On Mon, Jul 05, 2021 at 01:48:28PM +0300, Arseny Krasnov wrote:
On 04.07.2021 12:54, Michael S. Tsirkin wrote:
On Sun, Jul 04, 2021 at 12:23:03PM +0300, Arseny Krasnov wrote:
On 04.07.2021 11:30, Michael S. Tsirkin wrote:
On Sun, Jul 04, 2021 at 11:08:13AM +0300, Arseny Krasnov wrote:
This patchset modifies receive logic for SOCK_SEQPACKET.
Difference between current implementation and this version is that
now reader is woken up when there is at least one RW packet in rx
queue of socket and data is copied to user's buffer, while merged
approach wake up user only when whole message is received and kept
in queue. New implementation has several advantages:
1) There is no limit for message length. Merged approach requires
that length must be smaller than 'peer_buf_alloc', otherwise
transmission will stuck.
2) There is no need to keep whole message in queue, thus no
'kmalloc()' memory will be wasted until EOR is received.
Also new approach has some feature: as fragments of message
are copied until EOR is received, it is possible that part of
message will be already in user's buffer, while rest of message
still not received. And if user will be interrupted by signal or
timeout with part of message in buffer, it will exit receive loop,
leaving rest of message in queue. To solve this problem special
callback was added to transport: it is called when user was forced
to leave exit loop and tells transport to drop any packet until
EOR met.
Sorry about commenting late in the game. I'm a bit lost
SOCK_SEQPACKET
Provides sequenced, reliable, bidirectional, connection-mode transmission paths for records. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers part of more than one record. Record boundaries are visible to the receiver via the MSG_EOR flag.
it's supposed to be reliable - how is it legal to drop packets?
Sorry, seems i need to rephrase description. "Packet" here means fragment of record(message) at transport
layer. As this is SEQPACKET mode, receiver could get only whole message or error, so if only several fragments
of message was copied (if signal received for example) we can't return it to user - it breaks SEQPACKET sense. I think,
in this case we can drop rest of record's fragments legally.
Thank You
Would not that violate the reliable property? IIUC it's only ok to
return an error if socket gets closed. Just like e.g. TCP ...
Sorry for late answer, yes You're right, seems this is unwanted drop...
Lets wait for Stefano Garzarella feedback
It was the same concern I had with the series that introduced SEQPACKET
for vsock, which is why I suggested to wait until the message is
complete, before copying it to the user's buffer.
IIUC, with the current upstream implementation, we don't have this
problem, right?
I'm not sure how to fix this, other than by keeping all the fragments
queued until we've successfully copied them to user space, which is what
we should do without this series applied IIUC.
Stefano
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