Re: Ext2/3 32-bit stat() wrap for ~2TB files

[Andreas Dilger <adilger@clusterfs.com>]

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On Feb 11, 2005  20:52 +0000, Stephen C. Tweedie wrote:
> /*
>  * Maximal file size.  There is a direct, and {,double-,triple-}indirect
>  * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
>  * We need to be 1 filesystem block less than the 2^32 sector limit.
>  */
> 
> Trouble is, that limit *should* be an i_blocks limit, because i_blocks
> is still 32-bits, and (more importantly) is multiplied by the fs
> blocksize / 512 in stat(2) to return st_blocks in 512-byte chunks. 
> Overflow 2^32 sectors in i_blocks and stat(2) wraps.

I agree.  The problem AFAIR is that the i_blocks accounting is done in
the quota code, so it was a challenge to get it right, and the i_size
limit was easier to do.  Until now I don't think anyone has created
dense 2TB files, so the sparse limit was enough.

Fixing this to count i_blocks correctly would also allow us to have
larger sparse files (up to the indirect limit).

Note also that there was a patch to extend i_blocks floating around
(pretty small hack to use one of the reserved fields), and it might make
sense to get this into the kernel before we actually need it.

> But i_blocks includes indirect blocks as well as data, so for a
> non-sparse file we wrap stat(2) st_blocks well before the file is
> 2^32*512 bytes long.  Yet ext3_max_size() doesn't understand this:
> it simply caps the size with
> 
> 	if (res > (512LL << 32) - (1 << bits))
> 		res = (512LL << 32) - (1 << bits);

So, for the quick fix we could reduce this by the number of expected
[td]indirect blocks and submit that to 2.4 also.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://members.shaw.ca/adilger/             http://members.shaw.ca/golinux/


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