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

Michal Hocko
2021-02-19T10:36:16Z

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.

> +
> + /* 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;
> + }
> + if (pages < 0) {
> + switch (pages) {
> + case -EINTR:
> + case -ENOMEM:
> + return pages;
> + case -EHWPOISON:
> + /* Skip over any poisoned pages. */
> + start += PAGE_SIZE;
> + continue;
> + case -EBUSY:
> + case -EAGAIN:
> + continue;
> + default:
> + pr_warn_once("%s: unhandled return value: %ld\n",
> + __func__, pages);
> + return -ENOMEM;
> + }
> + }
> + start += pages * PAGE_SIZE;
> + }
> + return 0;
> +}
> +
> /*
> * Application wants to free up the pages and associated backing store.
> * This is effectively punching a hole into the middle of a file.
> @@ -934,6 +1001,8 @@ madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
> case MADV_FREE:
> case MADV_DONTNEED:
> return madvise_dontneed_free(vma, prev, start, end, behavior);
> + case MADV_POPULATE:
> + return madvise_populate(vma, prev, start, end);
> default:
> return madvise_behavior(vma, prev, start, end, behavior);
> }
> @@ -954,6 +1023,7 @@ madvise_behavior_valid(int behavior)
> case MADV_FREE:
> case MADV_COLD:
> case MADV_PAGEOUT:
> + case MADV_POPULATE:
> #ifdef CONFIG_KSM
> case MADV_MERGEABLE:
> case MADV_UNMERGEABLE:
> --
> 2.29.2
>

--
Michal Hocko
SUSE Labs