On 2025/1/12 6:12, Yu Zhao wrote:
> 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.
Hi, Yu, thank you for your review.
Maybe nr_reclaimed is the only value we need to accumulate? We only want
to retry folios that may have missed folio_rotate_reclaimable(), and
these folios should be reclaimed and freed. Therefore, we need to
accumulate nr_reclaimed. For the other fields in the stat, we should
just keep the values that were obtained the first time they were shrunk.
But I'm not sure if I'm missing something.
>
>> + 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).
>
As the message shows, I tested my case and it worked for my case. I
added logs, and they could identify the folios that have missed
folio_rotate_reclaimable(). I think it's the same for both MGLRU and
active/inactive LRU to identify the folios that may have missed
folio_rotate_reclaimable(). Or did I miss something again?
Thank you again.
Best regards,
Ridong
