On Sat, Jun 9, 2012 at 8:26 PM, Pranay Kumar Srivastava
pranay.shrivast...@hcl.com wrote:
Hi,
I've been trying to understand kernel_sendpage but I've not been able to
figure it out completely and hopefully someone else knows better so please
help me out on this.
I'm using kernel_sendpage for a TCP connection and it works well when
there are lesser number of kernel threads trying to send data using it.
Now the page I hand over to kernel_sendpage is reused again for reading
data from the socket and then processing it and then again resending the
processed data in the same page again. It's at maximum 2KB data and never
lesser than 120 bytes.
As I see it in the code, the page isn't copied in the skb frags array it's
just assigned and get_page is called to increment the page reference count.
(I don't free it anyway until the thread is stopped and it never is unless
it gets a signal).
Now I don't know wether kernel_sendpage will wait for the page to be sent
or it won't. I've tried with MSG_DONTWAIT and passing 0 for flags but after
every now and then the problem occurs at client which I'm describing below
with the best explanation I could think of...
When too many kernel threads are trying to send data using
kernel_sendpage, with NO MSG_DONTWAIT flag, then also it seems that this
call succeeds? However since I'm reusing the page the data can get
overridden by the next sock_recvmsg and when the network stack is ready to
send my page it gets garbage data at client?
The same issue I observed with MSG_DONTWAIT set even in that case the
client sometimes get garbage data.
So my query is,
To use kernel_sendpage what I need to do in order to be sure that network
stack indeed has sent my page and that I can reuse it for sock_recvmsg
again.
Thanks a lot for reading!
Regards,
Pranay Kr. Srivastava
pranay.shrivast...@hcl.com
Software Engineer
ERS,HCL Technologies
A-5, Sector 24, Noida 201301, U.P. (India)
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Hi,
I had a brief look at the kernel_sendpage() function. It can call either
sock-ops-sendpage() if this is set or calls
sock_no_sendpage() otherwise. If sock_no_sendpage() code path is executed
then,
sock_no_sendpage() (kunmaps the page)- kernel_sendmsg() - sock_sendmsg()
(waits for data to be synced). So, I think you are OK if this is the code
path.
You need to check this in case of sock-ops-sendpage().
- Rohan
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