linux-ext4 - mkfs'ing a 48-bit fs... or not.

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Date:	Mon, 03 Oct 2011 16:55:11 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	ext4 development <linux-ext4@...r.kernel.org>
Subject: mkfs'ing a 48-bit fs... or not.

Has anyone tried mke2fs at its limits?  The latest git tree seems to fail in several ways.
(Richard Jones reported the initial failure)

# truncate --size 1152921504606846976 reallybigfile 
# mke2fs -t ext4 reallybigfile

first,

Warning: the fs_type huge is not defined in mke2fs.conf

(when types "big" and "huge" got added, they never got a mke2fs.conf update?)

Then, I got:

reallybigfile: Not enough space to build proposed filesystem while setting up superblock

because:

        fs->group_desc_count = (blk_t) ext2fs_div64_ceil(
                ext2fs_blocks_count(super) - super->s_first_data_block,
                EXT2_BLOCKS_PER_GROUP(super));
        if (fs->group_desc_count == 0) {
                retval = EXT2_ET_TOOSMALL;

The div64_ceil returns > 2^32 (2^33, actually), and the cast to blk_t
(which should be dgrp_t?) turns that into a 0.

Trying it with "-O bigalloc" (which should be automatic at this size,
I think?) just goes away for a very long time, I'm not sure what it's
thinking about, or if it's in a loop somewhere (looking now).

I also came across this in ext2fs_initialize() in the bigalloc case:

                if (super->s_clusters_per_group > EXT2_MAX_CLUSTERS_PER_GROUP(super))
                        super->s_blocks_per_group = EXT2_MAX_CLUSTERS_PER_GROUP(super);
                super->s_blocks_per_group = EXT2FS_C2B(fs,
                                       super->s_clusters_per_group);

which seems to be incorrect; I doubt that you meant to set s_blocks_per_group under
a conditional, and then unconditionally set it immediately after.  I assume
that should be super->s_clusters_per_group in the first case?  I'll send a patch,
assuming so.

TBH I've kind of lost the thread on bigalloc, so just putting this out there for
now while I look into things a bit more.

-Eric
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