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Date:   Tue, 27 Feb 2018 22:11:04 -0800
From:   Andrei Vagin <avagin@virtuozzo.com>
To:     "Dmitry V. Levin" <ldv@altlinux.org>
Cc:     Pavel Emelyanov <xemul@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>, linux-mm@kvack.org,
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        linux-api@vger.kernel.org, criu@openvz.org, gdb@sourceware.org,
        devel@lists.open-mpi.org, rr-dev@mozilla.org,
        Arnd Bergmann <arnd@arndb.de>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Jann Horn <jannh@google.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrei Vagin <avagin@openvz.org>
Subject: Re: [PATCH v5 0/4] vm: add a syscall to map a process memory into a
 pipe
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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:
> > On 02/21/2018 03:44 AM, Andrew Morton wrote:
> > > On Tue,  9 Jan 2018 08:30:49 +0200 Mike Rapoport <rppt@linux.vnet.ibm.com> wrote:
> > > 
> > >> 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