Re: [PATCH net] tls: Skip tls_append_frag on zero copy size

[Maxim Mikityanskiy <maximmi@nvidia.com>]

On 2022-04-22 17:55, Jakub Kicinski wrote:
> On Thu, 21 Apr 2022 12:47:18 +0300 Maxim Mikityanskiy wrote:
>> On 2022-04-18 17:56, Maxim Mikityanskiy wrote:
>>> On 2022-04-14 13:28, Jakub Kicinski wrote:
>>>> I appreciate you're likely trying to keep the fix minimal but Greg
>>>> always says "fix it right, worry about backports later".
>>>>
>>>> I think we should skip more, we can reorder the mins and if
>>>> min(size, rec space) == 0 then we can skip the allocation as well.
>>>
>>> Sorry, I didn't get the idea. Could you elaborate?
>>>
>>> Reordering the mins:
>>>
>>> copy = min_t(size_t, size, max_open_record_len - record->len);
>>> copy = min_t(size_t, copy, pfrag->size - pfrag->offset);
>>>
>>> I assume by skipping the allocation you mean skipping
>>> tls_do_allocation(), right? Do you suggest to skip it if the result of
>>> the first min_t() is 0?
>>>
>>> record->len used in the first min_t() comes from ctx->open_record, which
>>> either exists or is allocated by tls_do_allocation(). If we move the
>>> copy == 0 check above the tls_do_allocation() call, first we'll have to
>>> check whether ctx->open_record is NULL, which is currently checked by
>>> tls_do_allocation() itself.
>>>
>>> If open_record is not NULL, there isn't much to skip in
>>> tls_do_allocation on copy == 0, the main part is already skipped,
>>> regardless of the value of copy. If open_record is NULL, we can't skip
>>> tls_do_allocation, and copy won't be 0 afterwards.
>>>
>>> To compare, before (pseudocode):
>>>
>>> tls_do_allocation {
>>>       if (!ctx->open_record)
>>>           ALLOCATE RECORD
>>>           Now ctx->open_record is not NULL
>>>       if (!sk_page_frag_refill(sk, pfrag))
>>>           return -ENOMEM
>>> }
>>> handle errors from tls_do_allocation
>>> copy = min(size, pfrag->size - pfrag->offset)
>>> copy = min(copy, max_open_record_len - ctx->open_record->len)
>>> if (copy)
>>>       copy data and append frag
>>>
>>> After:
>>>
>>> if (ctx->open_record) {
>>>       copy = min(size, max_open_record_len - ctx->open_record->len)
>>>       if (copy) {
>>>           // You want to put this part of tls_do_allocation under if (copy)?
>>>           if (!sk_page_frag_refill(sk, pfrag))
>>>               handle errors
>>>           copy = min(copy, pfrag->size - pfrag->offset)
>>>           if (copy)
>>>               copy data and append frag
>>>       }
>>> } else {
>>>       ALLOCATE RECORD
>>>       if (!sk_page_frag_refill(sk, pfrag))
>>>           handle errors
>>>       // Have to do this after the allocation anyway.
>>>       copy = min(size, max_open_record_len - ctx->open_record->len)
>>>       copy = min(copy, pfrag->size - pfrag->offset)
>>>       if (copy)
>>>           copy data and append frag
>>> }
>>>
>>> Either I totally don't get what you suggested, or it doesn't make sense
>>> to me, because we have +1 branch in the common path when a record is
>>> open and copy is not 0, no changes when there is no record, and more
>>> repeating code hard to compress.
>>>
>>> If I missed your idea, please explain in more details.
>>
>> Jakub, is your comment still relevant after my response? If not, can the
>> patch be merged?
> 
> I'd prefer if you refactored the code so tls_push_data() looks more
> natural.

I would be happy to improve the code, but I honestly didn't understand 
your idea. My attempt to understand it only made the code worse.

> But the patch is correct so if you don't want to you can
> repost.

OK, I'm resubmitting as is, but in case you find time to elaborate on 
your refactoring idea, I'm still open to suggestions.

Thanks.

> Sorry for the delay.