From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: linux-mm@kvack.org, linux-api@vger.kernel.org,
linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Michal Hocko <mhocko@suse.com>, Aaron Lu <aaron.lu@intel.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality
Date: Fri, 7 Jul 2017 20:45:34 +0300 [thread overview]
Message-ID: <20170707174534.wdfbciyfpovi52dy@node.shutemov.name> (raw)
In-Reply-To: <e328ff6a-2c4b-ec26-cc28-e24b7b35a463@oracle.com>
On Fri, Jul 07, 2017 at 10:29:52AM -0700, Mike Kravetz wrote:
> On 07/07/2017 03:23 AM, Kirill A. Shutemov wrote:
> > On Thu, Jul 06, 2017 at 09:17:26AM -0700, Mike Kravetz wrote:
> >> The mremap system call has the ability to 'mirror' parts of an existing
> >> mapping. To do so, it creates a new mapping that maps the same pages as
> >> the original mapping, just at a different virtual address. This
> >> functionality has existed since at least the 2.6 kernel.
> >>
> >> This patch simply adds a new flag to mremap which will make this
> >> functionality part of the API. It maintains backward compatibility with
> >> the existing way of requesting mirroring (old_size == 0).
> >>
> >> If this new MREMAP_MIRROR flag is specified, then new_size must equal
> >> old_size. In addition, the MREMAP_MAYMOVE flag must be specified.
> >
> > The patch breaks important invariant that anon page can be mapped into a
> > process only once.
>
> Actually, the patch does not add any new functionality. It only provides
> a new interface to existing functionality.
>
> Is it not possible to have an anon page mapped twice into the same process
> via system V shared memory? shmget(anon), shmat(), shmat.
> Of course, those are shared rather than private anon pages.
By anon pages I mean, private anon or file pages. These are subject to CoW.
> > What is going to happen to mirrored after CoW for instance?
> >
> > In my opinion, it shouldn't be allowed for anon/private mappings at least.
> > And with this limitation, I don't see much sense in the new interface --
> > just create mirror by mmap()ing the file again.
>
> The code today works for anon shared mappings. See simple program below.
>
> You are correct in that it makes little or no sense for private mappings.
> When looking closer at existing code, mremap() creates a new private
> mapping in this case. This is most likely a bug.
IIRC, existing code doesn't create mirrors of private pages as it requires
old_len to be zero. There's no way to get private pages mapped twice this
way.
--
Kirill A. Shutemov
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-07-07 17:45 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-06 16:17 [RFC PATCH 0/1] mm/mremap: add MREMAP_MIRROR flag Mike Kravetz
2017-07-06 16:17 ` [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality Mike Kravetz
2017-07-07 8:45 ` Anshuman Khandual
2017-07-07 17:14 ` Mike Kravetz
2017-07-09 7:23 ` Anshuman Khandual
2017-07-07 10:23 ` Kirill A. Shutemov
2017-07-07 17:29 ` Mike Kravetz
2017-07-07 17:45 ` Kirill A. Shutemov [this message]
2017-07-07 18:09 ` Mike Kravetz
2017-07-09 7:32 ` Anshuman Khandual
2017-07-10 16:22 ` Vlastimil Babka
2017-07-10 17:22 ` Mike Kravetz
2017-07-11 12:36 ` Michal Hocko
2017-07-11 18:23 ` Mike Kravetz
2017-07-11 21:02 ` Andrea Arcangeli
2017-07-11 21:57 ` Mike Kravetz
2017-07-11 23:31 ` Andrea Arcangeli
2017-07-12 11:46 ` Michal Hocko
2017-07-12 16:55 ` Mike Kravetz
2017-07-13 6:16 ` Michal Hocko
2017-07-13 16:01 ` Mike Kravetz
2017-07-13 16:30 ` Andrea Arcangeli
2017-07-13 18:11 ` Mike Kravetz
2017-07-13 20:33 ` Andrea Arcangeli
2017-07-07 8:19 ` [RFC PATCH 0/1] mm/mremap: add MREMAP_MIRROR flag Anshuman Khandual
2017-07-07 17:04 ` Mike Kravetz
2017-07-07 11:03 ` Anshuman Khandual
2017-07-07 17:12 ` Mike Kravetz
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=20170707174534.wdfbciyfpovi52dy@node.shutemov.name \
--to=kirill@shutemov.name \
--cc=aarcange@redhat.com \
--cc=aaron.lu@intel.com \
--cc=akpm@linux-foundation.org \
--cc=kirill.shutemov@linux.intel.com \
--cc=linux-api@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@suse.com \
--cc=mike.kravetz@oracle.com \
/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).