[RFC] memcpy_from_folio()

[Matthew Wilcox <willy@infradead.org>]

I think I have a good folio replacement for memcpy_from_page().  One of
the annoying things about dealing with multi-page folios is that you
can't kmap the entire folio, yet on systems without highmem, you don't
need to.  It's also somewhat annoying in the caller to keep track
of n/len/offset/pos/...

I think this is probably the best option.  We could have a loop that
kmaps each page in the folio, but that seems like excessive complexity.
I'm happy to have highmem systems be less efficient, since they are
anyway.  Another potential area of concern is that folios can be quite
large and maybe having preemption disabled while we copy 2MB of data
might be a bad thing.  If so, the API is fine with limiting the amount
of data we copy, we just need to find out that it is a problem and
decide what the correct limit is, if it's not folio_size().

 fs/ext4/verity.c           |   16 +++++++---------
 include/linux/highmem.h    |   29 +++++++++++++++++++++++++++++
 include/linux/page-flags.h |    1 +
 3 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
index e4da1704438e..afe847c967a4 100644
--- a/fs/ext4/verity.c
+++ b/fs/ext4/verity.c
@@ -42,18 +42,16 @@ static int pagecache_read(struct inode *inode, void *buf, size_t count,
 			  loff_t pos)
 {
 	while (count) {
-		size_t n = min_t(size_t, count,
-				 PAGE_SIZE - offset_in_page(pos));
-		struct page *page;
+		struct folio *folio;
+		size_t n;
 
-		page = read_mapping_page(inode->i_mapping, pos >> PAGE_SHIFT,
+		folio = read_mapping_folio(inode->i_mapping, pos >> PAGE_SHIFT,
 					 NULL);
-		if (IS_ERR(page))
-			return PTR_ERR(page);
-
-		memcpy_from_page(buf, page, offset_in_page(pos), n);
+		if (IS_ERR(folio))
+			return PTR_ERR(folio);
 
-		put_page(page);
+		n = memcpy_from_file_folio(buf, folio, pos, count);
+		folio_put(folio);
 
 		buf += n;
 		pos += n;
diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 9fa462561e05..9917357b9e8f 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h
@@ -414,6 +414,35 @@ static inline void memzero_page(struct page *page, size_t offset, size_t len)
 	kunmap_local(addr);
 }
 
+/**
+ * memcpy_from_file_folio - Copy some bytes from a file folio.
+ * @to: The destination buffer.
+ * @folio: The folio to copy from.
+ * @pos: The position in the file.
+ * @len: The maximum number of bytes to copy.
+ *
+ * Copy up to @len bytes from this folio.  This may be limited by PAGE_SIZE
+ * if the folio comes from HIGHMEM, and by the size of the folio.
+ *
+ * Return: The number of bytes copied from the folio.
+ */
+static inline size_t memcpy_from_file_folio(char *to, struct folio *folio,
+		loff_t pos, size_t len)
+{
+	size_t offset = offset_in_folio(folio, pos);
+	char *from = kmap_local_folio(folio, offset);
+
+	if (folio_test_highmem(folio))
+		len = min(len, PAGE_SIZE - offset);
+	else
+		len = min(len, folio_size(folio) - offset);
+
+	memcpy(to, from, len);
+	kunmap_local(from);
+
+	return len;
+}
+
 /**
  * folio_zero_segments() - Zero two byte ranges in a folio.
  * @folio: The folio to write to.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index bc09194d372f..bba2a32031a2 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -531,6 +531,7 @@ PAGEFLAG(Readahead, readahead, PF_NO_COMPOUND)
  * available at this point.
  */
 #define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))
+#define folio_test_highmem(__f)	is_highmem_idx(folio_zonenum(__f))
 #else
 PAGEFLAG_FALSE(HighMem, highmem)
 #endif