linux-kernel - Re: [PATCH] memory-hotplug: Fix bad area access on dissolve_free_huge

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

Message-ID: <20160921162715.GC24210@dhcp22.suse.cz>
Date:   Wed, 21 Sep 2016 18:27:16 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Rui Teng <rui.teng@...ux.vnet.ibm.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Santhosh G <santhog4@...ibm.com>
Subject: Re: [PATCH] memory-hotplug: Fix bad area access on
 dissolve_free_huge_pages()

On Wed 21-09-16 09:04:31, Dave Hansen wrote:
> On 09/21/2016 05:05 AM, Michal Hocko wrote:
> > On Tue 20-09-16 10:43:13, Dave Hansen wrote:
> >> On 09/20/2016 08:52 AM, Rui Teng wrote:
> >>> On 9/20/16 10:53 PM, Dave Hansen wrote:
> >> ...
> >>>> That's good, but aren't we still left with a situation where we've
> >>>> offlined and dissolved the _middle_ of a gigantic huge page while the
> >>>> head page is still in place and online?
> >>>>
> >>>> That seems bad.
> >>>>
> >>> What about refusing to change the status for such memory block, if it
> >>> contains a huge page which larger than itself? (function
> >>> memory_block_action())
> >>
> >> How will this be visible to users, though?  That sounds like you simply
> >> won't be able to offline memory with gigantic huge pages.
> > 
> > I might be missing something but Is this any different from a regular
> > failure when the memory cannot be freed? I mean
> > /sys/devices/system/memory/memory API doesn't give you any hint whether
> > the memory in the particular block is used and
> > unmigrateable.
> 
> It's OK to have free hugetlbfs pages in an area that's being offline'd.
>  If we did that, it would not be OK to have a free gigantic hugetlbfs
> page that's larger than the area being offlined.

That was not my point. I wasn't very clear probably. Offlining can fail
which shouldn't be really surprising. There might be a kernel allocation
in the particular block which cannot be migrated so failures are to be
expected. I just do not see how offlining in the middle of a gigantic
page is any different from having any other unmovable allocation in a
block. That being said, why don't we simply refuse to offline a block
which is in the middle of a gigantic page.
 
> It would be a wee bit goofy to have the requirement that userspace go
> find all the gigantic pages and make them non-gigantic before trying to
> offline something.

yes

-- 
Michal Hocko
SUSE Labs