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Message-ID: <3e160d54-5860-4616-bd32-5faff3afdb49@126.com>
Date: Mon, 16 Dec 2024 19:53:38 +0800
From: Ge Yang <yangge1116@....com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
21cnbao@...il.com, david@...hat.com, vbabka@...e.cz, liuzixing@...on.cn
Subject: Re: [PATCH V4] mm, compaction: don't use ALLOC_CMA in long term GUP
flow
在 2024/12/16 16:41, Baolin Wang 写道:
>
>
> On 2024/12/15 18:01, yangge1116@....com wrote:
>> From: yangge <yangge1116@....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@...r.kernel.org>
>> Signed-off-by: yangge <yangge1116@....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.
yes, I will change it in next version.
>
>> }
>> /*
>> * 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()?
Some places that use __isolate_free_page() don't have 'cc->alloc_flags',
which leads me to believe that no modification is required.
I will change it in next version.
>
>> }
>> 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;
>> /*
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