On 8/17/19 12:26 PM, Eric Dumazet wrote:
>
>
> On 8/17/19 4:19 PM, Jason Baron wrote:
>>
>>
>> On 8/17/19 12:26 AM, Eric Dumazet wrote:
>>> As Jason Baron explained in commit 790ba4566c1a ("tcp: set SOCK_NOSPACE
>>> under memory pressure"), it is crucial we properly set SOCK_NOSPACE
>>> when needed.
>>>
>>> However, Jason patch had a bug, because the 'nonblocking' status
>>> as far as sk_stream_wait_memory() is concerned is governed
>>> by MSG_DONTWAIT flag passed at sendmsg() time :
>>>
>>> long timeo = sock_sndtimeo(sk, flags & MSG_DONTWAIT);
>>>
>>> So it is very possible that tcp sendmsg() calls sk_stream_wait_memory(),
>>> and that sk_stream_wait_memory() returns -EAGAIN with SOCK_NOSPACE
>>> cleared, if sk->sk_sndtimeo has been set to a small (but not zero)
>>> value.
>>
>> Is MSG_DONTWAIT not set in this case? The original patch was intended
>> only for the explicit non-blocking case. The epoll manpage says:
>> "EPOLLET flag should use nonblocking file descriptors". So the original
>> intention was not to impact the blocking case. This seems to me like
>> a different use-case.
>>
>
> I guess the problem is how we define 'non-blocking' ...
>
> SO_SNDTIMEO can be used by application to implement a variation of
> non-blocking,
> by waiting for a socket event with a short timeout, to maybe recover
> from memory pressure conditions in a more efficient way than simply looping.
>
> Note that the man page for epoll() only _suggests_ to use nonblocking file
> descriptors.
>
> <quote>
> The suggested way to use epoll as an edge-triggered (EPOLLET)
> interface is as follows:
>
> i with nonblocking file descriptors; and
>
> ii by waiting for an event only after read(2) or
> write(2) return EAGAIN.
> </quote>
>
>
Ok, seems reasonable:
Acked-by: Jason Baron <jba...@akamai.com>
I found a similar pattern in net/smc/smc_tx.c, which I also just sent a
patch for.
Thanks,
-Jason
