Re: [PATCH V4] mm, compaction: don't use ALLOC_CMA in long term GUP flow

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/12/15 18:01, yangge1116@126.com wrote:
> From: yangge <yangge1116@126.com>
> 
> Since commit 984fdba6a32e ("mm, compaction: use proper alloc_flags
> in __compaction_suitable()") allow compaction to proceed when free
> pages required for compaction reside in the CMA pageblocks, it's
> possible that __compaction_suitable() always returns true, and in
> some cases, it's not acceptable.
> 
> 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.
> 
> During the start-up of the virtual machine, it will call
> pin_user_pages_remote(..., FOLL_LONGTERM, ...) to allocate memory.
> 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. Since there is 16G of free CMA memory on the NUMA
> node, watermark for order-0 always be met for compaction, so
> __compaction_suitable() always returns true, even if the node is
> unable to allocate non-CMA memory for the virtual machine.
> 
> For costly allocations, because __compaction_suitable() always
> returns true, __alloc_pages_slowpath() can't exit at the appropriate
> place, 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
> 
> In order to quickly fall back to remote node, we should remove
> ALLOC_CMA both in __compaction_suitable() and __isolate_free_page()
> in long term GUP flow. After this fix, starting a 32GB virtual machine
> with device passthrough takes only a few seconds.
> 
> Fixes: 984fdba6a32e ("mm, compaction: use proper alloc_flags in __compaction_suitable()")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: yangge <yangge1116@126.com>
> ---
> 
> V4:
> - rich the commit log description
> 
> V3:
> - fix build errors
> - add ALLOC_CMA both in should_continue_reclaim() and compaction_ready()
> 
> V2:
> - using the 'cc->alloc_flags' to determin if 'ALLOC_CMA' is needed
> - rich the commit log description
> 
>   include/linux/compaction.h |  6 ++++--
>   mm/compaction.c            | 18 +++++++++++-------
>   mm/page_alloc.c            |  4 +++-
>   mm/vmscan.c                |  4 ++--
>   4 files changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index e947764..b4c3ac3 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -90,7 +90,8 @@ extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
>   		struct page **page);
>   extern void reset_isolation_suitable(pg_data_t *pgdat);
>   extern bool compaction_suitable(struct zone *zone, int order,
> -					       int highest_zoneidx);
> +					       int highest_zoneidx,
> +					       unsigned int alloc_flags);
>   
>   extern void compaction_defer_reset(struct zone *zone, int order,
>   				bool alloc_success);
> @@ -108,7 +109,8 @@ static inline void reset_isolation_suitable(pg_data_t *pgdat)
>   }
>   
>   static inline bool compaction_suitable(struct zone *zone, int order,
> -						      int highest_zoneidx)
> +						      int highest_zoneidx,
> +						      unsigned int alloc_flags)
>   {
>   	return false;
>   }
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 07bd227..585f5ab 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -2381,9 +2381,11 @@ static enum compact_result compact_finished(struct compact_control *cc)
>   
>   static bool __compaction_suitable(struct zone *zone, int order,
>   				  int highest_zoneidx,
> +				  unsigned int alloc_flags,
>   				  unsigned long wmark_target)
>   {
>   	unsigned long watermark;
> +	bool use_cma;
>   	/*
>   	 * Watermarks for order-0 must be met for compaction to be able to
>   	 * isolate free pages for migration targets. This means that the
> @@ -2395,25 +2397,27 @@ static bool __compaction_suitable(struct zone *zone, int order,
>   	 * even if compaction succeeds.
>   	 * For costly orders, we require low watermark instead of min for
>   	 * compaction to proceed to increase its chances.
> -	 * ALLOC_CMA is used, as pages in CMA pageblocks are considered
> -	 * suitable migration targets
> +	 * In addition to long term GUP flow, ALLOC_CMA is used, as pages in
> +	 * CMA pageblocks are considered suitable migration targets
>   	 */
>   	watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
>   				low_wmark_pages(zone) : min_wmark_pages(zone);
>   	watermark += compact_gap(order);
> +	use_cma = !!(alloc_flags & ALLOC_CMA);
>   	return __zone_watermark_ok(zone, 0, watermark, highest_zoneidx,
> -				   ALLOC_CMA, wmark_target);
> +				   use_cma ? ALLOC_CMA : 0, wmark_target);

Why not just use 'alloc_flags & ALLOC_CMA' instead? then you can remove 
the 'use_cma' variable.

>   }
>   
>   /*
>    * compaction_suitable: Is this suitable to run compaction on this zone now?
>    */
> -bool compaction_suitable(struct zone *zone, int order, int highest_zoneidx)
> +bool compaction_suitable(struct zone *zone, int order, int highest_zoneidx,
> +				   unsigned int alloc_flags)
>   {
>   	enum compact_result compact_result;
>   	bool suitable;
>   
> -	suitable = __compaction_suitable(zone, order, highest_zoneidx,
> +	suitable = __compaction_suitable(zone, order, highest_zoneidx, alloc_flags,
>   					 zone_page_state(zone, NR_FREE_PAGES));
>   	/*
>   	 * fragmentation index determines if allocation failures are due to
> @@ -2474,7 +2478,7 @@ bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
>   		available = zone_reclaimable_pages(zone) / order;
>   		available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
>   		if (__compaction_suitable(zone, order, ac->highest_zoneidx,
> -					  available))
> +					  alloc_flags, available))
>   			return true;
>   	}
>   
> @@ -2499,7 +2503,7 @@ compaction_suit_allocation_order(struct zone *zone, unsigned int order,
>   			      alloc_flags))
>   		return COMPACT_SUCCESS;
>   
> -	if (!compaction_suitable(zone, order, highest_zoneidx))
> +	if (!compaction_suitable(zone, order, highest_zoneidx, alloc_flags))
>   		return COMPACT_SKIPPED;
>   
>   	return COMPACT_CONTINUE;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index dde19db..9a5dfda 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2813,6 +2813,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
>   {
>   	struct zone *zone = page_zone(page);
>   	int mt = get_pageblock_migratetype(page);
> +	bool pin;
>   
>   	if (!is_migrate_isolate(mt)) {
>   		unsigned long watermark;
> @@ -2823,7 +2824,8 @@ int __isolate_free_page(struct page *page, unsigned int order)
>   		 * exists.
>   		 */
>   		watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
> -		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
> +		pin = !!(current->flags & PF_MEMALLOC_PIN);
> +		if (!zone_watermark_ok(zone, 0, watermark, 0, pin ? 0 : ALLOC_CMA))
>   			return 0;

I wonder why not pass ‘cc->alloc_flags’ as a parameter for 
__isolate_free_page()?

>   	}
>   
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 5e03a61..33f5b46 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5815,7 +5815,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>   				      sc->reclaim_idx, 0))
>   			return false;
>   
> -		if (compaction_suitable(zone, sc->order, sc->reclaim_idx))
> +		if (compaction_suitable(zone, sc->order, sc->reclaim_idx, ALLOC_CMA))
>   			return false;
>   	}
>   
> @@ -6043,7 +6043,7 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
>   		return true;
>   
>   	/* Compaction cannot yet proceed. Do reclaim. */
> -	if (!compaction_suitable(zone, sc->order, sc->reclaim_idx))
> +	if (!compaction_suitable(zone, sc->order, sc->reclaim_idx, ALLOC_CMA))
>   		return false;
>   
>   	/*