From: Mike Kravetz <mike.kravetz@oracle.com>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>,
linux-mm@kvack.org, linux-api@vger.kernel.org,
linux-kernel@vger.kernel.org
Cc: 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 0/1] mm/mremap: add MREMAP_MIRROR flag
Date: Fri, 7 Jul 2017 10:04:04 -0700 [thread overview]
Message-ID: <e2650043-744b-3c24-01bc-18d3674ea491@oracle.com> (raw)
In-Reply-To: <6f1460ef-a896-aef4-c0dc-66227232e025@linux.vnet.ibm.com>
On 07/07/2017 01:19 AM, Anshuman Khandual wrote:
> On 07/06/2017 09:47 PM, 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 [1]. A comment
>> was added to the code to help preserve this feature.
>
>
> Is this the comment ? If yes, then its not very clear.
>
> /*
> * We allow a zero old-len as a special case
> * for DOS-emu "duplicate shm area" thing. But
> * a zero new-len is nonsensical.
> */
>
Yes, I believe that is the comment.
>>
>> The Oracle JVM team has discovered this feature and used it while
>> prototyping a new garbage collection model. This new model shows promise,
>> and they are considering its use in a future release. However, since
>> the only mention of this functionality is a single comment in the kernel,
>> they are concerned about its future.
>>
>> I propose the addition of a new MREMAP_MIRROR flag to explicitly request
>> this functionality. The flag simply provides the same functionality as
>> the existing undocumented 'old_size == 0' interface. As an alternative,
>> we could simply document the 'old_size == 0' interface in the man page.
>> In either case, man page modifications would be needed.
>
> Right. Adding MREMAP_MIRROR sounds cleaner from application programming
> point of view. But it extends the interface.
Yes. That is the reason for the RFC. We currently have functionality
that is not clearly part of a programming interface. Application programmers
do not like to depend on something that is not part of an interface.
>>
>> Future Direction
>>
>> After more formally adding this to the API (either new flag or documenting
>> existing interface), the mremap code could be enhanced to optimize this
>> case. Currently, 'mirroring' only sets up the new mapping. It does not
>> create page table entries for new mapping. This could be added as an
>> enhancement.
>
> Then how it achieves mirroring, both the pointers should see the same
> data, that can happen with page table entries pointing to same pages,
> right ?
Correct.
In the code today, page tables for the new (mirrored) mapping are created
as needed via faults. The enhancement would be to create page table entries
for the new mapping.
--
Mike Kravetz
--
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 17:04 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
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 [this message]
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=e2650043-744b-3c24-01bc-18d3674ea491@oracle.com \
--to=mike.kravetz@oracle.com \
--cc=aarcange@redhat.com \
--cc=aaron.lu@intel.com \
--cc=akpm@linux-foundation.org \
--cc=khandual@linux.vnet.ibm.com \
--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 \
/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).