linux-kernel - Re: [Question]: major faults are still triggered after mlockall when numa balancing

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Message-ID: <87wmujeqlg.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Wed, 15 Nov 2023 09:46:03 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        "zhangpeng (AS)" <zhangpeng362@...wei.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <lstoakes@...il.com>, <hughd@...gle.com>, <david@...hat.com>,
        <vbabka@...e.cz>, <peterz@...radead.org>, <mgorman@...e.de>,
        <mingo@...hat.com>, <riel@...hat.com>, <hannes@...xchg.org>,
        Nanyong Sun <sunnanyong@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Subject: Re: [Question]: major faults are still triggered after mlockall
 when numa balancing

"Yin, Fengwei" <fengwei.yin@...el.com> writes:

> On 11/13/2023 10:02 AM, Huang, Ying wrote:
>>>> There are other places in the kernel where the PTE is cleared, for
>>>> example, move_ptes() in mremap.c.  IIUC, we need to audit all them.
>>>>
>>>> Another possible solution is to check PTE again with PTL held before
>>>> reading in file data.  This will increase the overhead of major fault
>>>> path.  Is it acceptable?
>>> What if we check the PTE without page table lock acquired?
>> The PTE is zeroed temporarily only with PTL held.  So, if we acquire the
>> PTL in filemap_fault() and check the PTE, the PTE which is zeroed in
>> do_numa_page() will be non-zero now.  So we can avoid the major fault.
> Yes.
>
>> 
>> But, if we don't acquire the PTL, the PTE may still be zero.
> For do_numa_page()/change_pte_range(), it does very limit thing during
> PTE is cleared. Considering the code path of do_read_fault(), it's likely
> the PTE is none-zero.

It's possible per my understanding, although it doesn't feel good to
depend on some "race" condition.

> My concern to acquiring lock is that it brings extra PTL lock acquire/release
> for other more common cases.

Yes.  It will bring some overhead to acquire the PTL.

Anyway, some performance test is needed to compare the solution.

--
Best Regards,
Huang, Ying