This is a helper function for filesystems which support file tails.
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
---
include/linux/pagemap.h | 1 +
mm/filemap.c | 57 ++++++++++++++++++++++++++++++++++++++++++
2 files changed, 58 insertions(+)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c21b3ad1068c..618fe184c248 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -764,6 +764,7 @@ static inline struct page *grab_cache_page(struct address_space *mapping,
return find_or_create_page(mapping, index, mapping_gfp_mask(mapping));
}
+void folio_copy_tail(struct folio *, loff_t pos, void *src, size_t max);
struct folio *read_cache_folio(struct address_space *, pgoff_t index,
filler_t *filler, struct file *file);
struct folio *mapping_read_folio_gfp(struct address_space *, pgoff_t index,
diff --git a/mm/filemap.c b/mm/filemap.c
index 40be33b5ee46..b02d9c390d3c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -4145,3 +4145,60 @@ bool filemap_release_folio(struct folio *folio, gfp_t gfp)
return try_to_free_buffers(folio);
}
EXPORT_SYMBOL(filemap_release_folio);
+
+/**
+ * folio_copy_tail - Copy an in-memory file tail into a page cache folio.
+ * @folio: The folio to copy into.
+ * @pos: The file position of the first byte of data in the tail.
+ * @src: The address of the tail data.
+ * @max: The size of the buffer used for the tail data.
+ *
+ * Supports file tails starting at @pos that are a maximum of @max
+ * bytes in size. Zeroes the remainder of the folio.
+ */
+void folio_copy_tail(struct folio *folio, loff_t pos, void *src, size_t max)
+{
+ loff_t isize = i_size_read(folio->mapping->host);
+ size_t offset, len = isize - pos;
+ char *dst;
+
+ if (folio_pos(folio) > isize) {
+ len = 0;
+ } else if (folio_pos(folio) > pos) {
+ len -= folio_pos(folio) - pos;
+ src += folio_pos(folio) - pos;
+ max -= folio_pos(folio) - pos;
+ pos = folio_pos(folio);
+ }
+ /*
+ * i_size is larger than the number of bytes stored in the tail?
+ * Assume the remainder is zero-padded.
+ */
+ if (WARN_ON_ONCE(len > max))
+ len = max;
+ offset = offset_in_folio(folio, pos);
+ dst = kmap_local_folio(folio, offset);
+ if (folio_test_highmem(folio) && folio_test_large(folio)) {
+ size_t poff = offset_in_page(offset);
+ size_t plen = min(poff + len, PAGE_SIZE) - poff;
+
+ for (;;) {
+ memcpy(dst, src, plen);
+ memset(dst + plen, 0, PAGE_SIZE - poff - plen);
+ offset += PAGE_SIZE - poff;
+ if (offset == folio_size(folio))
+ break;
+ kunmap_local(dst);
+ dst = kmap_local_folio(folio, offset);
+ len -= plen;
+ poff = 0;
+ plen = min(len, PAGE_SIZE);
+ }
+ } else {
+ memcpy(dst, src, len);
+ memset(dst + len, 0, folio_size(folio) - len - offset);
+ }
+ kunmap_local(dst);
+ flush_dcache_folio(folio);
+}
+EXPORT_SYMBOL_GPL(folio_copy_tail);
--
2.39.1
