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SubjectRe: [PATCH 0/5] Page demotion for memory reclaim
On Thu, Mar 21, 2019 at 05:12:33PM -0700, Zi Yan wrote:
> > Yes, we may not want to migrate everything in the shrink_page_list()
> > pages. We might want to keep a page, so we have to do those checks first. At
> > the point we know we want to attempt migration, the page is already
> > locked and not in a list, so it is just easier to directly invoke the
> > new __unmap_and_move_locked() that migrate_pages() eventually also calls.
>
> Right, I understand that you want to only migrate small pages to begin with. My question is
> why not using the existing migrate_pages() in your patch 3. Like:
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a5ad0b35ab8e..0a0753af357f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1261,6 +1261,20 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> ; /* try to reclaim the page below */
> }
>
> + if (!PageCompound(page)) {
> + int next_nid = next_migration_node(page);
> + int err;
> +
> + if (next_nid != TERMINAL_NODE) {
> + LIST_HEAD(migrate_list);
> + list_add(&migrate_list, &page->lru);
> + err = migrate_pages(&migrate_list, alloc_new_node_page, NULL,
> + next_nid, MIGRATE_ASYNC, MR_DEMOTION);
> + if (err)
> + putback_movable_pages(&migrate_list);
> + }
> + }
> +
> /*
> * Anonymous process memory has backing store?
> * Try to allocate it some swap space here.
>
> Because your new migrate_demote_mapping() basically does the same thing as the code above.
> If you are not OK with the gfp flags in alloc_new_node_page(), you can just write your own
> alloc_new_node_page(). :)

The page is already locked, you can't call migrate_pages()
with locked pages. You'd have to surround migrate_pages with
unlock_page/try_lock_page, and I thought that looked odd. Further,
it changes the flow if the subsequent try lock fails, and I'm trying to
be careful about not introducing different behavior if migration fails.

Patch 2/5 is included here so we can reuse the necessary code from a
locked page context.

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