On Sat, Jan 11, 2025 at 2:25 AM Chen Ridong <chenridong@xxxxxxxxxxxxxxx> wrote:
>
> From: Chen Ridong <chenridong@xxxxxxxxxx>
>
> As commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
> while isolated") mentioned:
>
> The page reclaim isolates a batch of folios from the tail of one of the
> LRU lists and works on those folios one by one. For a suitable
> swap-backed folio, if the swap device is async, it queues that folio for
> writeback. After the page reclaim finishes an entire batch, it puts back
> the folios it queued for writeback to the head of the original LRU list.
>
> In the meantime, the page writeback flushes the queued folios also by
> batches. Its batching logic is independent from that of the page
> reclaim. For each of the folios it writes back, the page writeback calls
> folio_rotate_reclaimable() which tries to rotate a folio to the tail.
>
> folio_rotate_reclaimable() only works for a folio after the page reclaim
> has put it back. If an async swap device is fast enough, the page
> writeback can finish with that folio while the page reclaim is still
> working on the rest of the batch containing it. In this case, that folio
> will remain at the head and the page reclaim will not retry it before
> reaching there".
>
> The commit 359a5e1416ca ("mm: multi-gen LRU: retry folios written back
> while isolated") only fixed the issue for mglru. However, this issue
> also exists in the traditional active/inactive LRU and was found at [1].
The active/inactive LRU needs more careful thoughts due to its
complexity. Details below.
> It can be reproduced with below steps:
>
> 1. Compile with CONFIG_TRANSPARENT_HUGEPAGE=y
> 2. Mount memcg v1, and create memcg named test_memcg and set
> limit_in_bytes=1G, memsw.limit_in_bytes=2G.
> 3. Create a 1G swap file, and allocate 1.05G anon memory in test_memcg.
>
> It was found that:
>
> cat memory.limit_in_bytes
> 1073741824
> cat memory.memsw.limit_in_bytes
> 2147483648
> cat memory.usage_in_bytes
> 1073664000
> cat memory.memsw.usage_in_bytes
> 1129840640
>
> free -h
> total used free
> Mem: 31Gi 1.2Gi 28Gi
> Swap: 1.0Gi 1.0Gi 2.0Mi
>
> As shown above, the test_memcg used about 50M swap, but almost 1G swap
> memory was used, which means that 900M+ may be wasted because other memcgs
> can not use these swap memory.
>
> This issue should be fixed in the same way as mglru. Therefore, the common
> logic was extracted to the 'find_folios_written_back' function firstly,
> which is then reused in the 'shrink_inactive_list' function. Finally,
> retry reclaiming those folios that may have missed the rotation for
> traditional LRU.
>
> After change, the same test case. only 54M swap was used.
>
> cat memory.usage_in_bytes
> 1073463296
> cat memory.memsw.usage_in_bytes
> 1129828352
>
> free -h
> total used free
> Mem: 31Gi 1.2Gi 28Gi
> Swap: 1.0Gi 54Mi 969Mi
>
> [1] https://lore.kernel.org/linux-kernel/20241010081802.290893-1-chenridong@xxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-kernel/CAGsJ_4zqL8ZHNRZ44o_CC69kE7DBVXvbZfvmQxMGiFqRxqHQdA@xxxxxxxxxxxxxx/
> Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
> ---
>
> v6->v7:
> - fix conflict based on mm-unstable.
> - update the commit message(quote from YU's commit message, and add
> improvements after change.)
> - restore 'is_retrying' to 'skip_retry' to keep original semantics.
>
> v6: https://lore.kernel.org/linux-kernel/20241223082004.3759152-1-chenridong@xxxxxxxxxxxxxxx/
>
> mm/vmscan.c | 114 ++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 76 insertions(+), 38 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 01dce6f26..6861b6937 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -183,6 +183,9 @@ struct scan_control {
> struct reclaim_state reclaim_state;
> };
>
> +static inline void find_folios_written_back(struct list_head *list,
> + struct list_head *clean, struct lruvec *lruvec, int type, bool is_retrying);
> +
> #ifdef ARCH_HAS_PREFETCHW
> #define prefetchw_prev_lru_folio(_folio, _base, _field) \
> do { \
> @@ -1960,14 +1963,18 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> enum lru_list lru)
> {
> LIST_HEAD(folio_list);
> + LIST_HEAD(clean_list);
> unsigned long nr_scanned;
> - unsigned int nr_reclaimed = 0;
> + unsigned int nr_reclaimed, total_reclaimed = 0;
> + unsigned int nr_pageout = 0;
> + unsigned int nr_unqueued_dirty = 0;
> unsigned long nr_taken;
> struct reclaim_stat stat;
> bool file = is_file_lru(lru);
> enum vm_event_item item;
> struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> bool stalled = false;
> + bool skip_retry = false;
>
> while (unlikely(too_many_isolated(pgdat, file, sc))) {
> if (stalled)
> @@ -2001,22 +2008,47 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> if (nr_taken == 0)
> return 0;
>
> +retry:
> nr_reclaimed = shrink_folio_list(&folio_list, pgdat, sc, &stat, false);
>
> + sc->nr.dirty += stat.nr_dirty;
> + sc->nr.congested += stat.nr_congested;
> + sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> + sc->nr.writeback += stat.nr_writeback;
I think this change breaks the tests on the stats above, e.g.,
wakeup_flusher_threads(), because the same dirty/writeback folio can
be counted twice. The reason for that is that
folio_test_dirty/writeback() can't account for dirty/writeback buffer
heads, which can only be done by folio_check_dirty_writeback().
