linux-kernel - Re: Kernels v4.9+ cause short reads of block devices

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Message-ID: <ae3c9d95-5af1-eb2d-c0c1-32ae622f6c54@nazar.ca>
Date:   Wed, 23 Aug 2017 15:53:44 -0400
From:   Doug Nazar <nazard@...ar.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wei Fang <fangwei1@...wei.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernels v4.9+ cause short reads of block devices

On 8/23/17 3:37 PM, Linus Torvalds wrote:
> On Wed, Aug 23, 2017 at 12:15 PM, Doug Nazar <nazard@...ar.ca> wrote:
>> The following commits cause short reads of block devices, however writes are
>> still allowed.
>>
>> c2a9737f45e2 ("vfs,mm: fix a dead loop in truncate_inode_pages_range()")
>> d05c5f7ba164 ("vfs,mm: fix return value of read() at s_maxbytes")
>>
>> When e2fsck sees this, it thinks it's a bad sector and tries to write a
>> block of nulls which overwrites the valid data.
> Hmm. Block devices shouldn't have issues with s_maxbytes, and I'm
> surprised that nobody has seen that before.
>
>> Device is LVM over 2 x RAID-5 on an old 32bit desktop.
>>
>> RO    RA   SSZ   BSZ   StartSec            Size   Device
>> rw  4096   512  4096          0   9748044840960 /dev/Storage/Main
> .. and the problem may be as simple as just a missing initialization
> of s_maxbytes for blockdev_superblock.
>
> Does the attcahed trivial one-liner fix things for you?
>
> Al, if it really is this simple, how come nobody even noticed?
>
> Also, I do wonder if that check in do_generic_file_read() should just
> unconditionally use MAX_LFS_FILESIZE, since the whole point there is
> really about the index wrap-around, not about any underlying
> filesystem limits per se.
>
> And that's exactly what MAX_LFS_FILESIZE is - the maximum size that
> fits in the page index.

It's compiling now, but I think it's already set to MAX_LFS_FILESIZE.

[  169.095127] ppos=80180006000, s_maxbytes=7ffffffffff, 
magic=0x62646576, type=bdev

Doug