Mina Almasry <almasrymina@xxxxxxxxxx> writes:
> commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> reclaim"") enabled demotion in memcg reclaim, which is the right thing
> to do, however, it introduced a regression in the behavior of
> try_to_free_mem_cgroup_pages().
>
> The callers of try_to_free_mem_cgroup_pages() expect it to attempt to
> reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage
> of the cgroup should reduce by nr_pages. The callers expect
> try_to_free_mem_cgroup_pages() to also return the number of pages
> reclaimed, not demoted.
>
> However, what try_to_free_mem_cgroup_pages() actually does is it
> unconditionally counts demoted pages as reclaimed pages. So in practnice
> when it is called it will often demote nr_pages and return the number of
> demoted pages to the caller. Demoted pages don't lower the memcg usage,
> and so try_to_free_mem_cgroup_pages() is not actually doing what the
> callers want it to do.
>
> Various things work suboptimally on memory tiered systems or don't work
> at all due to this:
>
> - memory.high enforcement likely doesn't work (it just demotes nr_pages
> instead of lowering the memcg usage by nr_pages).
> - try_charge_memcg() will keep retrying the charge while
> try_to_free_mem_cgroup_pages() is just demoting pages and not actually
> making any room for the charge.
> - memory.reclaim has a wonky interface. It advertises to the user it
> reclaims the provided amount but it will actually often demote that
> amount.
>
> There may be more effects to this issue.
>
> To fix these issues I propose shrink_folio_list() to only count pages
> demoted from inside of sc->nodemask to outside of sc->nodemask as
> 'reclaimed'.
>
> For callers such as reclaim_high() or try_charge_memcg() that set
> sc->nodemask to NULL, try_to_free_mem_cgroup_pages() will try to
> actually reclaim nr_pages and return the number of pages reclaimed. No
> demoted pages would count towards the nr_pages requirement.
>
> For callers such as memory_reclaim() that set sc->nodemask,
> try_to_free_mem_cgroup_pages() will free nr_pages from that nodemask
> with either reclaim or demotion.
>
> Tested this change using memory.reclaim interface. I set up a test case where
> I allocate 500m in a cgroup, and then do:
>
> echo "50m" > memory.reclaim
>
> Without this fix, my kernel demotes 70mb and reclaims 4 mb
> (memory.current is reduced by about 4mb).
>
> With this fix, my kernel demotes all memory possible and reclaims 60mb
> (memory.current is reduced by about 60mb).
>
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>
> ---
>
> Changes in v3:
> - Reverted back to v1 implementation: "try to demote but don't count
> demoted pages unless they are demoted to outside the nodemask" as Ying
> suggested.
> - Made sure demotions that fall back to non next_demotion_target() are
> not counted as Wei suggested.
> - Updated comment in shrink_folio_list() as Ying suggested.
> - Added before/after for the test case in commit message since Ying
> asked.
> - Fixed call sites that don't provide sc->nodemask but expect demotion
> from a specific node as Ying pointed out.
>
> Cc: weixugc@xxxxxxxxxx
> Cc: ying.huang@xxxxxxxxx
>
> This is developed on top of mm-unstable largely because I want the
> solution to be compatible with the recently added nodes= arg on
> mm-unstable.
