From: Mike Kravetz <mike.kravetz@oracle.com>
To: Michal Hocko <mhocko@kernel.org>
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>,
Aaron Lu <aaron.lu@intel.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality
Date: Thu, 13 Jul 2017 09:01:54 -0700 [thread overview]
Message-ID: <21b264e7-b879-f072-03d2-f6f4aec5c957@oracle.com> (raw)
In-Reply-To: <20170713061651.GA14492@dhcp22.suse.cz>
On 07/12/2017 11:16 PM, Michal Hocko wrote:
> On Wed 12-07-17 09:55:48, Mike Kravetz wrote:
>> On 07/12/2017 04:46 AM, Michal Hocko wrote:
>>> On Tue 11-07-17 11:23:19, Mike Kravetz wrote:
>>>> On 07/11/2017 05:36 AM, Michal Hocko wrote:
>>> [...]
>>>>> Anyway the patch should fail with -EINVAL on private mappings as Kirill
>>>>> already pointed out
>>>>
>>>> Yes. I think this should be a separate patch. As mentioned earlier,
>>>> mremap today creates a new/additional private mapping if called in this
>>>> way with old_size == 0. To me, this is a bug.
>>>
>>> Not only that. It clears existing ptes in the old mapping so the content
>>> is lost. That is quite unexpected behavior. Now it is hard to assume
>>> whether somebody relies on the behavior (I can easily imagine somebody
>>> doing backup&clear in atomic way) so failing with EINVAL might break
>>> userspace so I am not longer sure. Anyway this really needs to be
>>> documented.
>>
>> I am pretty sure it does not clear ptes in the old mapping, or modify it
>> in any way. Are you thinking they are cleared as part of the call to
>> move_page_tables? Since old_size == 0 (len as passed to move_page_tables),
>> the for loop in move_page_tables is not run and it doesn't do much of
>> anything in this case.
>
> Dang. I have completely missed that we give old_len as the len
> parameter. Then it is clear that this old_len == 0 trick never really
> worked for MAP_PRIVATE because it simply fails the main invariant that
> the content at the new location matches the old one. Care to send a
> patch to clarify that and sent EINVAL or should I do it?
Sent a patch (in separate e-mail thread) to return EINVAL for private
mappings.
>> If adding hugetlbfs support to memfd_create works out, I would like to
>> see mremap(old_size == 0) support dropped. Nobody here (kernel mm
>> development) seems to like it. However, as you note there may be somebody
>> depending on this behavior. What would be the process for removing
>> such support? AFAIK, it is not documented anywhere. If we do document
>> the behavior, then we will certainly be stuck with it for a long time.
>
> I would rather document it than remove it. From the past we know that
> there are users and my experience tells me that once something is used
> it lives its life for ever basically. And moreover it is not like this
> costs us any maintenance burden to support the hack. Just make it more
> obvious so that we do not have to rediscover it each time.
I will put together a patch to add a description of (old_size == 0)
behavior to the man page.
--
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:[~2017-07-13 16:02 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 [this message]
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=21b264e7-b879-f072-03d2-f6f4aec5c957@oracle.com \
--to=mike.kravetz@oracle.com \
--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@kernel.org \
--cc=vbabka@suse.cz \
/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).