For MGLRU, it has been broken since day 1 and commit 1bc542c6a0d1
("mm/vmscan: wake up flushers conditionally to avoid cgroup OOM")
doesn't account for this either. I'll get around to that.
> + sc->nr.immediate += stat.nr_immediate;
> + total_reclaimed += nr_reclaimed;
> + nr_pageout += stat.nr_pageout;
> + nr_unqueued_dirty += stat.nr_unqueued_dirty;
> +
> + trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> + nr_scanned, nr_reclaimed, &stat, sc->priority, file);
> +
> + find_folios_written_back(&folio_list, &clean_list, lruvec, 0, skip_retry);
> +
> spin_lock_irq(&lruvec->lru_lock);
> move_folios_to_lru(lruvec, &folio_list);
>
> __mod_lruvec_state(lruvec, PGDEMOTE_KSWAPD + reclaimer_offset(),
> stat.nr_demoted);
> - __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> item = PGSTEAL_KSWAPD + reclaimer_offset();
> if (!cgroup_reclaim(sc))
> __count_vm_events(item, nr_reclaimed);
> __count_memcg_events(lruvec_memcg(lruvec), item, nr_reclaimed);
> __count_vm_events(PGSTEAL_ANON + file, nr_reclaimed);
> +
> + if (!list_empty(&clean_list)) {
> + list_splice_init(&clean_list, &folio_list);
> + skip_retry = true;
> + spin_unlock_irq(&lruvec->lru_lock);
> + goto retry;
> + }
> + __mod_node_page_state(pgdat, NR_ISOLATED_ANON + file, -nr_taken);
> spin_unlock_irq(&lruvec->lru_lock);
> + sc->nr.taken += nr_taken;
> + if (file)
> + sc->nr.file_taken += nr_taken;
>
> - lru_note_cost(lruvec, file, stat.nr_pageout, nr_scanned - nr_reclaimed);
> + lru_note_cost(lruvec, file, nr_pageout, nr_scanned - total_reclaimed);
>
> /*
> * If dirty folios are scanned that are not queued for IO, it
> @@ -2029,7 +2061,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> * the flushers simply cannot keep up with the allocation
> * rate. Nudge the flusher threads in case they are asleep.
> */
> - if (stat.nr_unqueued_dirty == nr_taken) {
> + if (nr_unqueued_dirty == nr_taken) {
> wakeup_flusher_threads(WB_REASON_VMSCAN);
> /*
> * For cgroupv1 dirty throttling is achieved by waking up
> @@ -2044,18 +2076,7 @@ static unsigned long shrink_inactive_list(unsigned long nr_to_scan,
> reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
> }
>
> - sc->nr.dirty += stat.nr_dirty;
> - sc->nr.congested += stat.nr_congested;
> - sc->nr.unqueued_dirty += stat.nr_unqueued_dirty;
> - sc->nr.writeback += stat.nr_writeback;
> - sc->nr.immediate += stat.nr_immediate;
> - sc->nr.taken += nr_taken;
> - if (file)
> - sc->nr.file_taken += nr_taken;
> -
> - trace_mm_vmscan_lru_shrink_inactive(pgdat->node_id,
> - nr_scanned, nr_reclaimed, &stat, sc->priority, file);
> - return nr_reclaimed;
> + return total_reclaimed;
> }
>
> /*
> @@ -4637,8 +4658,6 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> int reclaimed;
> LIST_HEAD(list);
> LIST_HEAD(clean);
> - struct folio *folio;
> - struct folio *next;
> enum vm_event_item item;
> struct reclaim_stat stat;
> struct lru_gen_mm_walk *walk;
> @@ -4668,26 +4687,7 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> scanned, reclaimed, &stat, sc->priority,
> type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>
> - list_for_each_entry_safe_reverse(folio, next, &list, lru) {
> - DEFINE_MIN_SEQ(lruvec);
> -
> - if (!folio_evictable(folio)) {
> - list_del(&folio->lru);
> - folio_putback_lru(folio);
> - continue;
> - }
> -
> - /* retry folios that may have missed folio_rotate_reclaimable() */
> - if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
> - !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> - list_move(&folio->lru, &clean);
> - continue;
> - }
> -
> - /* don't add rejected folios to the oldest generation */
> - if (lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
> - set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> - }
> + find_folios_written_back(&list, &clean, lruvec, type, skip_retry);
>
> spin_lock_irq(&lruvec->lru_lock);
>
> @@ -5706,6 +5706,44 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
>
> #endif /* CONFIG_LRU_GEN */
>
> +/**
> + * find_folios_written_back - Find and move the written back folios to a new list.
> + * @list: filios list
> + * @clean: the written back folios list
> + * @lruvec: the lruvec
> + * @type: LRU_GEN_ANON/LRU_GEN_FILE, only for multi-gen LRU
> + * @skip_retry: whether skip retry.
> + */
> +static inline void find_folios_written_back(struct list_head *list,
> + struct list_head *clean, struct lruvec *lruvec, int type, bool skip_retry)
> +{
> + struct folio *folio;
> + struct folio *next;
> +
> + list_for_each_entry_safe_reverse(folio, next, list, lru) {
> +#ifdef CONFIG_LRU_GEN
> + DEFINE_MIN_SEQ(lruvec);
> +#endif
> + if (!folio_evictable(folio)) {
> + list_del(&folio->lru);
> + folio_putback_lru(folio);
> + continue;
> + }
> +
> + /* retry folios that may have missed folio_rotate_reclaimable() */
> + if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
> + !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
Have you verified that this condition also holds for the
active/inactive LRU or did you just assume it? IOW, how do we know the
active/inactive LRU doesn't think this folio should be kept (and put
back to the head of the inactive LRU list).
