linux-ext4 - Re: [PATCH 11/28] ext4: correctly calculate number of blocks for fiemap

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Message-ID: <35b92b761003050846s55ac931dv93571c6b33c7b6f9@mail.gmail.com>
Date:	Fri, 5 Mar 2010 17:46:58 +0100
From:	Leonard Michlmayr <leonard.michlmayr@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 11/28] ext4: correctly calculate number of blocks for 
	fiemap

2010/3/5 Eric Sandeen <sandeen@...hat.com>
>
> > Thank you for pointing this out. I had not checked s_maxbytes.
> > Appearently s_maxbytes can be 1<<(32 + s_blocksize_bits) - 1. Therefore
> > the number of blocks in a file cannot be stored in a 32bit integer.
>
> For extent-based files it had better be....
>
> struct ext4_extent {
>        __le32  ee_block;       /* first logical block extent covers */
>        __le16  ee_len;         /* number of blocks covered by extent */
>
> The start block can't be more than 32 bits; this essentially limits
> the file size / maximum logical block to 2^32 blocks right?
>
> s_maxbytes comes out to 17592186044415
>
> 2^32 4k blocks is 17592186044416 bytes, or max byte offset 17592186044415
>
> What am I missing?  (confusion between max byte count and max
> byte offset, perhaps?)
>

Yes. The block offset has the maximum value 1<<32 - 1.
However the number of blocks may be 1<<32.
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