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SubjectRe: OOPS caused by ext2 changes
Dave Hansen wrote:
>
> Andrew Morton and I discused this earlier. I have some more information
> now. The problem: "dbench 64" run on a small (~120meg) partition with
> 1k block sizes produces Oopses.
>
> This changeset:
> http://linus.bkbits.net:8080/linux-2.5/patch@1.248.2.6?nav=index.html|ChangeSet|cset@1.248.2.6
> is the culprit. Without it applied, none of this happens.
>


It's vaguely surprising that that chunk is associated with the
problem.

However it seems that there's potential for a buffer reference
leak in ext2_get_branch:

while (--depth) {
bh = sb_bread(sb, le32_to_cpu(p->key));
if (!bh)
goto failure;
/* Reader: pointers */
if (!verify_chain(chain, p))
goto changed;
add_chain(++p, bh, (u32*)bh->b_data + *++offsets);
/* Reader: end */
if (!p->key)
goto no_block;
}
return NULL;
changed:
*err = -EAGAIN;
goto no_block;
failure:
*err = -EIO;
no_block:
return p;
}

See, sb_bread() bumps b_count, but on the `goto changed;'
branch we lose track of that buffer.

b_count is only 16 bits, so it's conceivable that the
count wraps to zero, and that is fatal.

It would be interesting to replace that `goto changed;'
with { __brelse(bh); goto changed; }. Plus maybe a
debug printk to see if we are indeed hitting that path.
I don't think this is the bug actually - if we were
leaking bh refs that easily we'd get `busy buffer'
whines at unmount. But it merits investigation.

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