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

[Andrew Morton <akpm@linux-foundation.org>]

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?

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