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Date: Fri, 12 May 2017 15:00:18 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
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Subject: Re: [patch v4 resend 2/2] kcmp: Add KCMP_EPOLL_TFD mode to compare
 epoll target files
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On Mon, 24 Apr 2017 18:39:28 +0300 Cyrill Gorcunov <gorcunov@gmail.com> wrote:

> With current epoll architecture target files are addressed
> with file_struct and file descriptor number, where the last
> is not unique. Moreover files can be transferred from another
> process via unix socket, added into queue and closed then
> so we won't find this descriptor in the task fdinfo list.
> 
> Thus to checkpoint and restore such processes CRIU needs to
> find out where exactly the target file is present to add it into
> epoll queue. For this sake one can use kcmp call where
> some particular target file from the queue is compared with
> arbitrary file passed as an argument.
> 
> Because epoll target files can have same file descriptor
> number but different file_struct a caller should explicitly
> specify the offset within.
> 
> To test if some particular file is matching entry inside
> epoll one have to
> 
>  - fill kcmp_epoll_slot structure with epoll file descriptor,
>    target file number and target file offset (in case if only
>    one target is present then it should be 0)
> 
>  - call kcmp as kcmp(pid1, pid2, KCMP_EPOLL_TFD, fd, &kcmp_epoll_slot)
>     - the kernel fetch file pointer matching file descriptor @fd of pid1
>     - lookups for file struct in epoll queue of pid2 and returns traditional
>       0,1,2 result for sorting purpose

That's quite a bit more code.  Is there a neat way of making it depend
on a new CONFIG_foo, then select CONFIG_foo if
CONFIG_CHECKPOINT_RESTORE?