mm/filemap: fix that first page is not mark accessed in filemap_read()

hulk inclusion
category: bugfix
bugzilla: 186896, https://gitee.com/src-openeuler/kernel/issues/I5FRAP
CVE: NA

--------------------------------

In filemap_read(), first page will not mark accessed if previous page
is equal to the current page:

if (iocb->ki_pos >> PAGE_SHIFT != ra->prev_pos >> PAGE_SHIFT))
	folio_mark_accessed(fbatch.folios[0]);

However, 'prev_pos' is set to 'ki_pos + copied' during last read, which
means 'prev_pos' can be equal to 'ki_pos' in this read, thus previous
page can be miscaculated.

Fix the problem by setting 'prev_pos' to the start offset of last read,
so that 'prev_pos >> PAGE_SHIFT' will be previous page as expected.

Fixes: 06c04442 ("mm/filemap.c: generic_file_buffered_read() now uses find_get_pages_contig")
Signed-off-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

mm/filemap.c

@@ -2549,10 +2549,11 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 				flush_dcache_page(pages[i]);
 
 			copied = copy_page_to_iter(pages[i], offset, bytes, iter);
-
-			written += copied;
-			iocb->ki_pos += copied;
-			ra->prev_pos = iocb->ki_pos;
+			if (copied) {
+				ra->prev_pos = iocb->ki_pos;
+				written += copied;
+				iocb->ki_pos += copied;
+			}
 
 			if (copied < bytes) {
 				error = -EFAULT;