Re: [PATCH v6 1/3] lib: zstd: Add kernel-specific API

[Nick Terrell <terrelln@fb.com>]

> On Dec 2, 2020, at 7:14 PM, Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote:
> 
> On Thu, Dec 03, 2020 at 01:42:03AM +0000, Nick Terrell wrote:
>> 
>> 
>>> On Dec 2, 2020, at 5:16 PM, Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote:
>>> 
>>> On Wed, Dec 02, 2020 at 12:32:40PM -0800, Nick Terrell wrote:
>>>> From: Nick Terrell <terrelln@fb.com>
>>>> 
>>>> This patch:
>>>> - Moves `include/linux/zstd.h` -> `lib/zstd/zstd.h`
>>>> - Adds a new API in `include/linux/zstd.h` that is functionally
>>>> equivalent to the in-use subset of the current API. Functions are
>>>> renamed to avoid symbol collisions with zstd, to make it clear it is
>>>> not the upstream zstd API, and to follow the kernel style guide.
>>>> - Updates all callers to use the new API.
>>>> 
>>>> There are no functional changes in this patch. Since there are no
>>>> functional change, I felt it was okay to update all the callers in a
>>>> single patch, since once the API is approved, the callers are
>>>> mechanically changed.
>>> [...]
>>>> --- a/lib/decompress_unzstd.c
>>>> +++ b/lib/decompress_unzstd.c
>>> [...]
>>>> static int INIT handle_zstd_error(size_t ret, void (*error)(char *x))
>>>> {
>>>> -	const int err = ZSTD_getErrorCode(ret);
>>>> -
>>>> -	if (!ZSTD_isError(ret))
>>>> +	if (!zstd_is_error(ret))
>>>> 		return 0;
>>>> 
>>>> -	switch (err) {
>>>> -	case ZSTD_error_memory_allocation:
>>>> -		error("ZSTD decompressor ran out of memory");
>>>> -		break;
>>>> -	case ZSTD_error_prefix_unknown:
>>>> -		error("Input is not in the ZSTD format (wrong magic bytes)");
>>>> -		break;
>>>> -	case ZSTD_error_dstSize_tooSmall:
>>>> -	case ZSTD_error_corruption_detected:
>>>> -	case ZSTD_error_checksum_wrong:
>>>> -		error("ZSTD-compressed data is corrupt");
>>>> -		break;
>>>> -	default:
>>>> -		error("ZSTD-compressed data is probably corrupt");
>>>> -		break;
>>>> -	}
>>>> +	error("ZSTD decompression failed");
>>>> 	return -1;
>>>> }
>>> 
>>> This looses diagnostics specificity - is this intended? At least the
>>> out-of-memory condition seems useful to distinguish.
>> 
>> Good point. The zstd API no longer exposes the error code enum,
>> but it does expose zstd_get_error_name() which can be used here.
>> I was thinking that the string needed to be static for some reason, but
>> that is not the case. I will make that change.
>> 
>>>> +size_t zstd_compress_stream(zstd_cstream *cstream,
>>>> +	struct zstd_out_buffer *output, struct zstd_in_buffer *input)
>>>> +{
>>>> +	ZSTD_outBuffer o;
>>>> +	ZSTD_inBuffer i;
>>>> +	size_t ret;
>>>> +
>>>> +	memcpy(&o, output, sizeof(o));
>>>> +	memcpy(&i, input, sizeof(i));
>>>> +	ret = ZSTD_compressStream(cstream, &o, &i);
>>>> +	memcpy(output, &o, sizeof(o));
>>>> +	memcpy(input, &i, sizeof(i));
>>>> +	return ret;
>>>> +}
>>> 
>>> Is all this copying necessary? How is it different from type-punning by
>>> direct pointer cast?
>> 
>> If breaking strict aliasing and type-punning by pointer casing is okay, then
>> we can do that here. These memcpys will be negligible for performance, but
>> type-punning would be more succinct if allowed.
> 
> Ah, this might break LTO builds due to strict aliasing violation.
> So I would suggest to just #define the ZSTD names to kernel ones
> for the library code.  Unless there is a cleaner solution...

I don’t want to do that because I want in the 3rd series of the patchset I update
to zstd-1.4.6. And I’m using zstd-1.4.6 as-is in upstream. This allows us to keep
the kernel version up to date, since the patch to update to a new version can be
generated automatically (and manually tested), so it doesn’t fall years behind
upstream again.

The alternative would be to make upstream zstd’s header public and
#define zstd_in_buffer ZSTD_inBuffer. But that would make zstd’s header
public, which would somewhat defeat the purpose of having a kernel wrapper.

These memcpy’s won’t hurt performance, since this function is called at most
every 4KB of input data in all the callers, though they are clunky.