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

[David Hildenbrand]

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