The patch titled Subject: mm, compaction: don't use ALLOC_CMA in long term GUP flow has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: yangge yangge1116@126.com Subject: mm, compaction: don't use ALLOC_CMA in long term GUP flow Date: Sun, 15 Dec 2024 18:01:07 +0800

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.

Link: https://lkml.kernel.org/r/1734256867-19614-1-git-send-email-yangge1116@126.c... Fixes: 984fdba6a32e ("mm, compaction: use proper alloc_flags in __compaction_suitable()") Signed-off-by: yangge yangge1116@126.com Cc: Baolin Wang baolin.wang@linux.alibaba.com Cc: David Hildenbrand david@redhat.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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(-)

--- a/include/linux/compaction.h~mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow +++ a/include/linux/compaction.h @@ -90,7 +90,8 @@ extern enum compact_result try_to_compac 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_suita }

static inline bool compaction_suitable(struct zone *zone, int order, - int highest_zoneidx) + int highest_zoneidx, + unsigned int alloc_flags) { return false; } --- a/mm/compaction.c~mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow +++ a/mm/compaction.c @@ -2379,9 +2379,11 @@ static enum compact_result compact_finis

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 @@ -2393,25 +2395,27 @@ static bool __compaction_suitable(struct * 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); }

/* * 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 @@ -2472,7 +2476,7 @@ bool compaction_zonelist_suitable(struct 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; }

@@ -2497,7 +2501,7 @@ compaction_suit_allocation_order(struct 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; --- a/mm/page_alloc.c~mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow +++ a/mm/page_alloc.c @@ -2812,6 +2812,7 @@ int __isolate_free_page(struct page *pag { struct zone *zone = page_zone(page); int mt = get_pageblock_migratetype(page); + bool pin;

if (!is_migrate_isolate(mt)) { unsigned long watermark; @@ -2822,7 +2823,8 @@ int __isolate_free_page(struct page *pag * 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; }

--- a/mm/vmscan.c~mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow +++ a/mm/vmscan.c @@ -5861,7 +5861,7 @@ static inline bool should_continue_recla 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; }

@@ -6089,7 +6089,7 @@ static inline bool compaction_ready(stru 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;

/* _

Patches currently in -mm which might be from yangge1116@126.com are

mm-compaction-dont-use-alloc_cma-in-long-term-gup-flow.patch