On Tue, Feb 27, 2018 at 05:18:18AM +0300, Dmitry V. Levin wrote:

On Mon, Feb 26, 2018 at 12:02:25PM +0300, Pavel Emelyanov wrote:

quoted

On 02/21/2018 03:44 AM, Andrew Morton wrote:

quoted

On Tue,  9 Jan 2018 08:30:49 +0200 Mike Rapoport [off-list ref] wrote:

quoted

This patches introduces new process_vmsplice system call that combines
functionality of process_vm_read and vmsplice.

All seems fairly strightforward.  The big question is: do we know that
people will actually use this, and get sufficient value from it to
justify its addition?

Yes, that's what bothers us a lot too :) I've tried to start with finding out if anyone 
used the sys_read/write_process_vm() calls, but failed :( Does anybody know how popular
these syscalls are?

Well, process_vm_readv itself is quite popular, it's used by debuggers nowadays,
see e.g.
$ strace -qq -esignal=none -eprocess_vm_readv strace -qq -o/dev/null cat /dev/null

For this case, there is no advantage from process_vmsplice().

But it can significantly optimize a process of generating a core file.
In this case, we need to read a process memory and save content into a
file. process_vmsplice() allows to do this more optimal than
process_vm_readv(), because it doesn't copy data into a userspace.

Here is a part of strace how gdb saves memory content into a core file:

10593 open("/proc/10193/mem", O_RDONLY|O_CLOEXEC) = 17
10593 pread64(17, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1048576, 140009356111872) = 1048576
10593 close(17)                         = 0
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 4096) = 4096
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1044480) = 1044480
10593 open("/proc/10193/mem", O_RDONLY|O_CLOEXEC) = 17
10593 pread64(17, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1048576, 140009357160448) = 1048576
10593 close(17)                         = 0
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 4096) = 4096
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1044480) = 1044480
10593 open("/proc/10193/mem", O_RDONLY|O_CLOEXEC) = 17
10593 pread64(17, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1048576, 140009358209024) = 1048576
10593 close(17)                         = 0
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 4096) = 4096
10593 write(16, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1044480) = 1044480
10593 open("/proc/10193/mem", O_RDONLY|O_CLOEXEC) = 17
10593 pread64(17, "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"..., 1048576, 140009359257600) = 1048576
10593 close(17)

It is strange that process_vm_readv() isn't used and that
/proc/10193/mem is opened many times.

BTW: "strace -fo strace-gdb.log gdb -p PID" doesn't work properly.

Thanks,
Andrei

-- 
ldv

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