Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory

Michal Hocko
2021-02-22T13:22:11Z

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