Re: [PATCH] mm/page_alloc: skip high atomic reservation at or below costly order

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, May 18, 2026 at 06:25:32PM -0700, JP Kobryn (Meta) wrote:
> We're seeing a pattern in production where 2MB THP order-9 allocations are
> failing due to fragmentation and triggering reclaim on systems with plenty
> of free memory. Over time, the success rate of these THP allocations do not
> increase at all.
> 
> Inspecting zone->vm_stat[NR_FREE_PAGES] via kprobe on compaction_suitable()
> indicated the given zone had sufficient free pages for order-9 allocations,
> yet they were going unused. Drilling down into the zone and inspecting
> /proc/pagetypeinfo revealed why. Order-9 blocks were accumulating in the
> zone's HighAtomic bucket (while zero were present in Movable). THP is
> unable to draw blocks from HighAtomic since that bucket is not in the
> fallback list.
> 
> The heuristic for reserving pageblocks in HighAtomic is that any atomic
> allocation greater than order-0 will result in the full pageblock being
> captured. This means that an order-1 atomic allocation will over-reserve by
> 256x, a full 512 pageblock.
> 
> Gate the reservation on order. Skip for allocations at or below
> PAGE_ALLOC_COSTLY_ORDER. This prevents smaller atomic allocations from
> reserving entire pageblocks, and significantly helps when THP is in use on
> a fragmented but otherwise healthy system.
> 
> Testing was performed using an A/B instagram workload receiving prod
> traffic. Each side had ~60 hosts with 64G memory. The patch resulted in
> several gains:
> 
> Unpatched
> HighAtomic pageblocks per host: 309-312 (1% of zone or 620MB),
>   ...all order-9 blocks in HighAtomic
> THP success rate: 1-6%
> Compaction success rate: 0-2%
> pgscan_kswapd (total across ~60 hosts, per minute): ~70.2M
> Atomic order-4+ allocations: 0
> 
> Patched
> HighAtomic pageblocks per host: 1
> THP success rate: 44-78%
> Compaction success rate: 24-47%
> pgscan_kswapd (total across ~60 hosts, per minute): ~29.9M
> Atomic order-4+ allocations: 0

This is an interesting patch. A couple of thoughts:

1. You disabled the highatomic reserve for this workload and it didn't
seem to matter. Presumably <costly orders don't need the protection.

2. Maxing out the reserves is odd. ALLOC_HIGHATOMIC allocations will
try reserved space first, and I'd expect things that are commonly
highatomic to be short-lived. Why don't we stop with a couple of
claimed highatomic blocks that get continuously recycled?

3. The impact on THP and compaction success rate is pretty
extreme. How can 1% of memory throw such a wrench into the gears?

Have you tried this with other workloads?