From: David Hildenbrand <david@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Khalid Aziz <khalid.aziz@oracle.com>,
"Longpeng (Mike,
Cloud Infrastructure Service Product Dept.)"
<longpeng2@huawei.com>,
Steven Sistare <steven.sistare@oracle.com>,
Anthony Yznaga <anthony.yznaga@oracle.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"Gonglei (Arei)" <arei.gonglei@huawei.com>
Subject: Re: [RFC PATCH 0/5] madvise MADV_DOEXEC
Date: Mon, 16 Aug 2021 18:13:20 +0200 [thread overview]
Message-ID: <97ed86a0-9fac-3dbc-0f9e-d669484c9485@redhat.com> (raw)
In-Reply-To: <YRqLc2W1P77tiSqj@casper.infradead.org>
On 16.08.21 17:59, Matthew Wilcox wrote:
> On Mon, Aug 16, 2021 at 05:01:44PM +0200, David Hildenbrand wrote:
>> On 16.08.21 16:40, Matthew Wilcox wrote:
>>> On Mon, Aug 16, 2021 at 04:33:09PM +0200, David Hildenbrand wrote:
>>>>>> I did not follow why we have to play games with MAP_PRIVATE, and having
>>>>>> private anonymous pages shared between processes that don't COW, introducing
>>>>>> new syscalls etc.
>>>>>
>>>>> It's not about SHMEM, it's about file-backed pages on regular
>>>>> filesystems. I don't want to have XFS, ext4 and btrfs all with their
>>>>> own implementations of ARCH_WANT_HUGE_PMD_SHARE.
>>>>
>>>> Let me ask this way: why do we have to play such games with MAP_PRIVATE?
>>>
>>> : Mappings within this address range behave as if they were shared
>>> : between threads, so a write to a MAP_PRIVATE mapping will create a
>>> : page which is shared between all the sharers.
>>>
>>> If so, that's a misunderstanding, because there are no games being played.
>>> What Khalid's saying there is that because the page tables are already
>>> shared for that range of address space, the COW of a MAP_PRIVATE will
>>> create a new page, but that page will be shared between all the sharers.
>>> The second write to a MAP_PRIVATE page (by any of the sharers) will not
>>> create a COW situation. Just like if all the sharers were threads of
>>> the same process.
>>>
>>
>> It actually seems to be just like I understood it. We'll have multiple
>> processes share anonymous pages writable, even though they are not using
>> shared memory.
>>
>> IMHO, sharing page tables to optimize for something kernel-internal (page
>> table consumption) should be completely transparent to user space. Just like
>> ARCH_WANT_HUGE_PMD_SHARE currently is unless I am missing something
>> important.
>>
>> The VM_MAYSHARE check in want_pmd_share()->vma_shareable() makes me assume
>> that we really only optimize for MAP_SHARED right now, never for
>> MAP_PRIVATE.
>
> It's definitely *not* about being transparent to userspace. It's about
> giving userspace new functionality where multiple processes can choose
> to share a portion of their address space with each other. What any
> process changes in that range changes, every sharing process sees.
> mmap(), munmap(), mprotect(), mremap(), everything.
Oh okay, so it's actually much more complicated and complex than I
thought. Thanks for clarifying that! I recall virtiofsd had similar
requirements for sharing memory with the QEMU main process, I might be
wrong.
"existing shared memory area" and your initial page table example made
me assume that we are simply dealing with sharing page tables of MAP_SHARED.
