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

David Hildenbrand
2021-02-19T11:12:09Z

On 19.02.21 12:04, Michal Hocko wrote:
> On Fri 19-02-21 11:43:48, David Hildenbrand wrote:
>> On 19.02.21 11:35, Michal Hocko wrote:
>>> On Wed 17-02-21 16:48:44, David Hildenbrand wrote:
>>> [...]
>>>
>>> I only got to the implementation now.
>>>
>>>> +static long madvise_populate(struct vm_area_struct *vma,
>>>> + struct vm_area_struct **prev,
>>>> + unsigned long start, unsigned long end)
>>>> +{
>>>> + struct mm_struct *mm = vma->vm_mm;
>>>> + unsigned long tmp_end;
>>>> + int locked = 1;
>>>> + long pages;
>>>> +
>>>> + *prev = vma;
>>>> +
>>>> + while (start < end) {
>>>> + /*
>>>> + * We might have temporarily dropped the lock. For example,
>>>> + * our VMA might have been split.
>>>> + */
>>>> + if (!vma || start >= vma->vm_end) {
>>>> + vma = find_vma(mm, start);
>>>> + if (!vma)
>>>> + return -ENOMEM;
>>>> + }
>>>
>>> Why do you need to find a vma when you already have one. do_madvise will
>>> give you your vma already. I do understand that you want to finish the
>>> vma for some errors but that shouldn't require handling vmas. You should
>>> be in the shope of one here unless I miss anything.
>>
>> See below, we might temporary drop the lock while not having processed all
>> pages
>>
>>>
>>>> +
>>>> + /* Bail out on incompatible VMA types. */
>>>> + if (vma->vm_flags & (VM_IO | VM_PFNMAP) ||
>>>> + !vma_is_accessible(vma)) {
>>>> + return -EINVAL;
>>>> + }
>>>> +
>>>> + /*
>>>> + * Populate pages and take care of VM_LOCKED: simulate user
>>>> + * space access.
>>>> + *
>>>> + * For private, writable mappings, trigger a write fault to
>>>> + * break COW (i.e., shared zeropage). For other mappings (i.e.,
>>>> + * read-only, shared), trigger a read fault.
>>>> + */
>>>> + tmp_end = min_t(unsigned long, end, vma->vm_end);
>>>> + pages = populate_vma_page_range(vma, start, tmp_end, &locked);
>>>> + if (!locked) {
>>>> + mmap_read_lock(mm);
>>>> + *prev = NULL;
>>>> + vma = NULL;
>>
>> ^ here
>>
>> so, the VMA might have been replaced/split/... in the meantime.
>>
>> So to make forward progress, I have to lookup again. (similar. but different
>> to madvise_dontneed_free()).
>
> Right. Missed that.

It would look more natural if we'd just be processing the whole range -
but then it would not fit into the generic infrastructure and would
result in even more code.

I decided to go with "process the passed range and treat the given VMA
as an initial VMA that is invalidated as soon as we drop the lock".

--
Thanks,

David / dhildenb