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

Michal Hocko
2021-02-19T11:05:08Z

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.
--
Michal Hocko
SUSE Labs