[PATCH v2 08/10] filemap: Add hwpoison handling to filemap_read()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

From: Jane Chu <jane.chu@oracle.com>

Add hwpoison handling to filemap_read() such that .read_iter() could
make best effort copying data out of clean pages without risking
MCE in case page cache contains HWpoison.

Signed-off-by: Jane Chu <jane.chu@oracle.com>
Co-developed-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
---
 mm/filemap.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 5af62e6abca5..c568d267c803 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2483,6 +2483,8 @@ static void filemap_get_read_batch(struct address_space *mapping,
 
 		if (!folio_batch_add(fbatch, folio))
 			break;
+		if (folio_has_hwpoisoned_page(folio))
+			break;
 		if (!folio_test_uptodate(folio))
 			break;
 		if (folio_test_readahead(folio))
@@ -2749,6 +2751,29 @@ static inline bool pos_same_folio(loff_t pos1, loff_t pos2, struct folio *folio)
 	return (pos1 >> shift == pos2 >> shift);
 }
 
+static size_t adjust_range_hwpoison(const struct folio *folio, size_t offset,
+		size_t bytes)
+{
+	const struct page *page = folio_page(folio, offset / PAGE_SIZE);
+	size_t safe_bytes;
+
+	if (!folio_has_hwpoisoned_page(folio))
+		return bytes;
+	if (is_page_hwpoison(page))
+		return 0;
+
+	/* Safe to read the remaining bytes in this page. */
+	safe_bytes = PAGE_SIZE - (offset % PAGE_SIZE);
+	page++;
+
+	/* Check each remaining page as long as we are not done yet. */
+	for (; safe_bytes < bytes; safe_bytes += PAGE_SIZE, page++)
+		if (is_page_hwpoison(page))
+			break;
+
+	return min(safe_bytes, bytes);
+}
+
 static void filemap_end_dropbehind_read(struct folio *folio)
 {
 	if (!folio_test_dropbehind(folio))
@@ -2862,7 +2887,8 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 			if (writably_mapped)
 				flush_dcache_folio(folio);
 
-			copied = copy_folio_to_iter(folio, offset, bytes, iter);
+			copied = adjust_range_hwpoison(folio, offset, bytes);
+			copied = copy_folio_to_iter(folio, offset, copied, iter);
 
 			already_read += copied;
 			iocb->ki_pos += copied;
-- 
2.47.3