linux-kernel - [PATCH] direct-io: don't introduce another read of inode->i

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Date:   Mon, 09 Jan 2017 16:42:58 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     axboe@...nel.dk, Chandan Rajendra <chandan@...ux.vnet.ibm.com>
Cc:     jack@...e.cz, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        anton@...ba.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] direct-io: don't introduce another read of inode->i_blkbits

Commit 20ce44d545844 ("do_direct_IO: Use inode->i_blkbits to compute
block count to be cleaned") introduced a regression: if the block size
of the block device is changed while a direct I/O request is being
setup, it can result in a panic.  See commit ab73857e354ab ("direct-io:
don't read inode->i_blkbits multiple times") for the reasoning, and
commit b87570f5d3496 ("Fix a crash when block device is read and block
size is changed at the same time") for a more detailed problem
description and reproducer.

Fixes: 20ce44d545844
Signed-off-by: Jeff Moyer <jmoyer@...hat.com>

---
Chandan, can you please test this to ensure this still fixes your problem?

diff --git a/fs/direct-io.c b/fs/direct-io.c
index b20adf9..c87bae4 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -905,8 +905,8 @@ static inline void dio_zero_block(struct dio *dio, struct dio_submit *sdio,
 static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
 			struct buffer_head *map_bh)
 {
-	const unsigned i_blkbits = dio->inode->i_blkbits;
 	const unsigned blkbits = sdio->blkbits;
+	const unsigned i_blkbits = blkbits + sdio->blkfactor;
 	int ret = 0;

 	while (sdio->block_in_file < sdio->final_block_in_request) {