linux-kernel - Re: [PATCH RFC] mm/madvise: introduce MADV

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Date:   Mon, 22 Feb 2021 14:19:02 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>, Jann Horn <jannh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Dave Hansen <dave.hansen@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, linux-alpha@...r.kernel.org,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to
 prefault/prealloc memory

On Mon 22-02-21 13:59:55, David Hildenbrand wrote:
> 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.

Ble, I have writen that opposit to the actual behavior. It will write
fault on writeable private mappings and only touch on read/only or
private mappings.

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

OK, then I must have misread your requirements. Maybe I just got lost in
all the combinations you have listed.
-- 
Michal Hocko
SUSE Labs