linux-kernel - Re: [PATCH 4/7] mm/hmm: properly handle migration pmd

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]

Date:   Tue, 28 Aug 2018 11:54:33 -0400
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Michal Hocko" <mhocko@...nel.org>
Cc:     "Jerome Glisse" <jglisse@...hat.com>, linux-mm@...ck.org,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        "Ralph Campbell" <rcampbell@...dia.com>,
        "John Hubbard" <jhubbard@...dia.com>
Subject: Re: [PATCH 4/7] mm/hmm: properly handle migration pmd

Hi Michal,

On 28 Aug 2018, at 11:45, Michal Hocko wrote:

> On Tue 28-08-18 17:42:06, Michal Hocko wrote:
>> On Tue 28-08-18 11:36:59, Jerome Glisse wrote:
>>> On Tue, Aug 28, 2018 at 05:24:14PM +0200, Michal Hocko wrote:
>>>> On Fri 24-08-18 20:05:46, Zi Yan wrote:
>>>> [...]
>>>>>> +	if (!pmd_present(pmd)) {
>>>>>> +		swp_entry_t entry = pmd_to_swp_entry(pmd);
>>>>>> +
>>>>>> +		if (is_migration_entry(entry)) {
>>>>>
>>>>> I think you should check thp_migration_supported() here, since PMD migration is only enabled in x86_64 systems.
>>>>> Other architectures should treat PMD migration entries as bad.
>>>>
>>>> How can we have a migration pmd entry when the migration is not
>>>> supported?
>>>
>>> Not sure i follow here, migration can happen anywhere (assuming
>>> that something like compaction is active or numa or ...). So this
>>> code can face pmd migration entry on architecture that support
>>> it. What is missing here is thp_migration_supported() call to
>>> protect the is_migration_entry() to avoid false positive on arch
>>> which do not support thp migration.
>>
>> I mean that architectures which do not support THP migration shouldn't
>> ever see any migration entry. So is_migration_entry should be always
>> false. Or do I miss something?
>
> And just to be clear. thp_migration_supported should be checked only
> when we actually _do_ the migration or evaluate migratability of the
> page. We definitely do want to sprinkle this check to all places where
> is_migration_entry is checked.

is_migration_entry() is a general check for swp_entry_t, so it can return
true even if THP migration is not enabled. is_pmd_migration_entry() always
returns false when THP migration is not enabled.

So the code can be changed in two ways, either replacing is_migration_entry()
with is_pmd_migration_entry() or adding thp_migration_supported() check
like Jerome did.

Does this clarify your question?

—
Best Regards,
Yan Zi

Download attachment "signature.asc" of type "application/pgp-signature" (517 bytes)