linux-kernel - Re: [RFC][PATCH] release mmap_sem before starting migration (Was Re: Need to take mmap_sem lock in move

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <74935449f7bd1248f959a526d56ca02a.squirrel@webmail-b.css.fujitsu.com>
Date:	Thu, 5 Feb 2009 22:23:45 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Swamy Gowda" <swamy@...ident.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Christoph Lameter" <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, brice.goglin@...ia.fr,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH] release mmap_sem before starting migration (Was
 Re: Need to take mmap_sem lock in move_pages.

Swamy Gowda wrote:
> KAMEZAWA Hiroyuki wrote:
>> On Wed, 4 Feb 2009 10:39:19 -0500 (EST)
>> Christoph Lameter <cl@...ux-foundation.org> wrote:
>>
>>> On Wed, 4 Feb 2009, KAMEZAWA Hiroyuki wrote:
>>>
>>> > mmap_sem can be released after page table walk ends.
>>>
>>> No. read lock on mmap_sem must be held since the migrate functions
>>> manipulate page table entries. Concurrent large scale changes to the
>>> page
>>> tables (splitting vmas, remapping etc) must not be possible.
>>>
>> Just for clarification:
>>
>> 1. changes in page table is not problem from the viewpoint of kernel.
>>   (means no panic, no leak,...)
>> 2. But this loses "atomic" aspect of migration and will allow unexpected
>>   behaviors.
>>   (means the page-mapping status after sys_move may not be what user
>> expects.)
>>
>>
>> Thanks,
>> -Kame
>>
>>
> But I can't understand how user can see different page->mapping , since
> new page->mapping still holds the anon_vma pointer which should still
> contain the changes in the vma list( due to split vma etc). But,
> considering it as a problem how is it avoided in case of hotremove?
>
I'm sorry page-mapping in my text is not page->mapping. Just means
process's memory map.

In my point of view, no problems (I wrote no problem in the kernel.)

One big difference between sys_move_pages and hot remove is
hot-remove retries many times but sys_move_pages() doesn't.
So, race/contention in migrate_page() will dramatically decrease
success-rate of page migration by system call.

In user side, sys_move_pages(), we may have to think more.
I wonder that there may be much more contentions of pte_lock and
page_lock() etc... if we remove mmap_sem.
The good point of mmap_sem is the waiter can sleep without any troubles
and nest of locks.

Thanks,
-Kame

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/