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>
