Thanks Kalesh for taking this to upstream.
On 8/2/2023 8:26 AM, Kalesh Singh wrote:
MGLRU has a LRU list for each zone for each type (anon/file) in each generation:
long nr_pages[MAX_NR_GENS][ANON_AND_FILE][MAX_NR_ZONES]; The min_seq (oldest generation) can progress independently for each type but the max_seq (youngest generation) is shared for both anon and file. This is to maintain a common frame of reference.
In order for eviction to advance the min_seq of a type, all the per-zone lists in the oldest generation of that type must be empty.
The eviction logic only considers pages from eligible zones for eviction or promotion.
scan_folios() {... for (zone = sc->reclaim_idx; zone >= 0; zone--) { ... sort_folio(); // Promote ... isolate_folio(); // Evict } ... }
Consider the system has the movable zone configured and default 4 generations. The current state of the system is as shown below (only illustrating one type for simplicity):
Type: ANON
Zone DMA32 Normal Movable Device
Gen 0 0 0 4GB 0
Gen 1 0 1GB 1MB 0
Gen 2 1MB 4GB 1MB 0
Gen 3 1MB 1MB 1MB 0
Now consider there is a GFP_KERNEL allocation request (eligible zone index <= Normal), evict_folios() will return without doing any work since there are no pages to scan in the eligible zones of the oldest generation. Reclaim won't make progress until triggered from a ZONE_MOVABLE allocation request; which may not happen soon if there is a lot of free memory in the movable zone. This can lead to OOM kills, although there is 1GB pages in the Normal zone of Gen 1 that we have not yet tried to reclaim.
This issue is not seen in the conventional active/inactive LRU since there are no per-zone lists.
If there are no (not enough) folios to scan in the eligible zones, move folios from ineligible zone (zone_index > reclaim_index) to the next generation. This allows for the progression of min_seq and reclaiming from the next generation (Gen 1).
As discussing offline, I think this can make system to spend too much time in scan_folios() in moving the pages from Gen-0 to Gen-1 of the other zone which can result into OOM is not active when necessary.
Qualcomm, Mediatek and raspberrypi [1] discovered this issue independently.
[1] https://github.com/raspberrypi/linux/issues/5395
Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation") Cc: stable@vger.kernel.org Cc: Yu Zhao yuzhao@google.com Cc: Andrew Morton akpm@linux-foundation.org Reported-by: Charan Teja Kalla quic_charante@quicinc.com Reported-by: Lecopzer Chen lecopzer.chen@mediatek.com Signed-off-by: Kalesh Singh kaleshsingh@google.com
We tested this patch on our systems for couple of weeks and aggressive OOM is not observed which otherwise is easily reproducible.
Tested-by: Charan Teja Kalla quic_charante@quicinc.com