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

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Date:   Sun, 27 Aug 2017 12:47:03 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Doug Nazar <nazard@...ar.ca>, Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wei Fang <fangwei1@...wei.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Dave Kleikamp <shaggy@...nel.org>
Subject: Re: Kernels v4.9+ cause short reads of block devices

On Wed, Aug 23, 2017 at 2:01 PM, Andreas Dilger <adilger@...ger.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.

                Linus