mm: add memcpy_from_file_folio()

This is the equivalent of memcpy_from_page().  It differs in that it takes
the position in a file instead of offset in a folio, it accepts the total
number of bytes to be copied (instead of the number of bytes to be copied
from this folio) and it returns how many bytes were copied from the folio,
rather than making the caller calculate that and then checking if the
caller got it right.

[akpm@linux-foundation.org: fix typo in comment]
Link: https://lkml.kernel.org/r/20230126201552.1681588-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index e225094..348701d 100644 (file)
--- 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)
 }
 
 /**
+ * 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_t(size_t, 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.
  * @start1: The first byte to zero.

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 69e93a0..a7e3a34 100644 (file)
--- 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