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

[Linus Torvalds]

On Wed, Aug 23, 2017 at 12:15 PM, Doug Nazar <nazard@xxxxxxxx> 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.

              Linus
 fs/block_dev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 9941dc8342df..4c3867c5298a 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -830,6 +830,7 @@ void __init bdev_cache_init(void)
 	if (IS_ERR(bd_mnt))
 		panic("Cannot create bdev pseudo-fs");
 	blockdev_superblock = bd_mnt->mnt_sb;   /* For writeback */
+	blockdev_superblock->s_maxbytes = MAX_LFS_FILESIZE;
 }
 
 /*