fuse: fix premature writetrhough request for large folio

When large folio is enabled and the initial folio offset exceeds
PAGE_SIZE, e.g. the position resides in the second page of a large
folio, after the folio copying the offset (in the page) won't be updated
to 0 even though the expected range is successfully copied until the end
of the folio.  In this case fuse_fill_write_pages() exits prematurelly
before the request has reached the max_write/max_pages limit.

Fix this by eliminating page offset entirely and use folio offset
instead.

Fixes: d60a601 ("fuse: support large folios for writethrough writes")
Reviewed-by: Horst Birthelmer <hbirthelmer@ddn.com>
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: lostjeffle

fs/fuse/file.c

@@ -1248,7 +1248,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_io_args *ia,
 {
 	struct fuse_args_pages *ap = &ia->ap;
 	struct fuse_conn *fc = get_fuse_conn(mapping->host);
-	unsigned offset = pos & (PAGE_SIZE - 1);
 	size_t count = 0;
 	unsigned int num;
 	int err = 0;
@@ -1275,7 +1274,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_io_args *ia,
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_folio(folio);
 
-		folio_offset = ((index - folio->index) << PAGE_SHIFT) + offset;
+		folio_offset = offset_in_folio(folio, pos);
 		bytes = min(folio_size(folio) - folio_offset, num);
 
 		tmp = copy_folio_from_iter_atomic(folio, folio_offset, bytes, ii);
@@ -1305,9 +1304,6 @@ static ssize_t fuse_fill_write_pages(struct fuse_io_args *ia,
 		count += tmp;
 		pos += tmp;
 		num -= tmp;
-		offset += tmp;
-		if (offset == folio_size(folio))
-			offset = 0;
 
 		/* If we copied full folio, mark it uptodate */
 		if (tmp == folio_size(folio))
@@ -1319,7 +1315,9 @@ static ssize_t fuse_fill_write_pages(struct fuse_io_args *ia,
 			ia->write.folio_locked = true;
 			break;
 		}
-		if (!fc->big_writes || offset != 0)
+		if (!fc->big_writes)
+			break;
+		if (folio_offset + tmp != folio_size(folio))
 			break;
 	}