linux-ext4 - Re: e2fsck discard errors

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Date:	Tue, 21 Feb 2012 08:28:10 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Phillip Susi <psusi@...ntu.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: e2fsck discard errors

On Mon, 20 Feb 2012, Phillip Susi wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> I was experimenting with e2fsck -E discard on image files and found
> some errors introduced by commit efa1a355.  The following section of
> code attempts to discard unused portions of the inode table:
> 
> 
> /* Discard inode table */
> if (ctx->options & E2F_OPT_DISCARD) {
>         blk64_t used_blks, blk, num;
> 
>         used_blks = DIV_ROUND_UP(
>                 (EXT2_INODES_PER_GROUP(fs->super) -
>                  group_free),
>                 EXT2_INODES_PER_BLOCK(fs->super));
> 
>         blk = ext2fs_inode_table_loc(fs, group) +
>                 used_blks;
>         num = fs->inode_blocks_per_group -
>                 used_blks;
>         e2fsck_discard_blocks(ctx, manager, blk, num);
> }
> 
> There are two problems with this.  The first is that trying to discard
> a count of zero blocks results in an -EINVAL, which silently halts (
> shouldn't this at least issue a warning? ) further discard attempts.
> The second I noticed after fixing that problem and had a bunch of
> valid inodes discarded.  It looks like the intent of this code is to
> free the part of the inode table that lies beyond the highest used
> inode, but instead it is using the count of free inodes, so when you
> have some free inodes followed by some used inodes, the used inodes at
> the end get discarded.
> 
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> 

Oops, thanks for noticing. You're right with both problems you're
describing. I'll make a patch so that we discard only unused parts of
inode table.

Thanks again!
-Lukas
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