It's actually something like a VMA container that you share between
processes. And whatever VMAs are currently inside that VMA container is
mirrored to other processes. I assume sharing page tables could actually
be an implementation detail, especially when keeping MAP_PRIVATE
(confusing in that context!) and other features that will give you
surprises (uffd) out of the picture.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-08-16 16:13 UTC|newest]
Thread overview: 74+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-27 17:11 [RFC PATCH 0/5] madvise MADV_DOEXEC Anthony Yznaga
2020-07-27 17:07 ` Eric W. Biederman
2020-07-27 18:00 ` Steven Sistare
2020-07-28 13:40 ` Christian Brauner
2020-07-27 17:11 ` [RFC PATCH 1/5] elf: reintroduce using MAP_FIXED_NOREPLACE for elf executable mappings Anthony Yznaga
2020-07-27 17:11 ` [RFC PATCH 2/5] mm: do not assume only the stack vma exists in setup_arg_pages() Anthony Yznaga
2020-07-27 17:11 ` [RFC PATCH 3/5] mm: introduce VM_EXEC_KEEP Anthony Yznaga
2020-07-28 13:38 ` Eric W. Biederman
2020-07-28 17:44 ` Anthony Yznaga
2020-07-29 13:52 ` Kirill A. Shutemov
2020-07-29 23:20 ` Anthony Yznaga
2020-07-27 17:11 ` [RFC PATCH 4/5] exec, elf: require opt-in for accepting preserved mem Anthony Yznaga
2020-07-27 17:11 ` [RFC PATCH 5/5] mm: introduce MADV_DOEXEC Anthony Yznaga
2020-07-28 13:22 ` Kirill Tkhai
2020-07-28 14:06 ` Steven Sistare
2020-07-28 11:34 ` [RFC PATCH 0/5] madvise MADV_DOEXEC Kirill Tkhai
2020-07-28 17:28 ` Anthony Yznaga
2020-07-28 14:23 ` Andy Lutomirski
2020-07-28 14:30 ` Steven Sistare
2020-07-30 15:22 ` Matthew Wilcox
2020-07-30 15:27 ` Christian Brauner
2020-07-30 15:34 ` Matthew Wilcox
2020-07-30 15:54 ` Christian Brauner
2020-07-31 9:12 ` Stefan Hajnoczi
2020-07-30 15:59 ` Steven Sistare
2020-07-30 17:12 ` Matthew Wilcox
2020-07-30 17:35 ` Steven Sistare
2020-07-30 17:49 ` Matthew Wilcox
2020-07-30 18:27 ` Steven Sistare
2020-07-30 21:58 ` Eric W. Biederman
2020-07-31 14:57 ` Steven Sistare
2020-07-31 15:27 ` Matthew Wilcox
2020-07-31 16:11 ` Steven Sistare
2020-07-31 16:56 ` Jason Gunthorpe
2020-07-31 17:15 ` Steven Sistare
2020-07-31 17:48 ` Jason Gunthorpe
2020-07-31 17:55 ` Steven Sistare
2020-07-31 17:23 ` Matthew Wilcox
2020-08-03 15:28 ` Eric W. Biederman
2020-08-03 15:42 ` James Bottomley
2020-08-03 20:03 ` Steven Sistare
[not found] ` <9371b8272fd84280ae40b409b260bab3@AcuMS.aculab.com>
2020-08-04 11:13 ` Matthew Wilcox
2020-08-03 19:29 ` Steven Sistare
2020-07-31 19:41 ` Steven Sistare
2021-07-08 9:52 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-07-08 12:48 ` Steven Sistare
2021-07-12 1:05 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-07-12 1:30 ` Matthew Wilcox
2021-07-13 0:57 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-08-13 19:49 ` Khalid Aziz
2021-08-14 20:07 ` David Laight
2021-08-16 0:26 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-08-16 8:07 ` David Laight
2021-08-16 6:54 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-08-16 8:02 ` David Hildenbrand
2021-08-16 12:07 ` Matthew Wilcox
2021-08-16 12:20 ` David Hildenbrand
2021-08-16 12:42 ` David Hildenbrand
2021-08-16 12:46 ` Matthew Wilcox
2021-08-16 13:24 ` David Hildenbrand
2021-08-16 13:32 ` Matthew Wilcox
2021-08-16 14:10 ` David Hildenbrand
2021-08-16 14:27 ` Matthew Wilcox
2021-08-16 14:33 ` David Hildenbrand
2021-08-16 14:40 ` Matthew Wilcox
2021-08-16 15:01 ` David Hildenbrand
2021-08-16 15:59 ` Matthew Wilcox
2021-08-16 16:06 ` Khalid Aziz
2021-08-16 16:15 ` Matthew Wilcox
2021-08-16 16:13 ` David Hildenbrand [this message]
2021-08-16 12:27 ` [private] " David Hildenbrand
2021-08-16 12:30 ` David Hildenbrand
2021-08-17 0:47 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-08-17 0:55 ` Matthew Wilcox
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=97ed86a0-9fac-3dbc-0f9e-d669484c9485@redhat.com \
--to=david@redhat.com \
--cc=anthony.yznaga@oracle.com \
--cc=arei.gonglei@huawei.com \
--cc=khalid.aziz@oracle.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=longpeng2@huawei.com \
--cc=steven.sistare@oracle.com \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).