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SubjectRe: [PATCH 09/14] mm: deactivations shouldn't bias the LRU balance
On Wed, May 20, 2020 at 07:25:20PM -0400, Johannes Weiner wrote:
> Operations like MADV_FREE, FADV_DONTNEED etc. currently move any
> affected active pages to the inactive list to accelerate their reclaim
> (good) but also steer page reclaim toward that LRU type, or away from
> the other (bad).
>
> The reason why this is undesirable is that such operations are not
> part of the regular page aging cycle, and rather a fluke that doesn't
> say much about the remaining pages on that list; they might all be in
> heavy use, and once the chunk of easy victims has been purged, the VM
> continues to apply elevated pressure on those remaining hot pages. The
> other LRU, meanwhile, might have easily reclaimable pages, and there
> was never a need to steer away from it in the first place.
>
> As the previous patch outlined, we should focus on recording actually
> observed cost to steer the balance rather than speculating about the
> potential value of one LRU list over the other. In that spirit, leave
> explicitely deactivated pages to the LRU algorithm to pick up, and let
> rotations decide which list is the easiest to reclaim.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Minchan Kim <minchan@kernel.org>
> Acked-by: Michal Hocko <mhocko@suse.com>
> ---
> mm/swap.c | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 5d62c5a0c651..d7912bfb597f 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -515,14 +515,12 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
>
> if (active)
> __count_vm_event(PGDEACTIVATE);
> - lru_note_cost(lruvec, !file, hpage_nr_pages(page));
> }
>
[]

mm/swap.c: In function 'lru_deactivate_file_fn':
mm/swap.c:504:11: warning: variable 'file' set but not used
[-Wunused-but-set-variable]
int lru, file;
^~~~

This?

diff --git a/mm/swap.c b/mm/swap.c
index fedf5847dfdb..9c38c1b545af 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -501,7 +501,7 @@ void lru_cache_add_active_or_unevictable(struct page *page,
static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
void *arg)
{
- int lru, file;
+ int lru;
bool active;

if (!PageLRU(page))
@@ -515,7 +515,6 @@ static void lru_deactivate_file_fn(struct page *page, struct lruvec *lruvec,
return;

active = PageActive(page);
- file = page_is_file_lru(page);
lru = page_lru_base_type(page);

del_page_from_lru_list(page, lruvec, lru + active);
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