On 2019/01/02 23:40, Jan Kara wrote:

I had a look into this and the only good explanation for this I have is
that sb->s_blocksize is different from (1 << sb->s_bdev->bd_inode->i_blkbits).
If that would happen, we'd get exactly the behavior syzkaller observes
because grow_buffers() would populate different page than
__find_get_block() then looks up.

However I don't see how that's possible since the filesystem has the block
device open exclusively and blkdev_bszset() makes sure we also have
exclusive access to the block device before changing the block device size.
So changing block device block size after filesystem gets access to the
device should be impossible. 

Anyway, could you perhaps add to your debug patch a dump of 'size' passed
to __getblk_slow() and bdev->bd_inode->i_blkbits? That should tell us
whether my theory is right or not. Thanks!

OK. Andrew, will you add (or fold into) this change?

From e6f334380ad2c87457bfc2a4058316c47f75824a Mon Sep 17 00:00:00 2001

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Thu, 3 Jan 2019 01:03:35 +0900
Subject: [PATCH] fs/buffer.c: dump more info for __getblk_gfp() stall problem

We need to dump more variables on top of
"fs/buffer.c: add debug print for __getblk_gfp() stall problem".

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Jan Kara <jack@suse.cz>
---
 fs/buffer.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 580fda0..a50acac 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c

@@ -1066,9 +1066,14 @@ static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
 #ifdef CONFIG_DEBUG_AID_FOR_SYZBOT
 		if (!time_after(jiffies, current->getblk_stamp + 3 * HZ))
 			continue;
-		printk(KERN_ERR "%s(%u): getblk(): executed=%x bh_count=%d bh_state=%lx\n",
+		printk(KERN_ERR "%s(%u): getblk(): executed=%x bh_count=%d bh_state=%lx "
+		       "bdev_super_blocksize=%lu size=%u "
+		       "bdev_super_blocksize_bits=%u bdev_inode_blkbits=%u\n",
 		       current->comm, current->pid, current->getblk_executed,
-		       current->getblk_bh_count, current->getblk_bh_state);
+		       current->getblk_bh_count, current->getblk_bh_state,
+		       bdev->bd_super->s_blocksize, size,
+		       bdev->bd_super->s_blocksize_bits,
+		       bdev->bd_inode->i_blkbits);
 		current->getblk_executed = 0;
 		current->getblk_bh_count = 0;
 		current->getblk_bh_state = 0;

-- 
1.8.3.1