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

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Message-ID: <32d4b46a-cf7c-4fd2-a86f-7e82b9b1a5e3@126.com>
Date: Wed, 8 Jan 2025 16:35:27 +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 2025/1/8 10:50, Baolin Wang 写道:
> 
> 
> On 2025/1/6 16:49, Ge Yang wrote:
>>
>>
>> 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.
> 
> Please consider unevictable folios that are not in the active/inactive 
> file/anon LRU lists, yet can still be migrated.
Ok, thanks.