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SubjectRe: [PATCH] sector_t overflow in block layer


On Thu, 18 May 2006, Andreas Dilger wrote:
> struct bio *bio;
> + unsigned long long sector;
> int ret = 0;
>
> BUG_ON(!buffer_locked(bh));
> BUG_ON(!buffer_mapped(bh));
> BUG_ON(!bh->b_end_io);
>
> + /* Check if we overflow sector_t when computing the sector offset. */
> + sector = (unsigned long long)bh->b_blocknr * (bh->b_size >> 9);

Ok so far, looks fine.

But what the heck is this:

> +#if !defined(CONFIG_LBD) && BITS_PER_LONG == 32
> + if (unlikely(sector != (sector_t)sector))
> +#else
> + if (unlikely(((bh->b_blocknr >> 32) * (bh->b_size >> 9)) >=
> + 0xffffffff00000000ULL))
> +#endif

I don't understand the #ifdef at all.

Why isn't that just a

if (unlikely(sector != (sector_t)sector))

and that's it? What does this have to do with CONFIG_LBD or BITS_PER_LONG,
or anything at all?

If the sector number fits in a sector_t, we're all good.

Linus
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