Re: [PATCH 3/4] mm/vmscan: Attempt to migrate page in lieu of discard

[Yang Shi]

On Fri, Oct 18, 2019 at 11:15 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 10/17/19 10:30 AM, Yang Shi wrote:
> >> +               if (!PageHuge(page)) {
> >> +                       int rc = migrate_demote_mapping(page);
> >> +
> >> +                       /*
> >> +                        * -ENOMEM on a THP may indicate either migration is
> >> +                        * unsupported or there was not enough contiguous
> >> +                        * space. Split the THP into base pages and retry the
> >> +                        * head immediately. The tail pages will be considered
> >> +                        * individually within the current loop's page list.
> >> +                        */
> >> +                       if (rc == -ENOMEM && PageTransHuge(page) &&
> >> +                           !split_huge_page_to_list(page, page_list))
> >> +                               rc = migrate_demote_mapping(page);
> > I recalled when Keith posted the patch at the first time, I raised
> > question about why not just migrating THP in a whole? The
> > migrate_pages() could handle this. If it fails, it just fallbacks to
> > base page.
>
> There's a pair of migrate_demote_mapping()s in there.  I expected that
> the first will migrate the whole THP and the second plus the split is
> only used if fails the whole migration.

Ah, that's right. I mis-read the first migrate_demote_mapping(). But,
why calling migrate_demote_mapping() twice for THP and base page (if
THP is failed) instead of calling migrate_pages() that does deal with
all the cases.

>
> Am I reading it wrong?