On Thu, Dec 17, 2020 at 01:52:41PM -0500, Pavel Tatashin wrote:
> +/*
> + * Verify that there are no unpinnable (movable) pages, if so return true.
> + * Otherwise an unpinnable pages is found return false, and unpin all pages.
> + */
> +static bool check_and_unpin_pages(unsigned long nr_pages, struct page
> **pages,
> + unsigned int gup_flags)
> +{
> + unsigned long i, step;
> +
> + for (i = 0; i < nr_pages; i += step) {
> + struct page *head = compound_head(pages[i]);
> +
> + step = compound_nr(head) - (pages[i] - head);
You can't assume that all of a compound head is in the pages array,
this assumption would only work inside the page walkers if the page
was found in a PMD or something.
> + if (gup_flags & FOLL_PIN) {
> + unpin_user_pages(pages, nr_pages);
So we throw everything away? Why? That isn't how the old algorithm worked
> @@ -1654,22 +1664,55 @@ static long __gup_longterm_locked(struct mm_struct
> *mm,
> struct vm_area_struct **vmas,
> unsigned int gup_flags)
> {
> - unsigned long flags = 0;
> + int migrate_retry = 0;
> + int isolate_retry = 0;
> + unsigned int flags;
> long rc;
>
> - if (gup_flags & FOLL_LONGTERM)
> - flags = memalloc_pin_save();
> + if (!(gup_flags & FOLL_LONGTERM))
> + return __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
> + NULL, gup_flags);
>
> - rc = __get_user_pages_locked(mm, start, nr_pages, pages, vmas, NULL,
> - gup_flags);
> + /*
> + * Without FOLL_WRITE fault handler may return zero page, which can
> + * be in a movable zone, and also will fail to isolate during migration,
> + * thus the longterm pin will fail.
> + */
> + gup_flags &= FOLL_WRITE;
Is &= what you mean here? |= right?
Seems like we've ended up in a weird place if FOLL_LONGTERM always
includes FOLL_WRITE. Putting the zero page in ZONE_MOVABLE seems like
a bad idea, no?
> + /*
> + * Migration may fail, we retry before giving up. Also, because after
> + * migration pages[] becomes outdated, we unpin and repin all pages
> + * in the range, so pages array is repopulated with new values.
> + * Also, because of this we cannot retry migration failures in a loop
> + * without pinning/unpinnig pages.
> + */
The old algorithm made continuous forward progress and only went back
to the first migration point.
> + for (; ; ) {
while (true)?
> + rc = __get_user_pages_locked(mm, start, nr_pages, pages, vmas,
> + NULL, gup_flags);
> + /* Return if error or if all pages are pinnable */
> + if (rc <= 0 || check_and_unpin_pages(rc, pages, gup_flags))
> + break;
So we sweep the pages list twice now?
> + /* Some pages are not pinnable, migrate them */
> + rc = migrate_movable_pages(rc, pages);
> +
> + /*
> + * If there is an error, and we tried maximum number of times
> + * bail out. Notice: we return an error code, and all pages are
> + * unpinned
> + */
> + if (rc < 0 && migrate_retry++ >= PINNABLE_MIGRATE_MAX) {
> + break;
> + } else if (rc > 0 && isolate_retry++ >= PINNABLE_ISOLATE_MAX) {
> + rc = -EBUSY;
I don't like this at all. It shouldn't be so flakey
Can you do migration without the LRU?
Jason
