Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough free memory in the lower memory tier

[Akinobu Mita]

2026年1月13日(火) 22:40 Michal Hocko <mhocko@xxxxxxxx>:
>
> On Tue 13-01-26 17:14:53, Akinobu Mita wrote:
> > On systems with multiple memory-tiers consisting of DRAM and CXL memory,
> > the OOM killer is not invoked properly.
> >
> > Here's the command to reproduce:
> >
> > $ sudo swapoff -a
> > $ stress-ng --oomable -v --memrate 20 --memrate-bytes 10G \
> >     --memrate-rd-mbs 1 --memrate-wr-mbs 1
> >
> > The memory usage is the number of workers specified with the --memrate
> > option multiplied by the buffer size specified with the --memrate-bytes
> > option, so please adjust it so that it exceeds the total size of the
> > installed DRAM and CXL memory.
> >
> > If swap is disabled, you can usually expect the OOM killer to terminate
> > the stress-ng process when memory usage approaches the installed memory
> > size.
> >
> > However, if multiple memory-tiers exist (multiple
> > /sys/devices/virtual/memory_tiering/memory_tier<N> directories exist) and
> > /sys/kernel/mm/numa/demotion_enabled is true, the OOM killer will not be
> > invoked and the system will become inoperable, regardless of whether MGLRU
> > is enabled or not.
> >
> > This issue can be reproduced using NUMA emulation even on systems with
> > only DRAM.  You can create two-fake memory-tiers by booting a single-node
> > system with "numa=fake=2 numa_emulation.adistance=576,704" kernel
> > parameters.
> >
> > The reason for this issue is that memory allocations do not directly
> > trigger the oom-killer, assuming that if the target node has an underlying
> > memory tier, it can always be reclaimed by demotion.
>
> Why don't we fall back to no demotion mode in this case? I mean we have
> shrink_folio_list:
>         if (!list_empty(&demote_folios)) {
>                 /* Folios which weren't demoted go back on @folio_list */
>                 list_splice_init(&demote_folios, folio_list);
>
>                 /*
>                  * goto retry to reclaim the undemoted folios in folio_list if
>                  * desired.
>                  *
>                  * Reclaiming directly from top tier nodes is not often desired
>                  * due to it breaking the LRU ordering: in general memory
>                  * should be reclaimed from lower tier nodes and demoted from
>                  * top tier nodes.
>                  *
>                  * However, disabling reclaim from top tier nodes entirely
>                  * would cause ooms in edge scenarios where lower tier memory
>                  * is unreclaimable for whatever reason, eg memory being
>                  * mlocked or too hot to reclaim. We can disable reclaim
>                  * from top tier nodes in proactive reclaim though as that is
>                  * not real memory pressure.
>                  */
>                 if (!sc->proactive) {
>                         do_demote_pass = false;
>                         goto retry;
>                 }
>         }
>
> to handle this situation no?

can_demote() is called from four places.
I tried modifying the patch to change the behavior only when can_demote()
is called from shrink_folio_list(), but the problem was not fixed
(oom did not occur).

Similarly, changing the behavior of can_demote() when called from
can_reclaim_anon_pages(), shrink_folio_list(), and can_age_anon_pages(),
but not when called from get_swappiness(), did not fix the problem either
(oom did not occur).

Conversely, changing the behavior only when called from get_swappiness(),
but not changing the behavior of can_reclaim_anon_pages(),
shrink_folio_list(), and can_age_anon_pages(), fixed the problem
(oom did occur).

Therefore, it appears that the behavior of get_swappiness() is important
in this issue.