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    SubjectCrash when IO is being submitted and block size is changed

    The kernel crashes when IO is being submitted to a block device and block
    size of that device is changed simultaneously.

    To reproduce the crash, apply this patch:

    --- linux-3.4.3-fast.orig/fs/block_dev.c 2012-06-27 20:24:07.000000000 +0200
    +++ linux-3.4.3-fast/fs/block_dev.c 2012-06-27 20:28:34.000000000 +0200
    @@ -28,6 +28,7 @@
    #include <linux/log2.h>
    #include <linux/cleancache.h>
    #include <asm/uaccess.h>
    +#include <linux/delay.h>
    #include "internal.h"
    struct bdev_inode {
    @@ -203,6 +204,7 @@ blkdev_get_blocks(struct inode *inode, s

    bh->b_bdev = I_BDEV(inode);
    bh->b_blocknr = iblock;
    + msleep(1000);
    bh->b_size = max_blocks << inode->i_blkbits;
    if (max_blocks)
    Use some device with 4k blocksize, for example a ramdisk.
    Run "dd if=/dev/ram0 of=/dev/null bs=4k count=1 iflag=direct"
    While it is sleeping in the msleep function, run "blockdev --setbsz 2048
    /dev/ram0" on the other console.
    You get a BUG at fs/direct-io.c:1013 - BUG_ON(this_chunk_bytes == 0);

    One may ask "why would anyone do this - submit I/O and change block size
    simultaneously?" - the problem is that udev and lvm can scan and read all
    block devices anytime - so anytime you change block device size, there may
    be some i/o to that device in flight and the crash may happen. That BUG
    actually happened in production environment because of lvm scanning block
    devices and some other software changing block size at the same time.

    I would like to know, what is your opinion on fixing this crash? There are
    several possibilities:

    * we could potentially read i_blkbits once, store it in the direct i/o
    structure and never read it again - direct i/o could be maybe modified for
    this (it reads i_blkbits only at a few places). But what about non-direct
    i/o? Non-direct i/o is reading i_blkbits much more often and the code was
    obviously written without consideration that it may change - for block
    devices, i_blkbits is essentially a random value that can change anytime
    you read it and the code of block_read_full_page, __block_write_begin,
    __block_write_full_page and others doesn't seem to take it into account.

    * put some rw-lock arond all I/Os on block device. The rw-lock would be
    taken for read on all I/O paths and it would be taken for write when
    changing the block device size. The downside would be a possible
    performance hit of the rw-lock. The rw-lock could be made per-cpu to avoid
    cache line bouncing (take the rw-lock belonging to the current cpu for
    read; for write take all cpus' locks).

    * allow changing block size only if the device is open only once and the
    process is singlethreaded? (so there couldn't be any outstanding I/Os). I
    don't know if this could be tested reliably... Another question: what to
    do if the device is open multiple times?

    Do you have any other ideas what to do with it?


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      Last update: 2012-06-28 06:42    [W:0.026 / U:6.948 seconds]
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