> On Dec 2, 2020, at 9:03 PM, Michał Mirosław <mirq-linux@xxxxxxxxxxxx> wrote:
>
> On Thu, Dec 03, 2020 at 03:59:21AM +0000, Nick Terrell wrote:
>> On Dec 2, 2020, at 7:14 PM, Michał Mirosław <mirq-linux@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote:
>>>>> On Wed, Dec 02, 2020 at 12:32:40PM -0800, Nick Terrell wrote:
>>>>>> From: Nick Terrell <terrelln@xxxxxx>
>>>>>>
>>>>>> 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.
>
> I thought the problem was API style spill-over from the library to other parts
> of the kernel. A header-only wrapper can stop this. I'm not sure symbol
> visibility (namespace pollution) was a concern.
Thanks for the review Michał! I have just submitted a new version of the patches
with the suggested changes!
Best,
Nick Terrell
> Best Regards
> Michał Mirosław
