On Mon, 2013-07-22 at 15:56 +0900, Alexandre Courbot wrote:
> When decompressing into memory, the output buffer length is set to some
> arbitrarily high value (0x7fffffff) to indicate the output is,
> virtually, unlimited in size.
>
> The problem with this is that some platforms have their physical memory
> at high physical addresses (0x80000000 or more), and that the output
> buffer address and its "unlimited" length cannot be added without
> overflowing. An example of this can be found in inflate_fast():
>
> /* next_out is the output buffer address */
> out = strm->next_out - OFF;
> /* avail_out is the output buffer size. end will overflow if the output
> * address is >= 0x80000104 */
> end = out + (strm->avail_out - 257);
>
> This has huge consequences on the performance of kernel decompression,
> since the following exit condition of inflate_fast() will be always
> true:
>
> } while (in < last && out < end);
>
> Indeed, "end" has overflowed and is now always lower than "out". As a
> result, inflate_fast() will return after processing one single byte of
> input data, and will thus need to be called an unreasonably high number
> of times. This probably went unnoticed because kernel decompression is
> fast enough even with this issue.
>
> Nonetheless, adjusting the output buffer length in such a way that the
> above pointer arithmetic never overflows results in a kernel
> decompression that is about 3 times faster on affected machines.
>
> Signed-off-by: Alexandre Courbot <acour...@nvidia.com>
This speeds up booting of my Versatile Express TC2 by 15 seconds when
starting on the A7 cluster :-)
Tested-by: Jon Medhurst <t...@linaro.org>
> ---
> lib/decompress_inflate.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/decompress_inflate.c b/lib/decompress_inflate.c
> index 19ff89e..d619b28 100644
> --- a/lib/decompress_inflate.c
> +++ b/lib/decompress_inflate.c
> @@ -48,7 +48,7 @@ STATIC int INIT gunzip(unsigned char *buf, int len,
> out_len = 0x8000; /* 32 K */
> out_buf = malloc(out_len);
> } else {
> - out_len = 0x7fffffff; /* no limit */
> + out_len = ((size_t)~0) - (size_t)out_buf; /* no limit */
> }
> if (!out_buf) {
> error("Out of memory while allocating output buffer");
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
