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

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Message-ID: <91a2d578-3ff2-41e1-a980-0d78c0663959@arm.com>
Date: Wed, 4 Sep 2024 17:01:48 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Dev Jain <dev.jain@....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 04/09/2024 16:41, Dev Jain wrote:
> 
> 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, I'll give it a review tomorrow. Although I suspect that Andrew will want
it based against mm-unstable once it gets into shape for merging.

> 
>>
>> Thanks,
>> Ryan
>>