linux-kernel - Re: [PATCH 1/3] mm-unstable: Multi-gen LRU: Fix per-zone reclaim

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Date:   Tue, 1 Aug 2023 20:07:38 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Kalesh Singh <kaleshsingh@...gle.com>
Cc:     akpm@...ux-foundation.org, surenb@...gle.com,
        android-mm@...gle.com, kernel-team@...roid.com,
        Charan Teja Kalla <quic_charante@...cinc.com>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 1/3] mm-unstable: Multi-gen LRU: Fix per-zone reclaim

On Tue, Aug 1, 2023 at 6:19 PM Kalesh Singh <kaleshsingh@...gle.com> 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).
>
> Qualcomm, Mediatek and raspberrypi [1] discovered this issue independently.
>
> [1] https://github.com/raspberrypi/linux/issues/5395
>
> Cc: Yu Zhao <yuzhao@...gle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Reported-by: Charan Teja Kalla <quic_charante@...cinc.com>
> Reported-by: Lecopzer Chen <lecopzer.chen@...iatek.com>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>

LGTM. But I think we need the Fixes tag and Cc stable.