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

[David Hildenbrand <david@redhat.com>]

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()).

-- 
Thanks,

David / dhildenb