Re: [PATCH v2] mm/page_alloc: fix defrag_mode for non-reclaimable allocations

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Thu, May 21, 2026 at 04:59:10PM -0700, Andrew Morton wrote:
> On Wed, 20 May 2026 12:22:28 +0000 Dmitry Ilvokhin <d@ilvokhin.com> wrote:
> 
> > When defrag_mode is enabled, ALLOC_NOFRAGMENT is enforced to prevent
> > migratetype fallbacks and keep pageblocks clean. The allocator relies on
> > reclaim and compaction to free pages of the correct type before allowing
> > fallback as a last resort.
> > 
> > However, non-reclaimable allocations such as GFP_ATOMIC cannot invoke
> > direct reclaim or compaction. With defrag_mode=1, these allocations hit
> > the !can_direct_reclaim bailout in __alloc_pages_slowpath() with
> > ALLOC_NOFRAGMENT still set, and fail without ever attempting a fallback.
> > 
> > This causes a large number of SLUB allocation failures for
> > skbuff_head_cache under network-heavy workloads, despite free memory
> > being available in other migratetype freelists.
> 
> That sounds painful.
> 
> > Clear ALLOC_NOFRAGMENT and retry for allocations that request kswapd
> > reclaim but cannot do direct reclaim themselves (GFP_ATOMIC).  Purely
> > speculative allocations like GFP_TRANSHUGE_LIGHT that don't set
> > __GFP_KSWAPD_RECLAIM are left to fail, since they have reasonable
> > fallbacks and should not cause fragmentation.
> 
> How serious is this to our users when running real-world workloads?

We observed it on a few of the Meta workloads that adopted
defrag_mode=1.

For the service under load there were 85509 SLUB allocation failures
messages in dmesg within 2 hours. All of them are GFP_ATOMIC allocations
for skbuff_head_cache, despite free pages being available in other
migratetype freelists (~13 GB free).

Since it is networking path from the practical point of view, this means
dropped packets, failed RPC requests, tail latency spikes and overall
service degradation.

> 
> > Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
> > 
> > Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
> > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>