Subject: Re: Kernels v4.9+ cause short reads of block devices
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Wei Fang <fangwei1@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
References: <1ae53e17-e455-4f17-0280-b0dae183a449@nazar.ca>
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 <ae3c9d95-5af1-eb2d-c0c1-32ae622f6c54@nazar.ca>
 <CA+55aFzWCSU6A0xmG+TC+K-tN3tZTOo_0dxYy5k=5K+DeTEx8A@mail.gmail.com>
From: Doug Nazar <nazard@nazar.ca>
Message-ID: <706d597b-37bd-7725-a164-9e2b19255fac@nazar.ca>
Date: Thu, 24 Aug 2017 06:03:33 -0400
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On 8/23/17 4:13 PM, Linus Torvalds wrote:

> Oh, right you are - I'm much too used to 64-bit, where
> MAX_LFS_FILESIZE is basically infinite, and was jusr assuming that it
> was something like the UFS bug we had not that long ago that was due
> to the 32-bit limit.
>
> But yes, on 32-bit, we are limited by the 32-bit index into the page
> cache, and we limit the index to 31 bits too, so we have (PAGE_SIZE <<
> 31) -1, which is that 7ffffffffff.

Yeah, it's an old organically grown storage server (install images, old 
vms, etc.) that
I can't convince myself it's worth upgrading.

> The *right* patch is likely to just this instead:
>
>    -#define MAX_LFS_FILESIZE       (((loff_t)PAGE_SIZE << (BITS_PER_LONG-1))-1)
>    +#define MAX_LFS_FILESIZE       (((loff_t)PAGE_SIZE <<
> BITS_PER_LONG)-PAGE_SIZE)
>
> which should make MAX_LFS_FILESIZE be 0xffffffff000 and you disk size
> should be ok.

That solves my issue. I'm curious if that check should also be in the 
write path. If the
check had been there too I wouldn't have ended up with any corruption.

I'm not sure if anything over 16TB will create/assemble (or if anybody 
else is crazy like me).

Doug