Re: [syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[Dileep Sankhla]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master
diff --git a/mm/readahead.c b/mm/readahead.c
index b415c9969176..8462e744a890 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -467,7 +467,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	struct address_space *mapping = ractl->mapping;
 	pgoff_t start = readahead_index(ractl);
 	pgoff_t index = start;
-	unsigned int min_order = mapping_min_folio_order(mapping);
+	unsigned int min_order;
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
 	unsigned int nofs;
@@ -475,6 +475,10 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	gfp_t gfp = readahead_gfp_mask(mapping);
 	unsigned int new_order = ra->order;
 
+	/* See comment in page_cache_ra_unbounded() */
+	nofs = memalloc_nofs_save();
+	filemap_invalidate_lock_shared(mapping);
+
 	trace_page_cache_ra_order(mapping->host, start, ra);
 	if (!mapping_large_folio_support(mapping)) {
 		ra->order = 0;
@@ -483,15 +487,14 @@ void page_cache_ra_order(struct readahead_control *ractl,
 
 	limit = min(limit, index + ra->size - 1);
 
+	min_order = mapping_min_folio_order(mapping);
+
 	new_order = min(mapping_max_folio_order(mapping), new_order);
 	new_order = min_t(unsigned int, new_order, ilog2(ra->size));
 	new_order = max(new_order, min_order);
 
 	ra->order = new_order;
 
-	/* See comment in page_cache_ra_unbounded() */
-	nofs = memalloc_nofs_save();
-	filemap_invalidate_lock_shared(mapping);
 	/*
 	 * If the new_order is greater than min_order and index is
 	 * already aligned to new_order, then this will be noop as index
@@ -516,8 +519,6 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	}
 
 	read_pages(ractl);
-	filemap_invalidate_unlock_shared(mapping);
-	memalloc_nofs_restore(nofs);
 
 	/*
 	 * If there were already pages in the page cache, then we may have
@@ -525,7 +526,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	 * situation below.
 	 */
 	if (!err)
-		return;
+		goto end;
 fallback:
 	/*
 	 * ->readahead() may have updated readahead window size so we have to
@@ -534,6 +535,9 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	if (ra->size > index - start)
 		do_page_cache_ra(ractl, ra->size - (index - start),
 				 ra->async_size);
+end:
+	filemap_invalidate_unlock_shared(mapping);
+	memalloc_nofs_restore(nofs);
 }
 
 static unsigned long ractl_max_pages(struct readahead_control *ractl,