linux-kernel - Re: [PATCH] mm: compaction: skip memory compaction when there are not enough migratable pages

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Message-ID: <2889f0bf-b0ae-4f1a-b91c-fb4b59eb2d97@126.com>
Date: Mon, 6 Jan 2025 16:49:58 +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, 21cnbao@...il.com,
 david@...hat.com, hannes@...xchg.org, liuzixing@...on.cn
Subject: Re: [PATCH] mm: compaction: skip memory compaction when there are not
 enough migratable pages



在 2025/1/6 16:12, Baolin Wang 写道:
> 
> 
> On 2025/1/4 16:58, yangge1116@....com wrote:
>> From: yangge <yangge1116@....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.
>>
>> 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. 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.
>> However, if there aren't enough migratable pages available, performing
>> memory compaction is also meaningless. Besides checking whether
>> the order-0 watermark is met, __compaction_suitable() also needs
>> to determine whether there are sufficient migratable pages available
>> for memory compaction.
>>
>> 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
>>
>> When the 16G of non-CMA memory on a single node is exhausted, we will
>> fallback to allocating memory on other nodes. In order to quickly
>> fallback to remote nodes, we should skip memory compaction when
>> migratable pages are insufficient. After this fix, it only takes a
>> few tens of seconds to start a 32GB virtual machine with device
>> passthrough functionality.
>>
>> Signed-off-by: yangge <yangge1116@....com>
>> ---
>>   mm/compaction.c | 19 +++++++++++++++++++
>>   1 file changed, 19 insertions(+)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index 07bd227..1c469b3 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -2383,7 +2383,26 @@ static bool __compaction_suitable(struct zone 
>> *zone, int order,
>>                     int highest_zoneidx,
>>                     unsigned long wmark_target)
>>   {
>> +    pg_data_t *pgdat = zone->zone_pgdat;
>> +    unsigned long sum, nr_pinned;
>>       unsigned long watermark;
>> +
>> +    sum = node_page_state(pgdat, NR_INACTIVE_FILE) +
>> +        node_page_state(pgdat, NR_INACTIVE_ANON) +
>> +        node_page_state(pgdat, NR_ACTIVE_FILE) +
>> +        node_page_state(pgdat, NR_ACTIVE_ANON);
>> +
>> +    nr_pinned = node_page_state(pgdat, NR_FOLL_PIN_ACQUIRED) -
>> +        node_page_state(pgdat, NR_FOLL_PIN_RELEASED);
>> +
>> +    /*
>> +     * Gup-pinned pages are non-migratable. After subtracting these 
>> pages,
>> +     * we need to check if the remaining pages are sufficient for memory
>> +     * compaction.
>> +     */
>> +    if ((sum - nr_pinned) < (1 << order))
>> +        return false;
>> +
> 
> IMO, using the node's statistics to determine whether the zone is 
> suitable for compaction doesn't make sense. It is possible that even 
> though the normal zone has long-term pinned pages, the movable zone can 
> still be suitable for compaction.
If all the memory used on a node is pinned, then this memory cannot be 
migrated anymore, and memory compaction operations would not succeed.
I haven't used movable zone before, can you explain why memory 
compaction is still necessary? Thank you.