[Devel] Re: [v13][PATCH 00/12] Implement eclone() system call

[Oren Laadan]

Queued for v19-rc2 to replace old clone-with-pids.

Oren.

Sukadev Bhattiprolu wrote:
> Andrew,
> 
> We ported the syscall to x86_64, powerpc and s390 and in the process hashed
> out couple of minor issues in the interface.
> 
> Can you please merge or let me know if there are other comments ?
> 
> ---
> 
> Subject: [v13][PATCH 00/12] Implement eclone() system call
> 
> To support application checkpoint/restart, a task must have the same pid it
> had when it was checkpointed.  When containers are nested, the tasks within
> the containers exist in multiple pid namespaces and hence have multiple pids
> to specify during restart.
> 
> This patchset implements a new system call, eclone() that lets a process
> specify the pids of the child process.
> 
> Patches 1 through 7 are helper patches needed for choosing a pid for the
> child process.
> 
> Patches 8 through 11 implement the eclone() system call on x86, x86_64, s390
> and powerpc.
> 
> Patch 12 documents the new system call, some/all of which will eventually
> go into a man page.
> 
> Changelog[v13]:
> 	- Implement sys_eclone() on x86_64, s390 and powerpc architectures
> 	- Reorg x86 implementation to enable sharing code with x86_64
> 	- [Arnd Bergmann] Remove the ->reserved1 field we now have args_size
> 	- [Nathan Lynch, Serge Hallyn]: Rename ->child_stack_base to
> 	  ->child_stack and ensure ->child_stack_size is 0 on architectures
> 	  that don't need the stack size.
> 	- Modify exmaple in Documentation to avoid unnecessary register copy.
> 
> Changelog[v12]:
> 	- Ignore ->child_stack_size when ->child_stack_base is NULL (PATCH 8)
> 	- Cleanup/simplify example in Documentation/eclone (PATCH 9).
> 	- Rename sys call to a shorter name, eclone()
> 
> Changelog[v11]:
> 	- [Dave Hansen] Move clone_args validation checks to arch-indpeendent
> 	  code.
> 	- [Oren Laadan] Make args_size a parameter to system call and remove
> 	  it from 'struct clone_args'
> 
> Changelog[v10]:
> 	- [Linus Torvalds] Use PTREGSCALL() implementation for clone rather
> 	  than the generic system call
> 	- Rename clone3() to clone_with_pids()
> 	- Update Documentation/clone_with_pids() to show example usage with
> 	  the PTREGSCALL implementation.
> 
> Changelog[v9]:
> 	- [Pavel Emelyanov] Drop the patch that made 'pid_max' a property
> 	  of struct pid_namespace
> 	- [Roland McGrath, H. Peter Anvin and earlier on, Serge Hallyn] To
> 	  avoid inadvertent truncation clone_flags, preserve the first
> 	  parameter of clone3() as 'u32 clone_flags' and specify newer
> 	  flags in clone_args.flags_high (PATCH 8/9 and PATCH 9/9)
> 	- [Eric Biederman] Generalize alloc_pidmap() code to simplify and
> 	  remove duplication (see PATCH 3/9].
> 	  
> Changelog[v8]:
> 	- [Oren Laadan, Louis Rilling, KOSAKI Motohiro]
> 	  The name 'clone2()' is in use - renamed new syscall to clone3().
> 	- [Oren Laadan] ->parent_tidptr and ->child_tidptr need to be 64bit.
> 	- [Oren Laadan] Ensure that unused fields/flags in clone_struct are 0.
> 	  (Added [PATCH 7/10] to the patchset).
> 
> Changelog[v7]:
> 	- [Peter Zijlstra, Arnd Bergmann]
> 	  Group the arguments to clone2() into a 'struct clone_arg' to
> 	  workaround the issue of exceeding 6 arguments to the system call.
> 	  Also define clone-flags as u64 to allow additional clone-flags.
> 
> Changelog[v6]:
> 	- [Nathan Lynch, Arnd Bergmann, H. Peter Anvin, Linus Torvalds]
> 	  Change 'pid_set.pids' to 'pid_t pids[]' so sizeof(struct pid_set) is
> 	  constant across architectures (Patches 7, 8).
> 	- (Nathan Lynch) Change pid_set.num_pids to unsigned and remove
> 	  'unum_pids < 0' check (Patches 7,8)
> 	- (Pavel Machek) New patch (Patch 9) to add some documentation.
> 
> Changelog[v5]:
> 	- Make 'pid_max' a property of pid_ns (Integrated Serge Hallyn's patch
> 	  into this set)
> 	- (Eric Biederman): Avoid the new function, set_pidmap() - added
> 	  couple of checks on 'target_pid' in alloc_pidmap() itself.
> 
> === IMPORTANT NOTE:
> 
> clone() system call has another limitation - all but one bits in clone-flags
> are in use and if more new clone-flags are needed, we will need a variant of
> the clone() system call. 
> 
> It appears to make sense to try and extend this new system call to address
> this limitation as well. The requirements of a new clone system call could
> then be summarized as:
> 
> 	- do everything clone() does today, and
> 	- give application an ability to choose pids for the child process
> 	  in all ancestor pid namespaces, and
> 	- allow more clone_flags
> 
> Contstraints:
> 
> 	- system-calls are restricted to 6 parameters and clone() already
> 	  takes 5 parameters, any extension to clone() interface would require
> 	  one or more copy_from_user().  (Not sure if copy_from_user() of ~40
> 	  bytes would have a significant impact on performance of clone()).
> 
> Based on these requirements and constraints, we explored a couple of system
> call interfaces (in earlier versions of this patchset). Based on input from
> Arnd Bergmann and others, the new interface of the system call is: 
> 
> 	struct clone_args {
> 		u64 clone_flags_high;
> 		u64 child_stack_base;
> 		u64 child_stack_size;
> 		u64 parent_tid_ptr;
> 		u64 child_tid_ptr;
> 		u32 nr_pids;
> 		u32 reserved0;
> 	};
> 
> 	sys_eclone(u32 flags_low, struct clone_args *cargs, int args_size,
> 			pid_t *pids)
> 
> Details of the struct clone_args and the usage are explained in the
> documentation (PATCH 12/12).
> 
> NOTE:
> 	While this patchset enables support for more clone-flags, actual
> 	implementation for additional clone-flags is best implemented as
> 	a separate patchset (PATCH 8/9 identifies some TODOs)
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> 
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