ext4: don't set dioread_nolock by default for blocksize < pagesize

Currently on calling echo 3 > drop_caches on host machine, we see
FS corruption in the guest. This happens on Power machine where
blocksize < pagesize.

So as a temporary workaound don't enable dioread_nolock by default
for blocksize < pagesize until we identify the root cause.

Also emit a warning msg in case if this mount option is manually
enabled for blocksize < pagesize.
Reported-by: NAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: NRitesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200327200744.12473-1-riteshh@linux.ibm.comSigned-off-by: NTheodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -2187,6 +2187,14 @@ static int parse_options(char *options, struct super_block *sb,
 		}
 	}
 #endif
+	if (test_opt(sb, DIOREAD_NOLOCK)) {
+		int blocksize =
+			BLOCK_SIZE << le32_to_cpu(sbi->s_es->s_log_block_size);
+		if (blocksize < PAGE_SIZE)
+			ext4_msg(sb, KERN_WARNING, "Warning: mounting with an "
+				 "experimental mount option 'dioread_nolock' "
+				 "for blocksize < PAGE_SIZE");
+	}
 	return 1;
 }
 
@@ -3793,7 +3801,6 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		set_opt(sb, NO_UID32);
 	/* xattr user namespace & acls are now defaulted on */
 	set_opt(sb, XATTR_USER);
-	set_opt(sb, DIOREAD_NOLOCK);
 #ifdef CONFIG_EXT4_FS_POSIX_ACL
 	set_opt(sb, POSIX_ACL);
 #endif
@@ -3843,6 +3850,10 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_li_wait_mult = EXT4_DEF_LI_WAIT_MULT;
 
 	blocksize = BLOCK_SIZE << le32_to_cpu(es->s_log_block_size);
+
+	if (blocksize == PAGE_SIZE)
+		set_opt(sb, DIOREAD_NOLOCK);
+
 	if (blocksize < EXT4_MIN_BLOCK_SIZE ||
 	    blocksize > EXT4_MAX_BLOCK_SIZE) {
 		ext4_msg(sb, KERN_ERR,