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

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Date:   Wed, 21 Sep 2016 09:04:31 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>
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 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.

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.