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SubjectRe: [PATCH] zero^H^H^H^Hsingle copy pipe
Date
From: "Ben LaHaise" <bcrl@redhat.com>
>
> Any particular reason for not using davem's single copy kiobuf based
> code?
>
The main problem is that map_user_kiobuf() locks pages into memory.
It's a bad idea for pipes. Either we must severely limit the maximum
amount of data in the direct-copy buffers, or we must add a swap file
based backing store. If I understand the BSD direct-pipe code correctly
it has a swap file based backing store. I think that's insane. And
limiting the direct copy buffers to a few kB defeats the purpose of
direct copy.
Instead pipe_write just queues (tsk,addr,len), and then pipe_read() uses
a modified access_process_vm() function to copy directly. Currently I've
copied the code from kernel/ptrace.c, but I plan to merge
copy_user_to_user() back into access_process_vm().

And the current pipe_{read,write} are a total mess with nested loops and
gotos. It's possible to create wakeup storms. I rewrote them as well ;-)

But the new code still contains one deadlock :-(
--
Manfred


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