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Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
To:     Michal Hocko <mhocko@suse.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Minchan Kim <minchan@kernel.org>, Jann Horn <jannh@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Dave Hansen <dave.hansen@intel.com>,
        Hugh Dickins <hughd@google.com>,
        Rik van Riel <riel@surriel.com>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Richard Henderson <rth@twiddle.net>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Matt Turner <mattst88@gmail.com>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>, Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>, linux-alpha@vger.kernel.org,
        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org
References: <20210217154844.12392-1-david@redhat.com>
 <640738b5-a47e-448b-586d-a1fb80131891@redhat.com>
 <YDOqA9nQHiuIrKBu@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <73f73cf2-1b4e-bfa9-9a4c-3192d7b7a5ec@redhat.com>
Date:   Mon, 22 Feb 2021 13:59:55 +0100
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On 22.02.21 13:56, Michal Hocko wrote:
> On Sat 20-02-21 10:12:26, David Hildenbrand wrote:
> [...]
>> Thinking about MADV_POPULATE vs. MADV_POPULATE_WRITE I wonder if it would be
>> more versatile to break with existing MAP_POPULATE semantics and directly go
>> with
>>
>> MADV_POPULATE_READ: simulate user space read access without actually
>> reading. Trigger a read fault if required.
>>
>> MADV_POPULATE_WRITE: simulate user space write access without actually
>> writing. Trigger a write fault if required.
>>
>> For my use case, I could use MADV_POPULATE_WRITE on anonymous memory and
>> RAM-backed files (shmem/hugetlb) - I would not have a minor fault when the
>> guest inside the VM first initializes memory. This mimics how QEMU currently
>> preallocates memory.
>>
>> However, I would use MADV_POPULATE_READ on any !RAM-backed files where we
>> actually have to write-back to a (slow?) device. Dirtying everything
>> although the guest might not actually consume it in the near future might be
>> undesired.
> 
> Isn't what the current mm_populate does?
>          if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE)
>                  gup_flags |= FOLL_WRITE;
> 
> So it will write fault to shared memory mappings but it will touch
> others.

Exactly. But for hugetlbfs/shmem ("!RAM-backed files") this is not what 
we want.

-- 
Thanks,

David / dhildenb