Re: [PATCH] mm: page_alloc: make defrag_mode retries follow the promoted order

[Andrew Morton]

On Tue, 29 Sep 2026 18:45:51 +0100 Kiryl Shutsemau <kirill@xxxxxxxxxxxxx> wrote:

> From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>
> 
> Since commit 7e8756d7ad22 ("mm: page_alloc: fix non-movable reclaim
> storm in defrag_mode"), direct reclaim and compaction for non-movable
> requests under defrag_mode run at pageblock_order, to produce the whole
> blocks that ALLOC_NOFRAGMENT needs. The retry decisions that follow
> still use the request order. An order-0 request can therefore retry
> indefinitely without ever reaching the ALLOC_NOFRAGMENT fallback:

7e8756d7ad22 is new in 7.3-rcX, so no cc:stable needed.

> - Reclaim at pageblock_order gives up after one pass as soon as a zone
>   looks compaction_ready(), and do_try_to_free_pages() then returns 1
>   even though nothing was reclaimed. It returns before the retry that
>   would reclaim memory.low-protected cgroups, so when most memory is
>   protected, the pass that did run finds next to nothing.
> 
> - Compaction at pageblock_order fails or is deferred.
> 
> - should_reclaim_retry() takes the reported progress as progress for
>   the order-0 request and resets no_progress_loops. The request
>   retries.
> 
> Order 1-3 requests loop the same way, and should_compact_retry() also
> checks their pageblock_order compaction result against the request
> order.
> 
> On a production host (64G, defrag_mode, memory.low covering most of the
> workload), 95% of direct reclaim runs were order-9 runs that returned 1
> with nothing reclaimed, at up to 60k runs per second. Across ~200M
> should_reclaim_retry() calls in a day, no_progress_loops never left 0.
> The spinning allocations were SLUB slab refills for inode and dentry
> caches. The time spent registers as memory pressure, and pressure-based
> OOM killing takes down both workloads and system services.

A production host running latest -rc?

> Treat promoted requests like costly orders:
> 
> - Reclaim progress does not reset no_progress_loops for them.
> 
> - should_compact_retry() checks the compaction result at the promoted
>   order. It does not retry COMPACT_SKIPPED, since the request can fall
>   back, and it does not escalate compaction to COMPACT_PRIO_SYNC_FULL.
> 
> When the fallback is taken, reset the retry counters, so that the
> fallback attempt gets a full retry budget before the OOM killer is
> considered.
> 
> In a VM reproducer (32G, defrag_mode, inode churn under memory.low):
> 
>                                         before    after
>     should_reclaim_retry() calls           63M     293k
>     peak memory pressure (PSI some avg10)  99%      12%
> 
> File creation runs 5.7x faster.
> 
> Fixes: 7e8756d7ad22 ("mm: page_alloc: fix non-movable reclaim storm in defrag_mode")
> Cc: <stable@xxxxxxxxxxxxxxx>

Please double-check?