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

[David Hildenbrand]

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