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SubjectRe: [PATCH net-next 1/2] tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive

On 04/24/2018 11:28 PM, Christoph Hellwig wrote:
> On Tue, Apr 24, 2018 at 10:27:21PM -0700, Eric Dumazet wrote:
>> When adding tcp mmap() implementation, I forgot that socket lock
>> had to be taken before current->mm->mmap_sem. syzbot eventually caught
>> the bug.
>> Since we can not lock the socket in tcp mmap() handler we have to
>> split the operation in two phases.
>> 1) mmap() on a tcp socket simply reserves VMA space, and nothing else.
>> This operation does not involve any TCP locking.
>> 2) setsockopt(fd, IPPROTO_TCP, TCP_ZEROCOPY_RECEIVE, ...) implements
>> the transfert of pages from skbs to one VMA.
>> This operation only uses down_read(&current->mm->mmap_sem) after
>> holding TCP lock, thus solving the lockdep issue.
>> This new implementation was suggested by Andy Lutomirski with great details.
> Thanks, this looks much more sensible to me.

Thanks Christoph

Note the high cost of zap_page_range(), needed to avoid -EBUSY being returned
from vm_insert_page() the second time TCP_ZEROCOPY_RECEIVE is used on one VMA.

Ideally a vm_replace_page() would avoid this cost ?

6.51% tcp_mmap [kernel.kallsyms] [k] unmap_page_range
5.90% tcp_mmap [kernel.kallsyms] [k] vm_insert_page
4.85% tcp_mmap [kernel.kallsyms] [k] _raw_spin_lock
4.50% tcp_mmap [kernel.kallsyms] [k] mark_page_accessed
3.51% tcp_mmap [kernel.kallsyms] [k] page_remove_rmap
2.99% tcp_mmap [kernel.kallsyms] [k] page_add_file_rmap
2.53% tcp_mmap [kernel.kallsyms] [k] release_pages
2.38% tcp_mmap [kernel.kallsyms] [k] put_page
2.37% tcp_mmap [kernel.kallsyms] [k] smp_call_function_single
2.28% tcp_mmap [kernel.kallsyms] [k] __get_locked_pte
2.25% tcp_mmap [kernel.kallsyms] [k] do_tcp_setsockopt.isra.35
2.21% tcp_mmap [kernel.kallsyms] [k] page_clear_age

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