy/kunmap occurred.
> > 
> > Eric Biggers, Matthew Wilcox, Christoph Hellwig, Dan Williams, and Al
> > Viro all suggested putting this code into helper functions.  Al Viro
> > further pointed out that these functions already existed in the iov_iter
> > code.[1]
> > 
> > Placing these functions in 'highmem.h' is suboptimal especially with the
> > changes being proposed in the functionality of kmap.  From a caller
> > perspective including/using 'highmem.h' implies that the functions
> > defined in that header are only required when highmem is in use which is
> > increasingly not the case with modern processors.  Some headers like
> > mm.h or string.h seem ok but don't really portray the functionality
> > well.  'pagemap.h', on the other hand, makes sense and is already
> > included in many of the places we want to convert.
> > 
> > Another alternative would be to create a new header for the promoted
> > memcpy functions, but it masks the fact that these are designed to copy
> > to/from pages using the kernel direct mappings and complicates matters
> > with a new header.
> > 
> > Lift memcpy_to_page(), memcpy_from_page(), and memzero_page() to
> > pagemap.h.
> > 
> > Also, add a memcpy_page(), memmove_page, and memset_page() to cover more
> > kmap/mem*/kunmap. patterns.
> > 
> > [1] https://lore.kernel.org/lkml/20201013200149.GI3576660@xxxxxxxxxxxxxxxxxx/
> >     https://lore.kernel.org/lkml/20201013112544.GA5249@xxxxxxxxxxxxx/
> > 
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Suggested-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> <SNIP>
> 
> > +static inline void memset_page(struct page *page, int val, size_t offset, size_t len)
> > +{
> > +       char *addr = kmap_atomic(page);
> > +       memset(addr + offset, val, len);
> > +       kunmap_atomic(addr);
> > +}
> 
> Other functions have (page, offset) pair. Insertion of 'val' in the middle here required
> to take a double look during review.

Let's be explicit here.  Your suggested order is:

	(page, offset, val, len)

right?  I think I would prefer that to (page, val, offset, len).