Re: [PATCH v1] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

["Huang, Ying" <ying.huang@intel.com>]

Wei Xu <weixugc@google.com> writes:

> On Fri, Dec 2, 2022 at 5:11 PM Mina Almasry <almasrymina@google.com> wrote:
>>
>> commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
>> reclaim"") enabled demotion in memcg reclaim, which is the right thing
>> to do, however, I suspect 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 practice
>> 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 I think try_to_free_mem_cgroup_pages() is not actually doing what
>> the callers want it to do.
>>
>> I suspect various things work suboptimally on memory 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 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. With this change,
>>
>>         echo "1m" > memory.reclaim
>>
>> Will cause freeing of 1m of memory from the cgroup regardless of the
>> demotions happening inside.
>>
>>         echo "1m nodes=0" > memory.reclaim
>>
>> Will cause freeing of 1m of node 0 by demotion if a demotion target is
>> available, and by reclaim if no demotion target is available.
>>
>> Signed-off-by: Mina Almasry <almasrymina@google.com>
>>
>> ---
>>
>> This is developed on top of mm-unstable largely because I need the
>> memory.reclaim nodes= arg to test it properly.
>> ---
>>  mm/vmscan.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 2b42ac9ad755..8f6e993b870d 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1653,6 +1653,7 @@ 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 = 0;
>>         unsigned int pgactivate = 0;
>>         bool do_demote_pass;
>>         struct swap_iocb *plug = NULL;
>> @@ -2085,7 +2086,17 @@ 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 = demote_folio_list(&demote_folios, pgdat);
>> +
>> +       /*
>> +        * Only count demoted folios as reclaimed if we demoted them from
>> +        * inside of the nodemask to outside of the nodemask, hence reclaiming
>> +        * pages in the nodemask.
>> +        */
>> +       if (sc->nodemask && node_isset(pgdat->node_id, *sc->nodemask) &&
>> +           !node_isset(next_demotion_node(pgdat->node_id), *sc->nodemask))
>
> next_demotion_node() is just the first demotion target node. Demotion
> can fall back to other allowed target nodes returned by
> node_get_allowed_targets().  When the page is demoted to a fallback
> node and this fallback node is in sc->nodemask, nr_demoted should not
> be added into nr_reclaimed, either.
>
> One way to address this issue is to pass sc->nodemask into
> demote_folio_list() and exclude sc->nodemask from the allowed target
> demotion nodes.

I don't think this is a good idea.  Because this may break the fast ->
slow -> storage aging order.  A warm page in fast memory node may be
reclaimed to storage directly, instead of being demoted to the slow
memory node.

If necessary, we can account "nr_demoted" in alloc_demote_page() and
to-be-added free_demote_page().

Best Regards,
Huang, Ying

>> +               nr_reclaimed += nr_demoted;
>> +
>>         /* 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 */
>> --
>> 2.39.0.rc0.267.gcb52ba06e7-goog