Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

["Zi Yan" <ziy@nvidia.com>]

On Fri Sep 4, 2026 at 10:11 AM EDT, Vlastimil Babka (SUSE) wrote:
> On 9/4/26 13:56, 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 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, 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. 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.
>> 
>> 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
>>   baseline (no patch)       59,408       -
>>   With this patch          155,409      +161.6%
>> 
>> Link: https://lore.kernel.org/all/20260403193535.9970-1-dipiets@amazon.it/T/#t [v1]
>> Link: https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@amazon.it/T/#u [v2]
>> Link: https://lore.kernel.org/all/20260710143437.12379-1-dipiets@amazon.it/T/#u [v3]
>> Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
>> Cc: stable@vger.kernel.org
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Dave Chinner <dgc@kernel.org>
>> Cc: Ritesh Harjani <ritesh.list@gmail.com>
>> Cc: linux-mm@kvack.org
>> Cc: linux-fsdevel@vger.kernel.org
>> Cc: linux-xfs@vger.kernel.org
>> Signed-off-by: Salvatore Dipietro <dipiets@amazon.it>
>
> I guess this will have to do until unlikely(we figure out a better API)...

We could add a "new_gfp = gfp_policy(gfp)" to adjust input gfp based on
various policies we currently have. Even better if callers can do that
instead.

>
> Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>
>> ---
>> v4: Clear __GFP_DIRECT_RECLAIM early in the slowpath and exempt
>>     __GFP_THISNODE so THP attempt keeps using direct compaction
>> v3: Move to mm/page_alloc.c, wake kcompactd instead of avoiding it
>> v2: Move from fs/iomap/buffered-io.c to mm/filemap.c
>> v1: Avoid compaction in iomap folio allocation
>> 
>>  mm/page_alloc.c | 18 +++++++++++++++---
>>  1 file changed, 15 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 12fac9084c48..542c2ec31061 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.
>> +	 */

Should this also be documented in gfp_types.h? It currently only says,

"__GFP_NORETRY: The VM implementation will try only very lightweight
memory direct reclaim to get some memory under memory pressure (thus it
can sleep)."

Otherwise,

Acked-by: Zi Yan <ziy@nvidia.com>

>> +	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,




-- 
Best Regards,
Yan, Zi