> ---
> mm/vmscan.c | 91 +++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 78 insertions(+), 13 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2b42ac9ad755..f324e80395c3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1555,13 +1555,18 @@ static void folio_check_dirty_writeback(struct folio *folio,
> mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
> }
>
> +struct demotion_control {
> + struct migration_target_control *mtc;
> + nodemask_t *demote_from_nodemask;
> + unsigned long nr_demoted_outside_nodemask;
> +};
> +
> static struct page *alloc_demote_page(struct page *page, unsigned long private)
> {
> struct page *target_page;
> nodemask_t *allowed_mask;
> - struct migration_target_control *mtc;
> -
> - mtc = (struct migration_target_control *)private;
> + struct demotion_control *dc = (struct demotion_control *)private;
> + struct migration_target_control *mtc = dc->mtc;
>
> allowed_mask = mtc->nmask;
> /*
> @@ -1576,13 +1581,31 @@ static struct page *alloc_demote_page(struct page *page, unsigned long private)
> mtc->nmask = NULL;
> mtc->gfp_mask |= __GFP_THISNODE;
> target_page = alloc_migration_target(page, (unsigned long)mtc);
> - if (target_page)
> + if (!target_page) {
> + mtc->gfp_mask &= ~__GFP_THISNODE;
> + mtc->nmask = allowed_mask;
> + target_page = alloc_migration_target(page, (unsigned long)mtc);
> + }
> +
> + if (!target_page)
> return target_page;
>
> - mtc->gfp_mask &= ~__GFP_THISNODE;
> - mtc->nmask = allowed_mask;
> + if (dc->demote_from_nodemask &&
> + !node_isset(page_to_nid(target_page), *dc->demote_from_nodemask))
Use mtc->nid directly?
> + dc->nr_demoted_outside_nodemask++;
>
> - return alloc_migration_target(page, (unsigned long)mtc);
> + return target_page;
> +}
> +
> +void free_demote_page(struct page *page, unsigned long private)
> +{
> + struct demotion_control *dc = (struct demotion_control *)private;
> +
> + if (dc->demote_from_nodemask &&
> + !node_isset(page_to_nid(page), *dc->demote_from_nodemask))
ditto
> + dc->nr_demoted_outside_nodemask--;
> +
> + folio_put(page_folio(page));
> }
>
> /*
> @@ -1590,7 +1613,8 @@ static struct page *alloc_demote_page(struct page *page, unsigned long private)
> * Folios which are not demoted are left on @demote_folios.
> */
> static unsigned int demote_folio_list(struct list_head *demote_folios,
> - struct pglist_data *pgdat)
> + struct pglist_data *pgdat,
> + nodemask_t *nodemask)
> {
> int target_nid = next_demotion_node(pgdat->node_id);
> unsigned int nr_succeeded;
> @@ -1608,6 +1632,12 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
> .nmask = &allowed_mask
> };
>
> + struct demotion_control dc = {
> + .mtc = &mtc,
> + .demote_from_nodemask = nodemask,
> + .nr_demoted_outside_nodemask = 0,
> + };
> +
> if (list_empty(demote_folios))
> return 0;
>
> @@ -1617,13 +1647,13 @@ static unsigned int demote_folio_list(struct list_head *demote_folios,
> node_get_allowed_targets(pgdat, &allowed_mask);
>
> /* Demotion ignores all cpuset and mempolicy settings */
> - migrate_pages(demote_folios, alloc_demote_page, NULL,
> - (unsigned long)&mtc, MIGRATE_ASYNC, MR_DEMOTION,
> + migrate_pages(demote_folios, alloc_demote_page, free_demote_page,
> + (unsigned long)&dc, MIGRATE_ASYNC, MR_DEMOTION,
> &nr_succeeded);
>
> __count_vm_events(PGDEMOTE_KSWAPD + reclaimer_offset(), nr_succeeded);
>
> - return nr_succeeded;
> + return dc.nr_demoted_outside_nodemask;
> }
>
> static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask)
> @@ -1643,7 +1673,12 @@ static bool may_enter_fs(struct folio *folio, gfp_t gfp_mask)
> }
>
> /*
> - * shrink_folio_list() returns the number of reclaimed pages
> + * shrink_folio_list() returns the number of reclaimed pages.
> + *
> + * Demoted pages are counted as reclaimed iff:
> + * (a) sc->nodemask arg is provided.
> + * (b) page has been demoted from a node inside sc->nodemask to a node
> + * outside sc->nodemask.
