Re: [PATCH v5] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

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

On Fri, 11 Sep 2026 14:21:02 +0000 Salvatore Dipietro <dipiets@amazon.it> wrote:

> Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
> introduced high-order folio allocations in the iomap buffered write
> path.  When memory is fragmented, each failed costly-order allocation
> enters __alloc_pages_slowpath() which runs direct compaction and
> drain_all_pages(), causing a 0.38x throughput drop on PostgreSQL
> pgbench (simple-update) with 1024 clients on a 96-vCPU arm64 system.
> 
> The root issue is that direct compaction is too expensive for hot
> allocation paths that have fallbacks to smaller allocations.
> __filemap_get_folio_mpol() already marks higher-order allocations with
> __GFP_NORETRY | __GFP_NOWARN, signalling that the caller can handle
> failure.  However, the page allocator still attempts full direct
> compaction for costly orders with __GFP_NORETRY, which is unnecessarily
> aggressive when the caller will simply retry at a lower order.
> 
> For costly-order allocations with __GFP_NORETRY, suppress direct reclaim
> for the whole slowpath by computing can_direct_reclaim (and hence
> can_compact) as false.  The !can_direct_reclaim check near the top of
> the slowpath then short-circuits to nopage: no direct reclaim, no direct
> compaction and no drain_all_pages() IPI across every CPU.  kswapd (and
> in turn kcompactd) is still woken further down for background
> defragmentation, so compaction keeps working for long-term system health
> while being removed from the latency-critical direct allocation path.
> 
> Allocations that also request __GFP_THISNODE are exempted.  That flag
> pairing identifies the local-node-first THP attempt issued by
> alloc_pages_mpol() (mempolicy.c), which relies on direct compaction to
> form transparent huge pages.  __GFP_NOFAIL is exempted as well, so a
> must-not-fail allocation is never made to fail.
> 
> Test environment:
>   Hardware:  AWS EC2 m8g.24xlarge (96 vCPU, arm64)
>              12x 1TB IO2 32000 IOPS RAID0 XFS
>   OS:        AL2023
>   Kernel:    v7.3-rc1
>   Database:  PostgreSQL 18.4
>   Workload:  pgbench simple-update, 1024 clients, 96 threads, 1200s
> 
> Results (average of 3 runs, TPS):
>   Config                                       Avg TPS   % vs baseline
>   AL2023 stock 6.1 kernel (pre-5d8edfb900d5)   136,942           n/a
>   v7.3-rc1 baseline (no patch)                  59,408             -
>   v7.3-rc1 + this patch                        161,994       +172.7%
> 
> The patch fully recovers the pre-5d8edfb900d5 performance, bringing
> throughput back above the pre-regression level and well clear of the
> ~59k baseline.  The AL2023 6.1 row runs a kernel that predates commit
> 5d8edfb900d5 ("iomap: Copy larger chunks from userspace"), so it is not
> directly comparable, but it shows the pre-regression level and confirms
> that the ~59k baseline is the anomaly and not the norm.

Thanks, I'll update mm.git's mm-hotfixes-unstable branch with this.


> v5: Derive the decision into a local flag instead of mutating gfp_mask

The acks from vbabka, hannes and hch were dropped.  Fair enough -
that's always a hard call.

Sashiko is worried:
	https://sashiko.dev/#/patchset/20260911142102.2294202-1-dipiets@amazon.it

That's different from Sashiko's v4 complaints:

	https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@amazon.it

does any of this look real?