linux-kernel - Re: [PATCH v2 0/2] Do not shatter hugezeropage on wp-fault

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: <98f73b2f-ec1c-4e81-bfb2-6e02ebc4cdae@arm.com>
Date: Wed, 4 Sep 2024 21:11:57 +0530
From: Dev Jain <dev.jain@....com>
To: Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org,
 david@...hat.com, willy@...radead.org, kirill.shutemov@...ux.intel.com
Cc: anshuman.khandual@....com, catalin.marinas@....com, cl@...two.org,
 vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
 dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org,
 jack@...e.cz, mark.rutland@....com, hughd@...gle.com,
 aneesh.kumar@...nel.org, yang@...amperecomputing.com, peterx@...hat.com,
 ioworker0@...il.com, jglisse@...gle.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v2 0/2] Do not shatter hugezeropage on wp-fault


On 9/4/24 17:06, Ryan Roberts wrote:
> Hi Dev,
>
> On 04/09/2024 11:09, Dev Jain wrote:
>> It was observed at [1] and [2] that the current kernel behaviour of
>> shattering a hugezeropage is inconsistent and suboptimal. For a VMA with
>> a THP allowable order, when we write-fault on it, the kernel installs a
>> PMD-mapped THP. On the other hand, if we first get a read fault, we get
>> a PMD pointing to the hugezeropage; subsequent write will trigger a
>> write-protection fault, shattering the hugezeropage into one writable
>> page, and all the other PTEs write-protected. The conclusion being, as
>> compared to the case of a single write-fault, applications have to suffer
>> 512 extra page faults if they were to use the VMA as such, plus we get
>> the overhead of khugepaged trying to replace that area with a THP anyway.
>>
>> Instead, replace the hugezeropage with a THP on wp-fault.
>>
>> v1->v2:
>>   - Wrap do_huge_zero_wp_pmd_locked() around lock and unlock
>>   - Call thp_fault_alloc() before do_huge_zero_wp_pmd_locked() to avoid
>>   - calling sleeping function from spinlock context
>>
>> [1]: https://lore.kernel.org/all/3743d7e1-0b79-4eaf-82d5-d1ca29fe347d@arm.com/
>> [2]: https://lore.kernel.org/all/1cfae0c0-96a2-4308-9c62-f7a640520242@arm.com/
>>
>> Dev Jain (2):
>>    mm: Abstract THP allocation
>>    mm: Allocate THP on hugezeropage wp-fault
>>
>>   include/linux/huge_mm.h |   6 ++
>>   mm/huge_memory.c        | 171 +++++++++++++++++++++++++++++-----------
>>   mm/memory.c             |   5 +-
>>   3 files changed, 136 insertions(+), 46 deletions(-)
>>
> What is the base for this? It doesn't apply on top of mm-unstable.

Sorry, forgot to mention, it applies on v6.11-rc5.

>
> Thanks,
> Ryan
>