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

David Hildenbrand
2021-02-22T13:02:11Z

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