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

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;

/*