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Subject: Re: [PATCH v3 0/2] mm,thp: Add filemap_huge_fault() for THP
To:     Song Liu <songliubraving@fb.com>,
        William Kucharski <william.kucharski@oracle.com>
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        Bob Kasten <robert.a.kasten@intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Chad Mynhier <chad.mynhier@oracle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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From:   William Kucharski <kucharsk@gmail.com>
Message-ID: <dc8d71ca-cbec-b924-386f-678ce528aff6@gmail.com>
Date:   Wed, 31 Jul 2019 02:58:21 -0600
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On 7/31/19 2:35 AM, Song Liu wrote:

> Could you please explain how to test/try this? Would it automatically map
> all executables to THPs?

Until there is filesystem support you can't actually try this, though I have 
tested it through some hacks during development and am also working on some 
other methods to be able to test this before large page filesystem read support 
is in place.

The end goal is that if enabled, when a fault occurs for an RO executable where 
the faulting address lies within a vma properly aligned/sized for the fault to 
be satisfied by mapping a THP, and the kernel can allocate a THP, the fault WILL 
be satisfied by mapping the THP.

It's not expected that all executables nor even all pages of all executables 
would be THP-mapped, just those executables and ranges where alignment and size 
permit. Future optimizations may include fine-tuning these checks to try to 
better determine whether an application would actually benefit from THP mapping.

 From some quick and dirty experiments I performed, I've seen that there are a 
surprising number of applications that may end up with THP-mapped pages, 
including Perl, Chrome and Firefox.

However I don't yet know what the actual vs. theoretical benefits would be.

     -- Bill