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

Michal Hocko
2021-02-22T13:00:26Z

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