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Date:   Mon, 22 Feb 2021 13:56:35 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.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
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory
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References: <20210217154844.12392-1-david@redhat.com>
 <640738b5-a47e-448b-586d-a1fb80131891@redhat.com>
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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.

-- 
Michal Hocko
SUSE Labs