Re: [PATCH v6] mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations

[Thadeu Lima de Souza Cascardo]

On Thu, Oct 01, 2026 at 08:21:51AM +0000, Salvatore Dipietro 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 reclaim, 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 reclaim and direct compaction are 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 enters the
> full blocking slowpath 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, clear
> __GFP_DIRECT_RECLAIM at the very start of the slowpath, before
> can_direct_reclaim, can_compact and the nofail checks are evaluated.
> This makes the entire slowpath treat the request as non-blocking: no
> direct reclaim, no direct compaction and no drain_all_pages() IPI
> across every CPU.  Reclaim and compaction move to the background
> instead: kswapd is still woken for reclaim, and it wakes kcompactd for
> defragmentation.  The system keeps its long-term health while the
> latency-critical direct allocation path no longer stalls.
> 
> 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.
> 
> Clearing __GFP_DIRECT_RECLAIM makes these allocations reach the
> !can_direct_reclaim bailout, which since commit 4c0ed883e051
> ("mm/page_alloc: fix defrag_mode for non-reclaimable allocations") drops
> ALLOC_NOFRAGMENT and retries instead of failing when defrag_mode is on.
> With vm.defrag_mode=1 a __GFP_NORETRY allocation could therefore succeed
> by fragmenting another migratetype's pageblock.  That is the wrong trade
> for a caller that has already communicated it can handle failure and has
> a cheap lower-order fallback, so exclude __GFP_NORETRY from the
> relaxation.  The exclusion is not restricted to costly orders: any
> __GFP_NORETRY caller has said it can cope with failure, so failing is
> preferable to fragmenting for all of them.
> 
> 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
>   v7.3-rc1 baseline (vm.defrag_mode=0)          58,602               -
>   v7.3-rc1 + this patch (vm.defrag_mode=0)     157,246         +168.3%
>   v7.3-rc1 baseline (vm.defrag_mode=1)         109,012               -
>   v7.3-rc1 + this patch (vm.defrag_mode=1)     157,778          +44.7%
> 
> Link: https://lore.kernel.org/all/20260403193535.9970-1-dipiets@xxxxxxxxx/T/#t [v1]
> Link: https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@xxxxxxxxx/T/#u [v2]
> Link: https://lore.kernel.org/all/20260710143437.12379-1-dipiets@xxxxxxxxx/T/#u [v3]
> Link: https://lore.kernel.org/all/20260904115629.3993331-1-dipiets@xxxxxxxxx/T/#u [v4]
> Link: https://lore.kernel.org/all/20260911142102.2294202-1-dipiets@xxxxxxxxx/T/#u [v5]
> Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Dave Chinner <dgc@xxxxxxxxxx>
> Cc: Ritesh Harjani <ritesh.list@xxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Dmitry Ilvokhin <d@xxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-xfs@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Acked-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Salvatore Dipietro <dipiets@xxxxxxxxx>
> ---
>  mm/page_alloc.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 12fac9084c48..64d09964ca77 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4784,10 +4784,10 @@ static inline struct page *
>  __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  						struct alloc_context *ac)
>  {
> -	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> -	bool can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask);
> -	bool nofail = gfp_mask & __GFP_NOFAIL;
>  	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
> +	bool can_direct_reclaim;
> +	bool can_compact;
> +	bool nofail;
>  	struct page *page = NULL;
>  	unsigned int alloc_flags;
>  	unsigned long did_some_progress;
> @@ -4802,6 +4802,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	bool can_retry_reserves = true;
>  	unsigned long alloc_start_time = jiffies;
>  
> +	/*
> +	 * Costly __GFP_NORETRY callers have a cheap fallback, so don't stall
> +	 * them in reclaim or compaction. __GFP_THISNODE callers are exempt.
> +	 */
> +	if (costly_order && (gfp_mask & __GFP_NORETRY) &&
> +	    !(gfp_mask & __GFP_THISNODE))
> +		gfp_mask &= ~__GFP_DIRECT_RECLAIM;
> +
> +	can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
> +	can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask);
> +	nofail = gfp_mask & __GFP_NOFAIL;
> +
>  	if (unlikely(nofail)) {
>  		/*
>  		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,

This would trigger a warning if __GFP_NOFAIL and __GFP_NORETRY were ever
used together, though that does not make sense and I haven't found a case
where it is used in practice.

Aside from either warning for the specific case of these flags being used
together or adding a comment right here to that effect, I don't think any
other change is needed, though.

Another comment about the general approach: TTM pool page allocator does
use __GFP_NORETRY and __GFP_DIRECT_RECLAIM for high orders and when we
previously suggested to drop __GFP_DIRECT_RECLAIM to avoid compaction, it
was frowned upon. In the end, a lot of other changes were made and for the
relevant cases, this does not seem to change its current behavior, amongst
other things, because it uses __GFP_THISNODE.

I did run some TTM GTT allocations with some fragmentation workload and I
haven't seen much change in the latency for these high order allocations,
so:

Tested-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxx>

> @@ -4918,8 +4930,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		 * Reclaim/compaction cannot run, so defrag_mode's strategy
>  		 * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow
>  		 * fallbacks rather than failing the allocation outright.
> +		 * Not for __GFP_NORETRY: those have a cheap lower order
> +		 * fallback, so failing beats fragmenting.
>  		 */
>  		if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) &&
> +		    !(gfp_mask & __GFP_NORETRY) &&
>  		    (gfp_mask & __GFP_KSWAPD_RECLAIM)) {
>  			alloc_flags &= ~ALLOC_NOFRAGMENT;
>  			goto retry;
> 
> base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
> -- 
> 2.50.1
> 
> 
> 
> 
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