From: Kirill Tkhai <ktkhai@virtuozzo.com>
To: Adam Borowski <kilobyte@angband.pl>
Cc: akpm@linux-foundation.org, dan.j.williams@intel.com,
mhocko@suse.com, keith.busch@intel.com,
kirill.shutemov@linux.intel.com, pasha.tatashin@oracle.com,
alexander.h.duyck@linux.intel.com, ira.weiny@intel.com,
andreyknvl@google.com, arunks@codeaurora.org, vbabka@suse.cz,
cl@linux.com, riel@surriel.com, keescook@chromium.org,
hannes@cmpxchg.org, npiggin@gmail.com,
mathieu.desnoyers@efficios.com, shakeelb@google.com, guro@fb.com,
aarcange@redhat.com, hughd@google.com, jglisse@redhat.com,
mgorman@techsingularity.net, daniel.m.jordan@oracle.com,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping
Date: Thu, 16 May 2019 17:25:06 +0300 [thread overview]
Message-ID: <14efd2c5-ffd1-84ad-b1d1-42f8ef44d7e2@virtuozzo.com> (raw)
In-Reply-To: <20190516134220.GB24860@angband.pl>
On 16.05.2019 16:42, Adam Borowski wrote:
> On Thu, May 16, 2019 at 04:10:07PM +0300, Kirill Tkhai wrote:
>> On 15.05.2019 22:38, Adam Borowski wrote:
>>> On Wed, May 15, 2019 at 06:11:15PM +0300, Kirill Tkhai wrote:
>>>> This patchset adds a new syscall, which makes possible
>>>> to clone a mapping from a process to another process.
>>>> The syscall supplements the functionality provided
>>>> by process_vm_writev() and process_vm_readv() syscalls,
>>>> and it may be useful in many situation.
>>>>
>>>> For example, it allows to make a zero copy of data,
>>>> when process_vm_writev() was previously used:
>>>
>>> I wonder, why not optimize the existing interfaces to do zero copy if
>>> properly aligned? No need for a new syscall, and old code would immediately
>>> benefit.
>>
>> Because, this is just not possible. You can't zero copy anonymous pages
>> of a process to pages of a remote process, when they are different pages.
>
> fork() manages that, and so does KSM. Like KSM, you want to make a page
> shared -- you just skip the comparison step as you want to overwrite the old
> contents.
>
> And there's no need to touch the page, as fork() manages that fine no matter
> if the page is resident, anonymous in swap, or file-backed, all without
> reading from swap.
Yes, and in case of you dive into the patchset, you will found the new syscall
manages page table entries in the same way fork() makes.
>>>> There are several problems with process_vm_writev() in this example:
>>>>
>>>> 1)it causes pagefault on remote process memory, and it forces
>>>> allocation of a new page (if was not preallocated);
>>>>
>>>> 2)amount of memory for this example is doubled in a moment --
>>>> n pages in current and n pages in remote tasks are occupied
>>>> at the same time;
>>>>
>>>> 3)received data has no a chance to be properly swapped for
>>>> a long time.
>>>
>>> That'll handle all of your above problems, except for making pages
>>> subject to CoW if written to. But if making pages writeably shared is
>>> desired, the old functions have a "flags" argument that doesn't yet have a
>>> single bit defined.
>
>
> Meow!
>
next prev parent reply other threads:[~2019-05-16 14:25 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-15 15:11 [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping Kirill Tkhai
2019-05-15 15:11 ` [PATCH RFC 1/5] mm: Add process_vm_mmap() syscall declaration Kirill Tkhai
2019-05-15 15:11 ` [PATCH RFC 2/5] mm: Extend copy_vma() Kirill Tkhai
2019-05-15 15:11 ` [PATCH RFC 3/5] mm: Extend copy_page_range() Kirill Tkhai
2019-05-15 15:11 ` [PATCH RFC 4/5] mm: Export round_hint_to_min() Kirill Tkhai
2019-05-15 15:11 ` [PATCH RFC 5/5] mm: Add process_vm_mmap() Kirill Tkhai
2019-05-15 18:29 ` Kees Cook
2019-05-16 12:54 ` Kirill Tkhai
2019-05-15 18:46 ` [PATCH RFC 0/5] mm: process_vm_mmap() -- syscall for duplication a process mapping Jann Horn
2019-05-16 13:02 ` Kirill Tkhai
2019-05-16 13:14 ` Jann Horn
2019-05-15 19:38 ` Adam Borowski
2019-05-16 13:10 ` Kirill Tkhai
2019-05-16 13:42 ` Adam Borowski
2019-05-16 14:25 ` Kirill Tkhai [this message]
2019-05-16 13:30 ` Michal Hocko
2019-05-16 13:52 ` Michal Hocko
2019-05-16 14:22 ` Kirill Tkhai
2019-05-16 13:32 ` Jann Horn
2019-05-16 13:56 ` Kirill Tkhai
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