> */
> static unsigned int shrink_folio_list(struct list_head *folio_list,
> struct pglist_data *pgdat, struct scan_control *sc,
> @@ -1653,6 +1688,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> LIST_HEAD(free_folios);
> LIST_HEAD(demote_folios);
> unsigned int nr_reclaimed = 0;
> + unsigned int nr_demoted_outside_nodemask = 0;
> unsigned int pgactivate = 0;
> bool do_demote_pass;
> struct swap_iocb *plug = NULL;
> @@ -2085,7 +2121,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> /* 'folio_list' is always empty here */
>
> /* Migrate folios selected for demotion */
> - nr_reclaimed += demote_folio_list(&demote_folios, pgdat);
> + nr_demoted_outside_nodemask =
> + demote_folio_list(&demote_folios, pgdat, sc->nodemask);
> +
> + if (sc->nodemask)
> + nr_reclaimed += nr_demoted_outside_nodemask;
> +
> /* Folios that could not be demoted are still in @demote_folios */
> if (!list_empty(&demote_folios)) {
> /* Folios which weren't demoted go back on @folio_list */
> @@ -2130,9 +2171,11 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> struct list_head *folio_list)
> {
> + nodemask_t nodemask = NODE_MASK_NONE;
Is it necessary for us to use NODEMASK_ALLOC/NODEMASK_FREE to save stack space?
Best Regards,
Huang, Ying
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .may_unmap = 1,
> + .nodemask = &nodemask
> };
> struct reclaim_stat stat;
> unsigned int nr_reclaimed;
> @@ -2140,6 +2183,12 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
> LIST_HEAD(clean_folios);
> unsigned int noreclaim_flag;
>
> + /*
> + * Set the nodemask in sc to indicate to shrink_folio_list() that we're
> + * looking for reclaim from this node.
> + */
> + node_set(zone->zone_pgdat->node_id, nodemask);
> +
> list_for_each_entry_safe(folio, next, folio_list, lru) {
> if (!folio_test_hugetlb(folio) && folio_is_file_lru(folio) &&
> !folio_test_dirty(folio) && !__folio_test_movable(folio) &&
> @@ -7031,12 +7080,20 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
> unsigned long zone_boosts[MAX_NR_ZONES] = { 0, };
> bool boosted;
> struct zone *zone;
> + nodemask_t nodemask = NODE_MASK_NONE;
> struct scan_control sc = {
> .gfp_mask = GFP_KERNEL,
> .order = order,
> .may_unmap = 1,
> + .nodemask = &nodemask,
> };
>
> + /*
> + * Set the nodemask in sc to indicate to kswapd_shrink_node() that we're
> + * looking for reclaim from this node.
> + */
> + node_set(pgdat->node_id, nodemask);
> +
> set_task_reclaim_state(current, &sc.reclaim_state);
> psi_memstall_enter(&pflags);
> __fs_reclaim_acquire(_THIS_IP_);
> @@ -7642,6 +7699,7 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> const unsigned long nr_pages = 1 << order;
> struct task_struct *p = current;
> unsigned int noreclaim_flag;
> + nodemask_t nodemask = NODE_MASK_NONE;
> struct scan_control sc = {
> .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> .gfp_mask = current_gfp_context(gfp_mask),
> @@ -7651,9 +7709,16 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
> .may_unmap = !!(node_reclaim_mode & RECLAIM_UNMAP),
> .may_swap = 1,
> .reclaim_idx = gfp_zone(gfp_mask),
> + .nodemask = &nodemask,
> };
> unsigned long pflags;
>
> + /*
> + * Set the nodemask in sc to indicate to shrink_node() that we're
> + * looking for reclaim from this node.
> + */
> + node_set(pgdat->node_id, nodemask);
> +
> trace_mm_vmscan_node_reclaim_begin(pgdat->node_id, order,
> sc.gfp_mask);
>
> --
> 2.39.0.rc0.267.gcb52ba06e7-goog
