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

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  Re: Kernels v4.9+ cause short reads of block devices Linus Torvalds <torvalds@linux-foundation.org> - 2017-08-27 21:50 +0200
    Re: Kernels v4.9+ cause short reads of block devices Dave Kleikamp <dave.kleikamp@oracle.com> - 2017-08-27 22:00 +0200
    Re: Kernels v4.9+ cause short reads of block devices Doug Nazar <nazard@nazar.ca> - 2017-08-29 12:50 +0200

#1720913 — Re: Kernels v4.9+ cause short reads of block devices

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.

                Linus

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#1720919

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
> 

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#1722318

On 8/27/17 3:47 PM, Linus Torvalds wrote:
> 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.

Not a problem... I got caught trying to find out why 4.13 hangs on a
warm reset. It has really slowed down my testing having towalk couple
of floors for every reboot.

Doug

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