mm/util: Add folio_mapping() and folio_file_mapping()

These are the folio equivalent of page_mapping() and page_file_mapping().
Add an out-of-line page_mapping() wrapper around folio_mapping()
in order to prevent the page_folio() call from bloating every caller
of page_mapping().  Adjust page_file_mapping() and page_mapping_file()
to use folios internally.  Rename __page_file_mapping() to
swapcache_mapping() and change it to take a folio.

This ends up saving 122 bytes of text overall.  folio_mapping() is
45 bytes shorter than page_mapping() was, but the new page_mapping()
wrapper is 30 bytes.  The major reduction is a few bytes less in dozens
of nfs functions (which call page_file_mapping()).  Most of these appear
to be a slight change in gcc's register allocation decisions, which allow:

   48 8b 56 08         mov    0x8(%rsi),%rdx
   48 8d 42 ff         lea    -0x1(%rdx),%rax
   83 e2 01            and    $0x1,%edx
   48 0f 44 c6         cmove  %rsi,%rax

to become:

   48 8b 46 08         mov    0x8(%rsi),%rax
   48 8d 78 ff         lea    -0x1(%rax),%rdi
   a8 01               test   $0x1,%al
   48 0f 44 fe         cmove  %rsi,%rdi

for a reduction of a single byte.  Once the NFS client is converted to
use folios, this entire sequence will disappear.

Also add folio_mapping() documentation.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: David Howells <dhowells@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index ba52f3a3478e..85607c6c0cba 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -320,6 +320,12 @@ struct vma_swap_readahead {
 #endif
 };
 
+static inline swp_entry_t folio_swap_entry(struct folio *folio)
+{
+	swp_entry_t entry = { .val = page_private(&folio->page) };
+	return entry;
+}
+
 /* linux/mm/workingset.c */
 void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
 void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg);