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

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Message-ID: <aWZLZzo5Hb1aFZ0j@tiehlicka>
Date: Tue, 13 Jan 2026 14:40:55 +0100
From: Michal Hocko <mhocko@...e.com>
To: Akinobu Mita <akinobu.mita@...il.com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, akpm@...ux-foundation.org,
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Subject: Re: [PATCH v4 3/3] mm/vmscan: don't demote if there is not enough
 free memory in the lower memory tier

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?

-- 
Michal Hocko
SUSE Labs