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Subject: Re: Kernels v4.9+ cause short reads of block devices
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Andreas Dilger <adilger@dilger.ca>
Cc: Doug Nazar <nazard@nazar.ca>, Al Viro <viro@zeniv.linux.org.uk>,
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        Wei Fang <fangwei1@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Mark Fasheh <mfasheh@versity.com>, Joel Becker <jlbec@evilplan.org>,
        Dave Kleikamp <shaggy@kernel.org>
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From: Dave Kleikamp <dave.kleikamp@oracle.com>
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Date: Sun, 27 Aug 2017 14:54:10 -0500
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On 08/27/2017 02:47 PM, Linus Torvalds wrote:
> On Wed, Aug 23, 2017 at 2:01 PM, Andreas Dilger <adilger@dilger.ca> wrote:
>>
>> Doug,
>> I noticed while checking for other implications of changing MAX_LFS_FILESIZE
>> that fs/jfs/super.c is also working around this limit.
> 
> Note to people: I just committed the patch to update MAX_LFS_FILESIZE.
> 
> I made it use the simpler (and clearer) calculation of
> 
>     ((loff_t)ULONG_MAX << PAGE_SHIFT)
> 
> for the 32-bit case, and I did *not* change any other users.
> 
> The jfs comment was a bit confusing, and talks about "wraps around" at
> 8TB, when that actually happens at 16TB. Yes, if you use a signed
> number for the index, it does wrap at 8TB, but you really shouldn't
> (and the code the jfs comment points to doesn't).
> 
> So I didn't touch that.  Nor did I touch:
> 
>> it also makes sense to fix jfs_fill_super() to
>> use MAX_LFS_FILESIZE instead of JFS rolling its own, something like:
>>
>>         /* logical blocks are represented by 40 bits in pxd_t, etc.
>>          * and page cache is indexed by long. */
>>         sb->s_maxbytes = min((u64)sb->s_blocksize) << 40,
>>                              MAX_LFS_FILESIZE);
> 
> which I agree should be modified. The new MAX_LFS_FILESIZE should be
> the right size, but the difference now is only one page less one byte.

I'll submit a patch to clean up jfs.

Thanks,
Shaggy

> 
>                 Linus
>