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

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Date:   Wed, 25 Apr 2018 06:01:02 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Christoph Hellwig <hch@...radead.org>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [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