Re: [2.4.23-pre3] Possible bug in fs/buffer.c

[Andreas Schwab <schwab@suse.de>]

Rolf Eike Beer <eike-kernel@sf-tec.de> writes:

> This is __put_unused_buffer_head from fs/buffer.c, lines 1156 to 1171:
>
>
> static void __put_unused_buffer_head(struct buffer_head * bh)
> {
> 	if (unlikely(buffer_attached(bh)))
> 		BUG();
> 	if (nr_unused_buffer_heads >= MAX_UNUSED_BUFFERS) {
> 		kmem_cache_free(bh_cachep, bh);
> 	} else {
> 		bh->b_dev = B_FREE;
> ===>		bh->b_blocknr = -1;		<===
> 		bh->b_this_page = NULL;
>
> 		nr_unused_buffer_heads++;
> 		bh->b_next_free = unused_list;
> 		unused_list = bh;
> 	}
> }
>
> In include/linux/fs.h "struct buffer_head" is defined this way:
>
> struct buffer_head {
>         /* First cache line: */
>         struct buffer_head *b_next;     /* Hash queue list */
>         unsigned long b_blocknr;        /* block number */
> ...
>
> So this line (and line 1205, which is the same) is either ugly (and someone
> meant ~0UL or something similar) or completely bogus.

It's neither ugly, nor bogus.  The only 100% reliable way to assign the
maximum value to an unsigned integer is to use -1.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
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