Re: [PATCH V2] mm: compaction: use the actual allocation context to determine the watermarks for costly order during async memory compaction

[Ge Yang <yangge1116@126.com>]

This patch has been revised based on Vlastimil's suggestions. Please 
continue to review it. Thank you.

在 2025/1/16 9:33, yangge1116@126.com 写道:
> From: yangge <yangge1116@126.com>
> 
> There are 4 NUMA nodes on my machine, and each NUMA node has 32GB
> of memory. I have configured 16GB of CMA memory on each NUMA node,
> and starting a 32GB virtual machine with device passthrough is
> extremely slow, taking almost an hour.
> 
> Long term GUP cannot allocate memory from CMA area, so a maximum of
> 16 GB of no-CMA memory on a NUMA node can be used as virtual machine
> memory. There is 16GB of free CMA memory on a NUMA node, which is
> sufficient to pass the order-0 watermark check, causing the
> __compaction_suitable() function to  consistently return true.
> 
> For costly allocations, if the __compaction_suitable() function always
> returns true, it causes the __alloc_pages_slowpath() function to fail
> to exit at the appropriate point. This prevents timely fallback to
> allocating memory on other nodes, ultimately resulting in excessively
> long virtual machine startup times.
> Call trace:
> __alloc_pages_slowpath
>      if (compact_result == COMPACT_SKIPPED ||
>          compact_result == COMPACT_DEFERRED)
>          goto nopage; // should exit __alloc_pages_slowpath() from here
> 
> We could use the real unmovable allocation context to have
> __zone_watermark_unusable_free() subtract CMA pages, and thus we won't
> pass the order-0 check anymore once the non-CMA part is exhausted. There
> is some risk that in some different scenario the compaction could in
> fact migrate pages from the exhausted non-CMA part of the zone to the
> CMA part and succeed, and we'll skip it instead. But only __GFP_NORETRY
> allocations should be affected in the immediate "goto nopage" when
> compaction is skipped, others will attempt with DEF_COMPACT_PRIORITY
> anyway and won't fail without trying to compact-migrate the non-CMA
> pageblocks into CMA pageblocks first, so it should be fine.
> 
> After this fix, it only takes a few tens of seconds to start a 32GB
> virtual machine with device passthrough functionality.
> 
> Link: https://lore.kernel.org/lkml/1736335854-548-1-git-send-email-yangge1116@126.com/
> Signed-off-by: yangge <yangge1116@126.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> 
> V2:
> - update code and message suggested by Vlastimil
> 
>   mm/compaction.c | 29 +++++++++++++++++++++++++----
>   1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 07bd227..3de7b67 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2490,7 +2490,8 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>    */
>   static enum compact_result
>   compaction_suit_allocation_order(struct zone *zone, unsigned int order,
> -				 int highest_zoneidx, unsigned int alloc_flags)
> +				 int highest_zoneidx, unsigned int alloc_flags,
> +				 bool async)
>   {
>   	unsigned long watermark;
>   
> @@ -2499,6 +2500,23 @@ compaction_suit_allocation_order(struct zone *zone, unsigned int order,
>   			      alloc_flags))
>   		return COMPACT_SUCCESS;
>   
> +	/*
> +	 * For unmovable allocations (without ALLOC_CMA), check if there is enough
> +	 * free memory in the non-CMA pageblocks. Otherwise compaction could form
> +	 * the high-order page in CMA pageblocks, which would not help the
> +	 * allocation to succeed. However, limit the check to costly order async
> +	 * compaction (such as opportunistic THP attempts) because there is the
> +	 * possibility that compaction would migrate pages from non-CMA to CMA
> +	 * pageblock.
> +	 */
> +	if (order > PAGE_ALLOC_COSTLY_ORDER && async &&
> +	    !(alloc_flags & ALLOC_CMA)) {
> +		watermark = low_wmark_pages(zone) + compact_gap(order);
> +		if (!__zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
> +					   0, zone_page_state(zone, NR_FREE_PAGES)))
> +			return COMPACT_SKIPPED;
> +	}
> +
>   	if (!compaction_suitable(zone, order, highest_zoneidx))
>   		return COMPACT_SKIPPED;
>   
> @@ -2534,7 +2552,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
>   	if (!is_via_compact_memory(cc->order)) {
>   		ret = compaction_suit_allocation_order(cc->zone, cc->order,
>   						       cc->highest_zoneidx,
> -						       cc->alloc_flags);
> +						       cc->alloc_flags,
> +						       cc->mode == MIGRATE_ASYNC);
>   		if (ret != COMPACT_CONTINUE)
>   			return ret;
>   	}
> @@ -3037,7 +3056,8 @@ static bool kcompactd_node_suitable(pg_data_t *pgdat)
>   
>   		ret = compaction_suit_allocation_order(zone,
>   				pgdat->kcompactd_max_order,
> -				highest_zoneidx, ALLOC_WMARK_MIN);
> +				highest_zoneidx, ALLOC_WMARK_MIN,
> +				false);
>   		if (ret == COMPACT_CONTINUE)
>   			return true;
>   	}
> @@ -3078,7 +3098,8 @@ static void kcompactd_do_work(pg_data_t *pgdat)
>   			continue;
>   
>   		ret = compaction_suit_allocation_order(zone,
> -				cc.order, zoneid, ALLOC_WMARK_MIN);
> +				cc.order, zoneid, ALLOC_WMARK_MIN,
> +				false);
>   		if (ret != COMPACT_CONTINUE)
>   			continue;
>